Alan W2AEW - 'Scopes for Dopes - Cycle25

live on youtube um so i know everybody's well behaved but i have to point that out um it's just setting up the stream on now um so okay okay thank you very much for joining uh yet another cycle 25 talk uh today i am super lucky to have alan w2aew i didn't even bust this call today um mr scope um and uh this came out of a conversation uh that uh i have to thank a few people uh for helping enable uh pat's uh one of them but uh basically what i was looking at was boy it's entirely selfish for me i have a scope i don't use it to its full capability i barely understand it boy and then i tripped over this wonderful deck that alan had done some years ago on scopes for dopes so explaining basics for oscilloscopes how to use them in the ham shack what are the basics around that what do you need to know uh to get started and so then i approached him and said hey could you update that for us and come talk to us so he was silly enough to say yes uh with that allen i will turn it over to you all righty well hey good morning and i see my my friend pat in here sitting in front of a wiggly square wave need a shorter ground to fix that yeah we got to talk about that on about 37 so let me do let me do a quick share of my presentation for us here and uh let's see make sure you all can see that i got it all righty so um yeah yeah the original the original version of this talk that i did was probably more than 10 years ago didn't include any of the digital stuff and it was done for a new jersey antique radio club at um at one of their meetings on what we used to be part of fort monmouth it was actually an area called evans area or camp evans and it's where um the us army signal corps during world war ii did a lot of their radar research and one of the guys that worked there prior to um founding tektronix as howard volume so it was just kind of interesting that we were doing a scope talk in a place where one of the founding members of tektronix actually worked before tektronix existed so uh kind of cool so we're going to call this basics of oscilloscopes and the hamshack our oscilloscopes and then the use in the hamshack here so um we'll just kind of get rolling all right so here's our agenda we're going to talk a bit about analog scopes first because that's what many of us have because they're you can almost get them for free these days because people want the digital stuff we'll talk about all the various controls and what they do right you mentioned that you've got a scope and you don't use it to its full potential i'll tell you the truth is that most people will use about you know 10 or 20 of what their scope can do especially today with the digital scopes with all the different features so i don't feel like you're not uh as long as you're you're using it for you know the applications that fit for your ham shock use and things like that great so don't worry about uh you might not ever need you know the the delayed dual time base or something like that but don't worry about it alan would you like questions as we go or do you want to hold those to the end i'm fine taking them as we go um it's just if we do them in the chat i won't see them so okay so what i'll ask is people just uh stay muted for most of it that'll keep side conversations from getting silly but come off mute and just call out the question from time to time or if you've got a question that you don't mind waiting to the end throw it in the chat and we'll deal with those at the end but if there's something that i i gotta talk you know on a slide that doesn't what something wasn't clear you have a question just give me a hug so so we'll talk about the basics of digital scopes some of the differences in terms of how they operate that type of thing we'll talk about probes and we'll talk about some examples of how you can use a scope in your hamshack so the cathode ray oscilloscope uh which is uh the old analog scopes uh i've just kind of got a pictorial picture here of of the the scope or you know the tube itself the crt and essentially it's kind of like a tv tube in the sense that you've got an electron gun firing an electron beam at the phosphor on the screen that glows as the beam is kind of swept through that beam is directed to go across and uh and up and down based on horizontal and vertical deflection plates and those horizontal vertical deflection plates are driven by amplifiers that gets input signals you know to to drive the beam back and forth through the horizontal circuit and then also to drive them vertically through your input and then the vertical amplifiers in there so that's kind of this is really a very basic block diagram of an analog scope you can think of it as really a just a live graphical voltmeter it's really all it does it displays essentially voltage variations over time and uh that's the simplest way to think about what a uh a cathode ray oscilloscope or any oscilloscope is uh the screen is typically broken up in with radicals uh and when we talk about things like volts per division or time per division we're talking about these kind of major divisions and it's typically a 10 by 10 or a 10 by 8 array of divisions uh on the scope so those major divisions are the ones that we're referring to and we say moles per division or you know microseconds per division or something like that and some of the other there's some other additional markings that can help with things like making rise and fall time measurements and then each of the major divisions are typically broken up with five dashes for minor divisions to help you to to make some estimations of voltage and things like that beyond just the major uh the major divisions so the display system uh typically its controls are you know are those are things like uh beam finder tracerotation focus and intensity and depending on the scope they'll look a little bit different there are some controls that you use more often like the trace intensity or maybe the focus you might use more often something like a trace rotation you don't use that often trace rotation refers to any tilt that there might be on the trace when there's no input ideally it's going to be exactly parallel to you know the horizontal grid lines or radical lines but uh because it's a magnetic deflection system depending on the magnetic field in your lab or where you're on your bench and where you put the scope you might get a little tilt on the display and this allows you to compensate for that some of the real older scopes actually with around crts didn't have a trace rotation you literally would loosen up the clamps holding the crt and you turn it to align it up but they'll look different on different scopes this is just another view of what it might look like and then there's one called beam finder the beam finder what it does is it collapses the deflection voltages for the horizontal and vertical down to the point where let's say you turn your scope on you're probing on your signal there's nothing on the screen well why is there nothing on the screen is there nothing on the screen because you're not triggering right or is there nothing on the screen because the horizontal and vertical adjustments aren't set right if the trace is way off the top or bottom or left to right of the screen hitting the beam fine will kind of collapse all those voltages so you can kind of see where it is oh it's way off to the left i need to adjust my horizontal position to the right to kind of get it on something like that but that's what the beam find is just a momentary push button to kind of you know bring bring the trace if there is one onto the screen somewhere so you can see which end it was you know thrown off the side of so the vertical system this is where this is the system that processes all the inputs that you put into the scope the signals you want to measure uh and and it also is typically what will define the bandwidth of the scope right the bandwidth of the scope is to think of the front of the front end of the scope as a low pass filter right it's going to pass signals from dc and then it's going to roll off at some point okay like a low-pass filter that roll-off point that 3db point is called the bandwidth of the scope and it's typically going to be dominated by the preamp and amplifier that is are inside the scope driving those vertical deflection plates but then there's other controls to control the position and coupling and sensitivity of the signal that gets applied to those vertical deflection plates to move the beam up or down okay so those controls again i've got a number up here to show what they look like typically in most scopes these days that you'll find you'll have some calibrated vertical scales and it says you can see it says volts per division here this one says volts per division ranging from you know 2 millivolts of division in this case up to 5 volts of division over here this one's 5 millivolts to 20 volts of division going around some of the older service scopes that you might find and i would say to steer away from are these uncalibrated scopes where you basically just have an adjustable vertical attenuator and a vernier control they're good for looking at relative amplitude waveforms and things like that and you could put a a known amplitude into it and adjust things to kind of get a calibrated thing but these days it's all there's almost no no reason to use those because you can get good calibrated vertical scales on scopes pretty easily these days so um it's often called the vertical attenuator control because if we go back to the previous slide you'll notice that we really have an attenuator ahead of a preamp okay so when we adjust the vertical scale we're not adjusting the gain of an amplifier okay we're adjusting the magnitude of a signal going into a preamp and the reason for that is much like if we think about a receiver design or a transmitter design if you when you tune your receiver you're not adjusting a tuned circuit you know to to dial in a particular frequency that you want to measure you're basically dying a local oscillator and a mix and a conversion stage to put the signal of interest you want into an if stage that does all the filtering and processing because it's a lot easier to do all the filtering and all that stuff on a fixed frequency okay so this is kind of similar to that it's a lot easier to set up and design the front end of a scope with a fixed amount of gain so you can get the the transient response the frequency response the linearity performance of that all set up with a fixed gain and then you just adjust the amplitude going into it okay so that's a lot easier to do than to have a variable gain amplifier that maintains its bandwidth and flatness across a wide amplitude range so that's why it's often called an attenuator control okay one thing i'll make a note of here is you'll notice one of the controls that you have see it says down here ac or dc coupling you see ac or dc coupling over here ac or dc coupling this is not like a dmm right whereas a dmm you select dc to measure dc voltages you select ac to measure ac voltages that's not the same thing here ac or dc coupling basically determines whether or not we essentially have a capacitor in series with the input or not and so what that allows us to do is let's say i'm looking at ripple on top of a power supply voltage so let's say if i had ground it was here and then this was where my power supply voltage was and i could see the ripple on it if i wanted to zoom in on that right i could go to a lower number of volts per division to get more sensitivity but then i might have to adjust my position to bring the signal back down on the screen right because if i this is say five volts or you know say two volts of division right now right one two three four and that's five volts if i set it to two volts of division this signal goes right up across up past the top of the screen so i have to bring my vertical position down to bring it back onto the screen again now eventually you'll run out of vertical position adjustment okay so what ac coupling allows you to do is throw a a series cat capacitor uh in that input path get rid of the dc component okay and then then you can adjust the volts per division however you want so now you're just focusing on the ac wiggle if you will the ripple that's on top of the power supply so it's funny that oftentimes if you're looking at dc signals looking for ripple you often will use ac coupling okay so a little counter intuitive but that's really what you would often do conversely let's say you're working on maybe like an audio amplifier okay and you're getting a little bit of clipping or something a little bit of distortion if you're probing around in that circuit it might be helpful to know when or why is it clipping is by clipping because the signal is being driven into the rail or am i clipping because i'm driving you know a voltage down and saturating a transistor or something like that so in that case it's helpful to not only see the ac signal but where it is in a dc standpoint so looking at those ac audio signals so an amplifier you might want dc coupling to understand both the bias and the um the ac wiggle on it so so the coupling control again not like a dmm it really controls how the signal gets processed through the scope whether you want to focus on just the wiggle and ignore and reject the dc component or you actually want to include the dc component what you're measuring so the last thing i'll mention too is i know notice on the front panel of the scope you will see notations like this for the input impedance of the scope most of the scopes or almost all of them will essentially be one mega ohm in parallel with some amount of capacitance this is one meg in parallel with 15 puff and there's some down here there's another one down here this is one meg and 25 puff and then some higher speed scopes will often have the option to switch in a 50 ohm termination for doing some rf work but it's typically only for scopes that are 500 megahertz bandwidth and above okay most of the time it's going to be one mega okay but the capacitance is important to note and we'll discuss this a little bit later when we talk about probes so but that is printed typically on the front panel of your scope all right any questions here everybody's awake okay no snoring that's good okay so more on vertical let's talk about vertical modes okay and these are some of the examples of the different ways vertical modes might appear there might be push buttons there might be an actual vertical mode switch and here i can see right above the vertical controls i see the vertical mode controls oh one more thing i should mention and i'll talk about it right here typically on the volts per division you'll see a center concentric knob and you see it says cal with a a right a right or counter or clockwise turning arrow what this does is if you turn that cowl knob all the way to the right it'll click and when it's in that clicked position now you're in that calibrated number of volts per division you do have the option of turning it to the left and and doing a kind of a vernier adjustment so if you wanted to make the the waveform on the screen a given size like maximize it on the screen you didn't really care about the actual volts per division you could use the variable for that and it also could be used for doing things like some rise time measurements and all but most of the time you're going to leave this thing fully clockwise in its clicked detented position to get the calibrated volts per division okay so meant to mention that on the previous slide so in terms of vertical modes right you can select view depending on how many channels you have channel one through channel four you can select one channel or the other you can select alt what alt does is on when the beam sweeps across it'll first sweep across showing you channel one the next sweep will show chat the channel two the next sweep will show channel one it'll alternate back and forth but often times at your sweep speeds are fast enough that you don't see this okay you just see two live traces on the screen and they're both on the screen because the persistence and the foster is going to leave them on there so it just looks like they're there there are instances though where if you're looking at relatively low sweep speeds like you're going along at 100 milliseconds you know 100 milliseconds per division which would give you one second it could go across the screen you'd actually see it alternate between channel 1 and channel two and if you want to look at both of those signals at the same time the alt mode really doesn't help you and that's where you'd want to use something called the chop mode what chop does is that relatively quickly tens of kilohertz will bounce back and forth between you know coupling to channel one and cupping the channel two but you don't see that okay what you'll see is essentially two to be looks like two beams going across the screen it's really one that's going back and forth with a blanking in between so you don't see it so chop is like alt for when you're looking at using lower sweep speeds on your scope alt is what you'd use for higher sweep speeds where you won't have any flicker between the going back and forth between multiple channels but both of these apply only when you're looking at more than one channel at a time simultaneously you'll also notice that there's an add mode where i can add two channels together well why would i want to do that hang on a sec there's also an invert mode where i can invert one channel now the add kind of makes sense now i can actually do an add an invert and actually do the difference measurement or maybe across the component because unlike a dmm where you take the leads from a dmm and you can poke them anywhere you want on the circuit right i want to look at the voltage across this resistor i want to look at the voltage across that you know the base to emitter or that transistor or something like that you can just really nearly poke that wherever you want you can't do that with a scope probe right because the scope probe one end of it is always ground it's tied to chassis ground so i can't make a differential measurement across two arbitrary points right i'm always making measurements with respect to ground so in order to make a measurement across a component i would actually use two probes both with respect to ground and do an add an invert so it's essentially taking channel one minus channel two which is basically like using a dmm across it so that's a kind of the easy way to make a differential measurement across a component by using the add and invert that's why it's typically on all these analog uh yeah your channels are more yeah so i think i think it's interesting to point out for people that haven't used scopes that's really uh something to be aware of that you don't want to go poking your ground lead trying to make a differential measurement you'll short something out you'll short something to ground potentially and that's a terrible problem to have when you're trying to troubleshoot oh yeah you'll let the smoke out that's for sure so that's a good point pat and it's it is something that i'll you know we'll talk a little bit about later too but um there are a couple of scopes and not many that have isolated channels where you can actually put the probe you know the the probe lead and its ground lead anywhere you want but 99 of the cases the ground lead on the scope is connected to chassis ground if you're working on a battery operated circuit then you can kind of get away with it because then there's you don't have any other ground connection but if you're working on anything that's plugged into you know plugged into the wall or a power supply you do have to kind of worry where ground is and always make sure you pro you only connect these scope probes ground lead to ground in your circuit okay so the horizontal system controls you know the sweeping of the beam across the screen okay and there's a bunch of little pieces to it here but in a sense there's a sweep generator which generates a voltage that is applied to the deflection plates to move the beam back and forth and that's very quickly swept back to the other side and does it again and there's a number of controls here we'll talk about in terms of you know how we actually control that on the analog scopes we call it a sweep time control or a time based control on the modern digital scopes unfortunately they call it horizontal scale okay because there is no sweeping in a digital scope because we're basically just digitizing signals but i you know i still call it you know sweep time or time base because that's really where where it comes from is basically creating as a ramp voltage that goes across so let's take a look at there's actually a couple of variations here that i want to talk about so the old service scopes that i mentioned earlier that didn't have the calibrated vertical they also don't have a calibrated horizontal and these were called recurrent sweep scopes so anytime you see an old scope that is for the horizontal has a frequency range like 10 hertz 100 hertz per kilohertz 10 kilo 10 kilo cycles is a dead giveaway that is kilo cycles and a vernier control like this this means that the horizontal is driven by essentially a saw a free-running sawtooth wave so it runs continuously it's free running there's a sync control that can be used to kind of try to injection lock a piece of your input signal onto this sawtooth to try to keep things somewhat synchronized but they're not calibrated as really older and really much simpler technology and again today almost no excuse to be using this anymore just if you see we're looking for a scope at a ham fest and it's got these types of controls as a recurrent sweep scope and just not going to be as useful for you as a triggered sweep scope which is what we're looking at here a trigger sweep scope the sweep essentially runs across the screen this voltage represents essentially a position across the screen horizontally and then it resets and then it waits until it gets triggered again and then it sweeps again so much much better technology in terms of control getting a nice stable waveform to look at on the screen and the sweep time is calibrated in you know milliseconds of microseconds or nanoseconds or seconds per division okay you don't have any of that really with the recurrent sweep scope so again because even these trigger sweep scopes have been around since the 60s there's really no excuse for using a recurrent sweep scope and getting limited by that anymore okay so uh let's talk about triggering again on the recurrence scopes it's called sync you know why do we want to uh trigger or synchronize we want to stabilize that waveform on the screen and what that does is you're trying to lock the sweep to some feature on a repetitive waveform okay so that we always start the sweep at the same point so as as you get multiple sweeps the waveform overlays on top of each other and you get a nice stable waveform to look at like the background on the back background behind path there okay so uh again with the old uh recurrent sweep scope so there was a sync control and what that would do is take a portion of the input signal and tr and inject it into that horizontal oscillator to try to injection lock it and that's the only control you had it's just horrible okay so and this is kind of what it looked like you select an internal sink uh sync source and you the sync amplitude controlled how much of the input signal you could inject into that that oscillator and you'd have to mess around with the horizontal oscillator vernier and this control to get a stable waveform it's just horrible don't bother don't don't use those okay use a triggered sweep scope okay the trigger sweep scope again the sweep starts when the input signal meets a trigger criteria that we'll talk about here in a minute and it's get a whole lot more powerful and much much much easier to get a stable waveform on the screen for you to look at okay so let's talk a little bit about the trigger controls one little neat trick that i'll give to you is that on most old older tektronix analog scopes and a lot of other manufactured kind of followed suit um sometimes people the triggering thing is the thing that can get most people kind of host up how to set it up i don't know what to do or whatever and the the secret is if there's a set of controls like these lever controls here bring them all to the top normal trigger mode you know positive sweep auto trigger dc couple auto triggered and you know internal trigger mode throw all those trigger buttons up to the top and you'll most likely get a trace you can start working with this is kind of a little trick it's almost universally true so let's talk about triggering so again triggering is going to be used it's going to pick off signals from the vertical inputs because we want to look at our input that we're sending into the scope and basically lock on a portion of that and then send off the sweep to start at that at that location on our waveform so you typically can get signals from the input or an external a separate input to trigger we can talk about coupling and the sources and that's going to basically provide an input like like the trigger on a gun to kick off the horizontal suite the trigger source is typically internal which means that i'm going to pick off a portion of my i'm going to use my input signal as my reference of what i want to trigger on that's 99 of the time that's what you're doing you might have another signal source you want to trigger from so you want to look at waveforms with respect to some other signal and that could be an external trigger source uh in some cases if you're looking at doing things uh that are related to the line frequency or power supply for excuse me ac line frequency like you're looking for ripple or something like that that's synchronous to the ac line you can simply do a line trigger and it's also a way of just getting a nice good consistent trigger if you just want to get a nice waveform it's updating all the time but you don't want to have mess around try to trigger on just the signal itself because it doesn't matter a line trigger is a nice handy way to do that okay so that's our source selections the trigger mode we'll talk about auto here in a moment but there's auto trigger mode there's normal trigger mode which basically says i'm going to get a sweep when i get a trigger that's what you're using most of the time you might actually use auto most of the time but again we're going to talk about that in a minute a single trigger would just give you a single trace in response to a trigger and stop now for an old analog scope that's not terribly useful unless you've got a scope camera there to kind of capture what that trace is but it's there and also a lot of the older scopes have a tv trigger mode that allows you to trigger on uh like the ntsc composite video signal to look at either horizontal lines or vertical vertical sync pulses a trigger on those and then the similar to the vertical section uh and vertical uh controls there's also trigger coupling so you may choose to do you know dc reject or ac coupling you know of the signal into the trigger so you're only going to trigger on a portion of the wiggle of the signal you could do dc coupling and then sometimes there's some other features to do high frequency reject and things like that but all that becomes pretty scope dependent and now the digital scopes will have a lot more trigger choices available for you as well but most of the time you can use a very simple trigger to get what you want to get okay so again the trigger really controls when that sweep is going to happen on an analog scope on a digital scope it more determines what feature i want to capture in the memory to go analyze so let's talk about auto trigger here's a picture of my 465 scope and there's the auto trigger button right there it's not necessarily what you think it is auto trigger does not automatically set up your trigger settings that's not what it does okay so it's not like an auto set button and but it does provide a very useful feature so it's really a usability aid because let's say if we didn't have this auto trigger and i'm trying to look at a waveform and i got a blank screen why do i have a blank screen do i have a blank screen because my adjustments are all out of whack or am i not just triggering it right i don't know okay just can i just comment on this one um one easy way of explaining that would be to assume that it's an auto free run in other words the signal will just free run across the screen i think that's probably an easy way to explain it that's exactly right so what happens with auto and it's exactly as you described the scope waits for a trigger if it doesn't get a trigger within about 50 to 100 milliseconds then it self triggers and you know it does it just says okay i'm going to issue a trigger anyway so that kind of is a free run type mode saying okay the trigger you told me to do doesn't exist so i'm just going to kind of free run here but if during while i'm free running i actually you actually get the trigger adjustment right then i'll actually lock on your signal so what this does is it puts a trace on the scope even if the trigger event is not satisfied so you can see what's going on so you say well i should be triggered but i'm not my waveform is sliding back and forth or something and that gives you the clues like okay the waveform's on the screen it's the right i've got this the vertical settings right i might want to play with the horizontal settings but then i might have to adjust my trigger level to kind of get myself triggered right but it gives you a way of getting something on the screen okay and and to give you an a to figure out how to correct what you might have set up wrong in the trigger otherwise if you have the trigger setup wrong you have a blank screen so it really is kind of an auto free run okay but it's an auto free one that will will switch back to a triggered mode once the trigger condition gets satisfied okay so if your trace is kind of rolling or is unsynchronized it's likely that you're seeing these auto trigger sweeps and you're not actually triggering on your signal itself okay so the the most basic controls and the most common ones are just doing what's called a an edge trigger okay where you're triggering on uh this crossing through a threshold voltage that you set up with the trigger level control so here's four examples where this is positive slope i've got the level turned up to a slightly higher positive voltage so you can see we're starting the sweep on the sine wave you know something above ground right because i turn this positive and i'm i'm starting to sweep on the positive slope when i go from below that level to above it if i switch the slope down to negative i'd start at the same point but then i trigger on the portion of the waveform that's going down through that and similarly if i move the trigger level down say below zero okay below ground now you can see the voltage is starting or the waveform is starting below the center of the screen and then the slope determines which way we're going from that okay because it's just a continuous continuously running sine wave it all depends on where you take the picture of it right this helps you determine what portion of the picture you want the waveform to start at but this is the very the most common set of controls that you'll use and oftentimes you're just setting up you know positive slope leave the trigger and then the slow the level in the middle and that's it but it depends on what your waveforms look like you may mess around with that to get what you want okay but 99 of the time you're probably just going to use a simple edge trigger okay so with that um let's kind of talk a bit about the differences between analog and digital scopes so before i do that if there's any other questions on the analog scope features you know vertical horizontal or trigger let's cover those now before we jump into digital one thing i would like to put forward as a suggestion that is that when you for instance want to look at a signal let's say you're getting close to the bandwidth or close to the edge of the bandwidth if you put a signal and say if let's start at 50 kilohertz as an example and you you fill up the full screen with you you've got 10 you're taking up all 10. but when you get to the actual bandwidth frequency 200 megs you've reduced the signal the signal will reduce down to approximately 7.2 divisions right and i think that's something you need to be aware of or people need to be aware of yeah there's in the old analog scopes uh we had what was called the old five times rule because this is kind of like a gaussian single pole roll-off on the old analog scopes so the bandwidth rating like i said is when you're three db downs you're gonna be about point you're gonna be about 70 percent of the amplitude so if i had 100 megahertz scope i put 100 megahertz signal into it the voltage that i read is going to be is not going to read the full voltage it's going to be attenuated down to about 70 percent of the true value because of the pass filter characteristics and the old rule of thumb was that if you wanted to get within two percent of accuracy of your vertical amplitude for a signal the scope should have five times the bandwidth of the frequency of that signal okay so for 100 megahertz scope you're only going to get within two percent of uh the amplitude of a signal up to 20 megahertz right and then below that above 20 megahertz the signal amplitude is going to start dropping it doesn't mean you can't see a signal beyond 100 megahertz or even at 150 megahertz you'll still be able to see it it's just going to be lower in amplitude so you can't believe the amplitude all right because of that low pass filter characteristic so that's a good point paul thanks while we're talking about filters uh typically what what is the frequency on the the ac coupling high pass filter that's typically down in the tens of hertz okay uh for for the vertical the vertical ac coupling is down in the tens of hertz so that you could look at a 60 cycle thing with no problem you wouldn't be able to look at something that's varying at like 2 hertz or 10 hertz or something like that you'll only see a little bit of it so it's typically down in the tens of hertz because it doesn't have to be a huge capacitor because looking into a one mega ohm input impedance right so um but it's typically going to be down in that tens of hertz type of value okay all right any more questions on analog before we go okay so let's talk a little bit about analog versus digital so here's a simple very simplified block diagram of a digital scope now the same it's the same basic front end right we're still going to have an input attenuator input preamps and things like that but rather than going off and applying that to a vertical amplifier drive deflection plates we're applying that signal into an analog to digital converter which is basically taking samples of that of that waveform okay and sending those samples off to be processed the trigger system will be looking at it in some scopes the trigger system will be looking at the digital samples in some cases the trigger system might be pulling data off of the analog input it all depends on the scope ultimately all those samples that you know those pictures the samples of the voltages over time are stored into memory and then processed to throw on the display so open all of this so you we have this concept of sample rate right how often are we going to be grabbing and taking a picture of uh the voltage at that instant in time so that's called our sample rate and then our sample interval is really one over that sample rate and typically the sample rate is going to be many many times more than the bandwidth of the scope right and we know from nyquist that it has to be at least two times the bandwidth of the scope but in many cases it's much more than that like a typical 100 mega scope may have a giga sample per second sample rate okay so how do we deal with all those samples so um the sample rate the memory depth to put those samples in and the horizontal scale are all interrelated as you might imagine if i wanted to look at the sine wave right if i sample it 10 times then i need 10 samples of memory but if i sample it 100 times now i need a 100 samples of membrane if i want to sample this 10 000 times and i need memory to store 10 000 samples okay so you can see that the sample rate and the acquisition length in time really determine how much memory we have to have to store these waveform samples and a lot of the modern you know even the inexpensive digital scopes will have mega samples of memory i was like well i got mega samples of memory my display doesn't have mega pixels of display horizontally so how do we deal with that so far the acquired samples are then processed into display points so i might have samples in memory that are much much tighter in terms of interval than the the samples that are going to be used for display so most scopes will actually take the samples from the waveform memory and then compress them in some way to put them on the display and in most cases with the modern digital scopes they'll take all these samples that appear within a display interval a pixel if you will and then maybe cloud them all into a whole column if you will of pixels in the display and then again and again and then kind of do an intensity grade to kind of emulate what the phosphor did on the old analog scopes to give you kind of an intensity graded display but there are other display modes that are available like a peak detect mode as well as a peak to peak or envelope mode so lots of ways of doing additional processing on those could be millions of samples to display them on the maybe thousand points that you've got across the screen in terms of the waveform display so this gives you the ability to get really good resolution on your measurements but also get good visibility of your waveform as well so the way the scopes all do this it's a whole topic for a whole nother talk but it's it is something that you'll find and you almost have to kind of play with the scopes yourself to see is this going to work for what i wanted to do but in most cases they're really designed to really somewhat emulate what the old analog scopes did so some digital scopes have got some pretty significant advantages right we can capture a waveform and do a single shot on that zoom in on it see what's going on like here's a stair-step waveform it's got a little bit of a glitch we could zoom in on that glitch and see what it looks like these are things you very you can do in some sense if you've got a live waveform on an analog scope with things like a dual delay time base but it's really simple to do on a digital scope you capture the waveform zoom in really simple to see what's going on also something that digital scopes give you that is much harder to do on an analog scope is pre-trigger information like what happened before my trigger event right well an analog scope can't do that right remember a trigger kicks off the sweep so you can't see what happened before it on a digital scope the scope is sampling all the time and when you when you define a trigger the trigger just says okay i'm sampling all the time when you find this thing i want to capture the data around that and i said around that purposely because that trigger position can be anywhere on the screen so i can look at pre-trigger information or post trigger information see what led up to an event and see what happened after it that kind of thing is a big advantage on on digital scopes that analog scopes typically couldn't do there's often many many different types of triggers if you're looking at like digital buses we can trigger on a bus address or something like that and of course there's also automatic measurements peak-to-peak measurements voltage measurements rise and fall time measurements frequency measurements these can be made all automatically without you having to count divisions and do things like that so uh so that's some of the advantages that the digital scope brings you so more features oftentimes you can do some waveform math okay so i can take one waveform and another waveform in this particular case i got a waveform here where i'm measuring the voltage on either side of a resistor in series with the base of a trip this is actually an experiment i was doing to look at switching time of the transistor so this is like the output of the collector this is these these two voltages here channel two and channel three where the voltages on either side of a resistor in series with the base and then i did a little bit of math to actually do you know this voltage minus that one divided by that resistor value to give me a waveform that represented this current okay so these kind of things you can do with the digital scope that you can't do with an analog with doing things like math okay so there's other advanced processing features like like a fourier transform to give you the spectral content of a signal things like that again features that are in a digital scope that an analog scope doesn't have okay so they say well hey that's all great why don't i just always go with a digital scope well there are some disadvantages to digital scopes as well one is that they can appear noisy they're not necessarily more noisy than an analog scope but an analog scope you know the trace on the crt is drawn by repetitive you know sweeps of an electron beam and it's only those areas that get overwritten with each sweep that light up the brightest any little noise disturbances that cause a momentary deviation from where that sweep is going are going to be dimmer so they don't show up they're not as prominent on the digital scope since you capture or display everything right it looks noisier because those things that would be really dim on an analog scope show up sometimes at an equal brightness or near equal brightness depending on how the color or the intensity grading is set up so they can appear noisy but they're not always probably the biggest disadvantage is that digital scopes can aliase the waveform okay this is where the sample rate is not fast enough for the particular waveform they say hey but alan you just told me that 100 megahertz scope samples at a giga sample per second well yeah it does but if you adjusting the horizontal scale down to look at something like 10 milliseconds of division it's not going to be using the full sample rate when it stores the waveform because you might need megabytes of storage so it'll cut the sample rate of the stored waveform it might cut it down to you know to something that is too low for your waveform and you can get aliasing so here look we look at this red sine wave let's say that's our normal input signal but if i was looking at it at a slow time base okay or a you know a large you know seconds per division type of value the sample rate might be too low so if i look at i got these samples at these green points here on the waveform you can see that i'm not sampling each cycle except but once right and if i stitch those points together which is what the scope will do it'll show you a waveform that is not your original waveform and i call this the wagon wheel effect if you're ever watching the old westerns on tv and you watch the horse drawn wagons and sometimes the wagon wheels look like they're turning backwards all right it's all because of the frame rate the 30 30 frame per second or 60 frames per second of the movie is taking pictures of that wheel when it's in different positions and it's not taking pictures of it fast enough it's taking pictures of it you know at some sub rate those pictures might be at positions where the wheel hasn't completed a rotation it looks like the wheels are going backwards that's an aliasing effect that's exactly the same thing that can happen in a scope and this is of particular importance when we start talking about rf applications that we might use a scope for a good example is let's say we're uh we want to look at the modulated rf envelope like a station monitor so i might be transmitting it you know on 7.2 megahertz looking at my single sideband signal but i want to look at the rf envelope from my my my voice from the microphone you know you'd have to take the microphone out of your drawer i know you're doing all cw but so you have to dial down to you know maybe a millisecond of division or 500 microseconds of division and by doing that the sample rate of the stored waveform may come down where it's going to be lower than the rf carrier and you're going to alias the signal okay so that's something to kind of be thoughtful of when we're looking at if you if your application is going to be looking at monitoring your rf envelope an analog scope is actually better so oftentimes if you're looking at your your signal and it looks like it's triggered but you're still getting this wandering waveform it's not steady it could be that you're aliasing okay uh depending on how you got the scope set up and how much memory you're using the update rate can be slow with the responsiveness of this scope can be slow where an analog scope is instantaneous as you make adjustments and changes okay they can be a bit pricey because uh it's a newer technology and they also can generally have if you ever use xy mode we'll talk a little bit about xy mode later the xy mode on digital scopes is generally pretty lousy compared to analog scopes where it's actually pretty good okay so um that's probably all we're going to talk about in terms of you know just the digital scopes themselves in terms of features because the controls are very much the same you still have the same vertical controls same coupling controls horizontal is just called scale instead of sweep speed but it's still calibrated in so many volts per division and the trigger controls are virtually the same except like i said there's a lot more triggers available to you but as hobbyists we're generally just going to use an edge trigger so um so with that let's talk a bit about probing which is kind of universal regardless of whether we're talking about an analog or digital scope yeah why do we want to use probes well obviously to connect our signal to to connect the signal in our circuit to the scope to go look at it right and the idea ideally we want to minimize loading you know when we go to measure something right the heisenberg uncertainty principle says that you know just the act of observing something will alter it in some way right we want to minimize the amount that we're altering the signal okay so the probes help us do that most probes that we're going to be looking at are passive probes like 1x and 10x probes are going to talk about next but oftentimes there are also active probes like current probes high voltage probes there's also rf probes like near field probes and things like that that we we might use or even just a simple wire going to the front of the scope and wrap around your coax oftentimes is enough to pick up the rf signal that you're transmitting to look at it so you can almost call that approved it's really just more than a pickup wire okay so let's first talk about the 1x and 10x passive probes a 1x probe is really just a direct connection to the scope input all right and so it's really simple there's nothing to it it's just it just has a nice convenient end on it with a little hook tip or maybe a little spring-loaded hook like we call a witch's hat or as a sharp little probe tip like a probe and then a ground lead with an alligator clip on it or something very simple connection uh the problem is is that they can have fairly excessive capacitive loading because remember we talked about the scope input we showed those examples i've got 15 20 25 picofarads of income input capacitance and on top of that the capacitance from the scope lead itself so you'd be talking about you know 100 150 picofarads uploading and depending on what you if you're looking at an audio circuit you probably don't care but if you're looking at anything in an if an r or an rf signal um 100 picofarads might be significant it might actually affect the operation of your circuit you're trying to look at the output of a crystal oscillator or something like that 100 pico power is certainly going to affect uh that the to the tuned circuit if you will of that that uh that oscillator okay so that's why most of the time we're actually not going to use 1x probes we're going to use 10x probes now 10x probes will i essentially isolate the probe or the the circuit from that cable capacitance and the scope capacitance by putting a nine mega ohm resistor right at the probe tip okay so okay so so now the circuit is going to see nine mega ohms before that capacitance so the capacitance is now nine mega ohms away and it's not going to necessarily be as much of a an impact on the circuit that's good but now that 9 meg ohm resistor eventually is going to form a voltage divider with that one mega ohm input impedance of the scope so it's a 10x voltage divider so we're going to reduce our signal amplitude by a factor of 10. now in some cases the scope will automatically detect this depending on the scope and the probe manufacturer sometimes they kind of work together you plug the probe in it knows it's a 10x probe and adjusts the vertical scale by itself and in other cases you may have to tell the scope i'm using a 10x probe so it adjusts the vertical scale appropriately okay so otherwise your voltage rings will be off by a factor of 10 you won't know why okay the good thing is is that it reduces the effect of that capacity of loading on the circuit the bad is that we have to compensate it so more on this next what does that mean okay but again most of the time we're going to be using 10x probe so understanding compensation it's not a bad thing you just have to know what it is and do it so let's talk about probe compensation so here's a simple model of say from here over is the probe so i've got the probe tip okay i've got the nine mega ohm resistor there i've got a coaxial cable between here and the scope okay the scope has got the the one mega ohm input impedance and the parallel capacitance all right so if you think about that ignore the capacitors i've got a 10x voltage divider with the resistors but this capacitor we can't get rid of right so as frequency goes up at some point the reactance of this capacitor is going to go down below one meg and now i've got a low pass filter between my nine mega ohm resistor and the input capacitance so that low pass filter will start rolling the bandwidth off well that's no good right i'm using this probe to prevent problems with you know frequency response by the capacitive loading so what we do is we actually put a capacitor in parallel with the nine mega ohm resistor and with that and what you do is you adjust this capacitor so now these capacitors have got the same ten to one ratio in fact it's the other way around instead of ten to one going this way or nine to one going this way it's nine to one going the other but uh but what we're doing is setting up that same ratio so as the frequency goes up okay as the frequency goes up at low frequencies the 10x attenuation is dominated by the resistors as the frequency gets up past 10 or 20 kilohertz now the attenuation gets dominated by this capacitive voltage divider between cp and the adjustment capacitor and the input the probes the scope's input capacitance now because the all scopes don't have the same input capacitance we have to make this capacitor adjustable okay and we the scopes provide you a tool to do that right on the front panel of the scope you'll see a a probe calibrator or probe compensation signal on and basically it's typically a one kilohertz square wave now square wave is nice because at one kilohertz the one kilohertz frequency is to the point where the resistor voltage divider is going to work but because i've got fast edges on it there's higher frequency content right as you know from square waves you've got energy at all of the odd harmonics so those auto harmonics are higher and higher in frequency and they'll get affected more by the capacitive voltage divider so what you do is you hook your scope probe up to that compensation signal and you adjust that capacitor on the probe to get a perfect looking square wave if it's not properly adjusted if it's under compensated okay like if this capacitor was too big then you'll you'll make the edges slower okay it'll look like this so you adjust that to properly compensate if the capacitor is adjusted the other way you'll be overcompensated right and you may overshoot you know the signal that's that's when this capacitor is actually too big you'll overshoot the signal okay so you plug into the the probe in and you look at that square wave and then you simply adjust that compensation adjustment a little screwdriver adjustment on the probe until the square wave looks good and what you've done then is given a nice flat frequency response of the probe so that you can now believe the amplitudes that you see of the signals that are you know 10 or 20 kilohertz or higher using that probe okay this is something if you ever you know if you look at the probe sometimes the little adjustment is on the the probe itself the part that you hold in your hand sometimes that adjustment is on a compensation box that's attached to the front of the scope where it plugs in so you have to look at your particular probe it's usually a very tiny little screwdriver adjustment um it's either a hex or like a phillips or a straight and you often want to adjust that with a non-conductive screwdriver if you can or if you have all you have is a conductive you adjust it pull it out make sure it's good adjust it again but the good thing is if you only have one scope you just have to make adjustments on your probes once and then forget about it okay but the problem is some people forget about it from the get-go and never do it okay so again most scopes have got that calibration signal for that purpose okay so now this is something i alluded to earlier looking at the pat's background picture is the probe ground now as i mentioned that's connected to chassis ground on your scope all right so whatever point you connect that to you're going to ground and connect the ground on your circuit so as long as it is ground you're okay if it's not ground you're going to short that node to ground so if it's a battery operated circuit again may not matter but if it's not then you do have to watch where you put that ground lead but the other thing to worry about is that ground lead is an inductor in in essentially the return path we have this loop if you will okay so that kind of represents an inductance in the in the ground path so that inductance coupled with the input capacitance of the probe can create a tank circuit and cause ringing so you want to keep it short okay if you can you're working looking at audio signals things like that we don't matter but if you're looking at digital signals that have fast edges or you're looking at rf signals we want to keep that as short as possible so a lot of the ground the probes are designed that if you yank this witch's head off you'll have a probe tip that looks like this with a little concentric round ring like a coaxial type of rigid coaxial probe and you can either make your own or the probes will often come with new that might get lost a little spring ground clip they can slip on there so now i've got a much shorter ground lead and just as an example here's a a fast rising edge measured with a five inch ground lead length and you can see the ringing on that edge okay that's all due to that ground lead length that same exact signal probe with this i've minimized that ringing because i minimized the inductance in that ground lead length so again all you need to worry about if you're dealing with digital signals probing clocks or data signals and things like that that have high frequency content or if you're actually looking at rf or if signals you might want to really consider how long that ground lead is and how that's going to affect what you're looking at so let's talk a little bit about xy mode xy mode is where uh you might have one channel that actually drives the horizontal position of the beam instead of the horizontal sweep oscillator or the horizontal uh sweep circuit you might have another one channel that drives that and channel another channel that drives the vertical so this is kind of like the etch-a-sketch mode of a scope right where you have the two knobs you move the beam anywhere you want based on those two values you know this is this is actually a picture that i uh i think i took a similar picture of this i used to do this when i was a kid i had a scope when i was in high school i had a scope in my bedroom i hooked into the back of my stereo and this is kind of what it looked like when i had the left channel going on the one channel on the right going onto another channel and you know listening to pink floyd or something like that you just get this mess okay but you kind of a little bit see kind of anything that's kind of along the diagonal it was coming out of both speakers but i think it deviated from that was on the left or the right channel so it's kind of neat but it could be used for doing frequency comparison because if the x and y are driven with the same signal you'll get a diagonal line if there's a phase shift between those signals that'll turn into an ellipse or a circle or a diagonal going the other way if there's a frequency difference between them you'll get like figure eight patterns or other things like this these are called lissajou patterns i've got a video on that as well so that's a crude way of making frequency measurements you could also do things like a curve tracer there's a very common old old circuit called an octopus which is similar to this where i've got a little step down transformer and i use these two probes to put across a circuit and i'm essentially measuring the voltage and the current uh by with two probes here one looking across a resistor one looking across the device itself and you essentially can trace out like a short circuit would look like a vertical bar an open looks like a horizontal bar a resistor looks like a sloped line capacitor will look like an ellipse you know diodes and junctions will kind of have a wiggle to them you know that type of thing and like have you ever heard of the old huntron tracker it was a little circuit a little piece of equipment that was used oftentimes in tv service centers because you could you might kind of know at particular points in a circuit if i put this little curve trace or this huntron tracker in different nodes this is what that waveform would look like and if it deviated from that you knew something was wrong didn't really tell you much about the operation of the circuit but just told you if things were wrong or not okay but you can build one of these little circuits it was called an octopus because it had eight leads when he counted everything that was going out that octopus circuit and you build a little something like that and some of the old analog scopes actually have a component tester built in which is basically that okay another thing you can do with xy mode a little more sophisticated is actually you know maybe you do a frequency response sweep let's say i had a sweep function generator okay i had the ramp output the frequency ramp go out to one input okay and then look at say the the voltage coming out of say the if transformer as i apply this sweep signal into the device i'm testing and actually look at the frequency response as i go across so more of a sophisticated you know use for x y mode if you really like playing with scopes there's better ways of doing this these days but this is what xy mode is typically used for and again the digital scopes are pretty lousy at xy mode because it's all sampled points as opposed to a continuously moving beam so uh xy mode is generally handled much better another common way for for xy mode i think i'll talk about later is looking at um like a trapezoid pattern like an old station model will have a trapezoid pattern to look at things like uh linearity of your amplifier or something like that so that's that's a essentially an xy application of the scope so again analog or digital again in many cases digital is better you know automatic measurements storage into memory zooming in etc but for an rf envelope or things like that analog is definitely better okay like at slow sweep speeds to see audio the rf can get under sampled so here's like a single sideband rf envelope on an analog scope to me i get a lot livelier view and a more intuitive view of what's going on with the rf compared to looking at that on a digital scope some digital scopes don't have enough you know intensity grading to make it look like you know i've got really nice intensity grading here and see what's going on you know the update rate is going to be instantaneous on an analog scope it might not update quite as fast on a digital scope so there's a lot of reasons why for doing rf work and analog scope is oftentimes much better so with that before we jump into scopes of the ham shack let's take another quick little break to see if there's any questions on the scopes in particular and then we'll just talk about a couple of applications where you can use a scope and a shack this is bad go ahead paul is it worth mentioning anything to do with rice time and bandwidth um the fact that you know a scope will only handle up to a certain rise time depending on the bandwidth of the scope yep yeah so the the old-school uh figure of merit for that is is a factor called 0.35 like 0.35 divided by the bandwidth will tell you the rise time of the scope and vice versa 0.35 divided by the rise time the scope tells you the bandwidth of the scope okay and that kind of that relationship holds true for most of the analog scopes and most of the lower end digital scopes is when you get into the really high end high speed digital scopes where that factor changes to more like a 0.4 or 0.45 because of the digital signal processing that's going on because remember the faster things move in the time domain the more bandwidth they occupy in the frequency domain right so like a square wave we said has got spectral components or frequency components at the fundamental of the square wave and at the odd harmonics the more more odd harmonic components like the fifth sixth you know the sixth the seventh the ninth the eleventh the thirteenth etc the higher you go the faster the edge will be so um again so if you're looking at a well i got i got a 10 megahertz clock that's that's that's you know driving my arduino or something like that well that 10 megahertz clock might have two nanosecond edges right in order to look at those two nanosecond edges i made it i might need a couple of hundred megahertz on my scope in order to accurately see that okay even though the clock signal itself is only 10 meg okay so that's kind of the relationship there so thanks for that paul pat you had something i i did well first of all i'd like to say thanks i i've seen this presentation a couple of times and and i've given one like it a couple times and always learn something every time i hear from you and i really appreciate your youtube channel it's been a great learning resource for myself so i really appreciate it my question was you know just when with low frequency just trying to make some dumb stupid measurements you talked about probes why can't i just take a b and c and maybe cut the end off of it and just you know use the wires off of that what's the effect of that well that's essentially a 1x probe it just doesn't have a nice convenient probe tip on it okay for low frequency work uh and for and for relatively low impedance circuit work it's fine okay now let's talk about both of those right because at low frequencies the maybe 100 or 150 picofarads of capacitance that is going to be represent is going to be seen looking into that scope because what what's the impedance looking into that that coax the end of the coax there you got the probe capacitor the lead capacitance right we're certainly talking about frequencies that are low enough where we don't have to worry about transmission line effects of a you know three or four foot lead you know piece of you know coax so we're just it's just going to look capacitive so we're going to see essentially the one mega ohm resistance of the scope in parallel with the scope's input capacitance and the the coax capacitance so maybe 100 picofarads or more so if you're dealing with low frequencies the reactants of 100 picofarad capacitance might represent thousands of ohms so who cares right it may not matter but if you're looking at like a like a crystal mic element which has a very high input impedance you know 100 picofarads might affect its frequency response right it might roll it off okay because it's like putting 100 picofarad capacitor on that spot in the circuit so you have to kind of think about all right if i'm looking at the circuit how would this a circuit be affected by sticking a one mega ohm resistor from this point to ground and sticking 100 150 picofarad capacitor from this point to ground and if your conclusion is that well the frequency is low enough that i'm not going to care it's dc it's audio it's not going to care or the impedance i'm not dealing with circuit that has impedances in the megomes of resistance right so i'm dealing with very high impedance circuits even though i might have a you know a 100 picofarad capacitor it might represent a few hundred kilo ohms of capacitive reactants but if the circuit impedance that i'm looking at is is a couple of mega ohms then that's going to dominate that node impedance it might affect the performance so that's where you have to think about it is whether or not sticking a 100 puff capacitor in that spot in the circuit is going to affect its operation and that's where you can draw the line to say well i can use this little hunk of coax or i really need a 10x probe to isolate that capacitor i think so yeah okay all right so let's move on a little bit so a couple of places to use a ham shack a scope and a hamshack uh like a transmit signal monitor so again looking at some most station monitors like i've got a sitting over here next to me is a kenwood sm 220 you can look at the modulated rf envelope we'll look at things like flat topping checking your your am modulation depth looking at adjusting your psk31 audio levels things like that or do like a trapezoidal monitor for amplifier linearity we can do some analysis on coax through like a time domain reflectometer we'll talk about that here in a minute also obviously circuit troubleshooting uh equipment debug and repair and even measuring like unknown capacitors inductors as ways we can actually use a scope to help us do that even though most of us have got dmms that can do some of that now anyway but but what fun is that when you can use a scope so um so let's take a look at monitoring your tx output uh the typically you're gonna have to somehow sample your rf signal okay you can use a you could build or use a resistive sampler um i've got a company where you're basically just you know tapping off of the line with a higher value resistor so that it doesn't disturb the swr but it also attenuates a signal a bit to go into the scope you can use a capacitive divider that's typically what the station monitors do and also can minimize the loading um you could also just use a pickup coil or an antenna you could stick an antenna on the front of the scope and have it pick up something um and or or like even just a little coil of wire around you you know as a pickup coil that could be your scope pick off point and your your signal is going through this way a capacitive divider it could be something as simple as this i've just i've got a coaxial line going through here i've got a little stub going in here and depending on how and it's nothing's connected to that center coax line but the closer i adjust that piston the more coupling i'll get that's kind of how this kind of coupler here works as well and another one that's really super simple is if you got a a trans match in your in your shack oftentimes they've got multiple antenna ports and there's almost always at least one that you're not using hook your scope up to that port but just don't ever switch to that port okay you're just using that port as essentially a pickup point so even though you're switched to a different port to a different antenna there's going to be enough wires hanging off of that unused port to act as an antenna to pick up enough rf to be visible on your scope or if you just want to poke a hole in the back of your tuner and uh you know bolt in a panel mount b and c connector with nothing at the other end maybe except a short stub of wire like a pickup antenna you could use that uh to couple rf signal into your scope okay you could also build a detector or mod or even buy a detector demodulator circuit something similar to this that can then follow the amplitude of your signal so not only look at the rf envelope but also then get another waveform that is just the rf amplitude to kind of like follow the audio or something like that and this is typically what's done to do like a linearity measurement where you might look at the um the envelope of the signal coming out of the amp and on on say on x or y and none on the other one look at the input to the amp from the rf standpoint and you build essentially a trapezoidal you know view to make sure you get good linear operation of of your amplifier okay so this is kind of one idea and very common way to use a scope and a shack so monitoring the rf envelope you might want to look for things like flat topping look at a psk envelope to be sure that you're not over driving it too hard you might want to look at am modulation depth and that's quite simple it's just a you look at the the the peak voltage of the envelope and then the the trough and then this simple formula tells you the the uh am modulation depth and this could be this is like a single sideband signal here we're just looking to be sure we're not clipping up at the top and that type of thing and once you've got it got a you can even once you've got something coupled into your scope at a given frequency if you know how much power you're putting out you can actually get some calibrated numbers you know you kind of figure out okay this is this is 100 watts 50 watts wouldn't be half of this right because this is showing voltage right so 50 watts would be kind of more down and out of this area and things like that because remember it's a square law type thing but that's that's what the rf envelope might look like on your scope and a very very common thing to use uh and it's just kind of cool to look at so here's an example using what we call tdr time domain reflectometry if we inject a pulse into a transmission line a piece of coax and if we don't have any load at the output of that coax what's going to happen we're going to get a reflection right that's that's w we've got the highest w r we got we got a 50 ohm line that's not terminating the 50 ohms the signal gets reflected back so by putting a pulse generator into a into a hunk of collax at the same point we measure with a scope we're going to initially see that signal generator uh create kind of this initial step but then when that pulse reaches the end and then reflects back it's going to cause a secondary step to pop up here and of course this delta t is related to the transmission line length and you can calculate that length or that might be like a distance to fault you might have a break in a line you want to see how far down the line it is if you know the velocity factor of your coax you can calculate that distance default by measuring what that time is you could even put in a variable load here and adjust it until the reflection goes away and if you do that you can you can figure out what the impedance is i might have a hunk of coax that i don't know is this 50 ohm line or is it 75 online you can do that by playing with the load until the reflection reflected pulse goes away then you know you've matched the line of pieces okay i've got some of the math in here if you actually wanted to do some of these calculations i won't go over this right now but but it is something you can do and i show this by doing this with a pulse generator okay but if you have a digital scope you could even replace this with a 9 volt battery and a serious resistor and you literally just you know hook up the 9 volt battery have a serious resistor and just momentarily touch the input of the coax right here and that'll inject a signal there'll be a voltage drop across the the series resistor okay because it looks like 50 ohms initially and then you get the reflection back so in a single shot i can get this waveform because i do a stored waveform so without even having a pulse generator just a 9 volt battery and a 50 ohm resistor and a bare wire just touch it there and i can get the same waveform and do these same measurements so super cheap you know easy way to do a time domain loctometer how about measuring capacitors and inductors well we can use again use a nice fast edge pulse generator you can you can build one out of a little logic circuit pretty simple and we've got a couple of circuits to actually measure capacitors and inductors so for capacitors we just want to measure the time constant right so if we have a known series resistor and we improve what's going on over here we put that fast fast pulse in we can see how long it takes that to come up and uh it takes 63 percent of the amplitude is what you'll get after uh one rc time constant so we know rc we can actually just measure the calculate the capacitor and this is kind of convenient out of scope because if you use that vernier good vertical control and adjust the amplitude of the waveform to be eight divisions tall so from this point to this point to be eight divisions tall the 63 point is when we cross the fifth division super simple you back measure that distance in time and you can calculate what that capacitor is so it's just cool just fun stuff inductors what we're going to do is just create a little tank circuit we'll just couple ac couple into a tank circuit with you can play with the values here but you say unknown capacitor put your own known inductor there that creates a tank circuit that will ring at the resonant frequency so what you do is you make you you bang this in there you measure this and you measure the frequency on your scope and then once you know that frequency you can calculate the inductance from this formula here so it's a little bit of math but to me it's fun ways of using your scope okay so xy mode we talked about already and oh it looks like i did some cut and paste issues with my my uh powerpoint so i think we've got a couple here that we're going to go through again let's just go through these again well that's that my fast edge pulse generator there's this is one you can build a 74 ac 14 is a really pretty fast uh schmidt trigger logic circuit and uh being cmos it can drive relatively low impedance load so you parallel all these extra gates up uh you can drive that 50 ohm load pretty easily it'll give you about a two nanosecond rise time and it'll operate from anywhere from three to five volts so that's kind of a neat thing okay uh so that we talked about uh making those measurements so anyway there's uh that that's kind of what we had to talk about um just a little reminder on my youtube channel just youtube.comcall and you've got my email address here if you've got any questions about how to do any of this stuff but hopefully this this gives you a little bit of an idea of some of the things you can do with your scope how to use it go pull it out blow the dust off it and play with it you're not going to hurt anything so thanks alan that's awesome um we've had some folks uh we've had a couple folks drop off for the uh naqp ready contest but um they were frantically texting me telling me uh to make sure that i told you how much how much they enjoy your youtube channel oh great okay um hey any questions from the team andre not really a question but more of a comment anybody that is interested in probing line-powered equipment to check with their volt meter at what potential the chassis is at before they put the ground lead on especially tube powered equipment and they didn't have the polarized plugs and they could have a hot chassis so yeah good point it's uh seriously lets the smoke out of everything yeah should we not ask you why you know that high school my school lab didn't check gop went bang i got burned yeah interesting yeah yeah the isolation transformers yep great learn the hard way hey dan you had a question uh comment actually alan thank you i learned something really good today great um looking at that calibration waveform and i had almost exactly the waveform that you showed where it's it's kind of got a rounded leading edge yeah and then you're talking about probes i went looked at my probe and lo and behold little adjustment in there that'll likely be that capacitor exactly it is i never knew that so thank you yep have everybody got this one little nugget like that that's great but uh people just forget about that and they said why do i have this screwdriver here i'm never taking my scoop my probe apart i don't need that i can't tell you alan how many times i've adjusted the little screw on my 465 scope and uh i know i knew what i was aiming for but i never knew what happened inside when i did it yep yeah so the other thing to think about too is if you get an old scope you pick up a scope from a ham fest they say oh now i need to find probes take a note of what that input capacitance is because then when you go to buy a probe it'll tell you what capacitance range it can compensate for okay so if you pick up a scope and it says it's got you know 15 picofarads of input capacitance and you're going to buy a probe and the probe says it can compensate from 20 to 30 picofarads you won't be able to compensate that probe on that scope so it's important to kind of know that if you're probe shopping john you were muted or saying something oh no okay okay harry did you have a question or comment there oh i just uh i just wanted to say thank you i thought it was a terrific uh presentation um i sold instruments for over 30 years so um i never did sell tech okay maybe for two weeks but then the aca got terminated it was one of the late things i did i sold for brody and hp in the past but okay i work for yeah yeah i found that on linkedin so anyway i just wanted to say you know this is probably one of the best sessions i've ever seen on scopes in my 30 odd years oh thank you thank you just just a terrific job thank you thank you okay any questions okay just one question to go back to that the probe compensation again so the um the capacitance is listed that's listed on the um machine on the uh scope yeah and then the probe has to what's on the probe has to be within that range yes its range has to contain that that's a good question so when you when you're looking when you're probe shopping there will be two specs that have something to do with capacitance one will specify the compensation range okay and the compensation range needs to include the actual capacitance of your scope the other thing that you'll see on the scope spec page is the input capacitance of the probe and what that is is that's what when you're taking that probe and touching it on a circuit that's what your circuit is going to see that really has nothing to do with the compensation range so it's a compensation range that has to ensure that your scope capacitance is in that range now for the tip for that tip capacitance what are you looking for for an ideal does it depend on whether what frequencies you're looking at or yeah well i mean oftentimes it's going to be what it's going to be right and uh obviously the lower the better most 10x probes are in the neighborhood of 10 to 15 picofarads input capacitance there are some higher frequency passive probes that are down you know in the you know three or four pico fires but they're typically pretty expensive but most 10x probes are going to have you know eight 10 12 15 pico powers of input capacitance okay and that's just what they're going to be so there isn't you know and the wider bit higher bandwidth probes will be a little bit less but the important thing to match up with your scope is to ensure that that compensation range includes your scope

Channel: Cycle 25

Views: 1,639

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Id: CFUPPhxSCzk

Length: 79min 15sec (4755 seconds)

Published: Sat Feb 27 2021