How to Use an Oscilloscope

[Music] an oscilloscope is another important tool when it comes to analyzing and debugging circuits they let you see electrical signals and you often hear it abbreviated as Oh smoke or sometimes just scope they're particularly handy when it comes to measuring things like amplitude frequency and transient signals that your multimeter just can't catch in older cathode ray oscilloscope the electrical signal under test changed the voltage between a set of vertical deflection plates that caused an electron beam to move up and down as it swept across a phosphorescent screen but modern digital scopes take measurements using an analog to digital converter to sample the signal and then draw the waveform on a screen while all these buttons and knobs may seem daunting at first don't worry I'll show you which ones you need to care about so you can begin taking measurements to start let's look at the user interface the first thing you'll likely notice when looking at a scope is the display this is where your waveform and any information about it will be drawn here we have two ports marked with colored bands that accept probes with BNC connectors you'll commonly see scopes with one two or four channels this one happens to have two the vertical section controls the voltage scale on the display for each channel independently the horizontal section controls the time scale on the screen for both channels you can manually adjust how the scope triggers with this section which tells the scope to start measuring when it sees a particular part of a signal the menu section contains various settings some of which we'll use later like saving an image you'll also be pressing single run and stop quite often to start and stop measuring as it turns out scope probes are more than just some pieces of wire the most common probes you'll find our passive voltage probe which means that they measure the voltage difference between the ground clip and the tip of the probe there are no active electronics in the probe just some cable resistors and capacitors if you were to look inside our oscilloscope you would find that each of the input ports has a 1 mega ohm resistor and a 16 Pico farad capacitor connected in parallel from the signal input to ground just connecting wires to the port would work for low frequencies but since they look like antennas they're liable to pick up noise and inject it into our circuit or measurement to fix that we use a coaxial cable to provide extra shielding now if you look at most scope probes you'll see a little switch labeled One X on one side and 10x on the other this refers to the amount the probe attenuates the signal if you switch it to one X it is exactly like our drawing the tip is connected directly to the inner wire of the coax cable and the ground clip is connected to the shielding while this works well for signals with frequencies less than a few megahertz coax cable can act like a capacitor and add more than 100 Pico farad's to your test circuit this can adversely affect many circuits especially if you're trying to capture higher frequencies to remedy this many probes have the option of enabling an inline and 9 mega ohm resistor to isolate your circuit from the cable this is what the 10x setting on the probe does the 9 mega ohm resistor in the probe and the 1 mega ohm resistor in the scope act as a 10 to 1 voltage divider it's important to note that 10x does not mean 10 times gain it means the signal is reduced by a factor of 10 the problem with adding a resistor in line with our signal is that with the capacitance in the coax and scope it creates a low-pass filter and high-frequency signals will appear smaller to fix this 10x probes include an adjustable capacitor in parallel with the 9 mega ohm resistor used to compensate for the low-pass filter this will keep the signal attenuation more or less the same across all frequencies now I recommend leaving the probe on 10x as that's the best compromise between signal attenuation and having the probe effect the circuit you're trying to measure however that does mean we need to tell the oscilloscope that we're using a 10x probe and we need to tune the compensation capacitor inside the probe you'll likely only need to tune your probe once but it's a good idea to check them every now and then to make sure that the compensation capacitor didn't get adjusted by your lab partner or so spooky goat to start select 10x setting on your probe and plug it into the oscilloscope turn on the scope and let it boot up we'll need to make some initial changes to the settings first make sure channel 2 is off press the channel 1 button to see the input setting to make sure you're on DC coupling as we want to see all parts of our signal press the button next to probe highlight 10x using the top left knob and press the knob to select it press the trigger menu button make sure type is set to edge and source is channel 1 check that slope is set to up edge these telescopes that we want to start our measurement on a rising edge of a signal on channel 1 most scopes come with some kind of built-in frequency generator so you can test your probes and tune the compensation capacitor this one happens to have a 1 kilohertz square wave generator we can use attach the ground clip to the ground tab and hook the probe around the signal tab you should see something appear on the Scopes display now you'll need to adjust the horizontal and vertical scale knob so you can clearly see the waveform note that turning the knobs clockwise decreases the time or voltage scale which has the effect of zooming in on the waveform if you want to move the waveform up or down you can adjust a smaller knob above the channel 1 button just note that this moves the zero volt level for channel 1 as well if the waveform seem to be moving or jittering this is because the scope has no idea when on the waveform to start measuring but we can fix that by adjusting the trigger level use the knob in the trigger section to raise the level to somewhere between the min and Max of the waveform this tells the scope to begin measuring as soon as it sees a rising edge perform one sweep on the screen to draw the waveform and then repeat the process note that digital scopes like this can store and display information before the trigger because this is a periodic waveform the image appears static on the screen now these highs and loads should appear completely flat as we assume the scope is outputting a near-perfect square wave if not it's because the compensation capacitor in the probe has been tuned so we're actually measuring the signal incorrectly using a tiny flat-head screwdriver adjust the screw head in the probe until the square wave has straight edges note that some probes have the compensation adjustment through near the probes plug instead now that we have everything set up let's take some measurement to make some sample waveforms I've got a mini gen board attached to an Arduino Pro Mini I can adjust the type of waveform by pressing this button and adjust the frequency by turning this knob to start attached the ground clip to the negative post on the output and attach the probe tip to the positive post just make sure they don't short each other out oh and if you don't have something to clip to you can usually remove the hook end of most scope probes to get a fine tip that you can press two component leads and traces let's take a look at a sine wave as you can see we're zoomed in a bit too much on the time scale adjust the horizontal scale so we get a clear sine wave now adjust the trigger level to stabilize the waveform by pressing the measure button to scope will perform some automatic measuring to give us things like peak to peak voltage and the frequency of the waveform you can also see that there is about a 1.5 volt DC offset on this signal if you have a non periodic signal or something the auto measuring features can't handle you can use cursors to manually measure waveforms press the cursors button and press the top menu button to turn them on you can measure vertically which correspond to voltage in this case or you can change them to measure time select cursor a with the menu button and use the adjustment knob to move the first line select cursor B and move it to the point you want to measure here you can see the time between the top and bottom Peaks is about 1.1 6 milliseconds all that works great for periodic signals but what if you have something that only shows up sporadically for example whenever I make an adjustment on this knob the Arduino sends a digital message over spy to the mini gen board let's try to capture that signal find an appropriate common line to attach the ground clip and attach the probe to the most line when we adjust the knob on the breadboard we can see the message but it disappears quickly since it's not periodic it's hard to capture in view so we need to tell the scope to only perform a single sweep once it detects a rising or falling edge and not keep sampling and drawing on the display to do that we first have to set our trigger to falling edge since this line is normally high and driven low set the trigger level to something below the default high voltage we'll set it to about half the voltage now press the single button which tells the scope to start measuring once it sees the trigger condition and to stop measuring after it's made a single sweep across the display adjust the frequency knob on the breadboard again and you'll see our spy data appear on the scope you can zoom in hit the single button again and recapture the message to measure it and examine the bit to make sure the message looks correct as mentioned earlier many scopes have more than one input here I've attached channel 1 to mosy and channel 2 to s CK which is the spike lock now when I do a single shot capture I can see how the clock signal lines up with the data bit and let's say I want to save this image to my computer so I can use it in things like blog posts and lab reports I can plug in a thumb drive press the save recall button and press save wait for the progress bar to show that it's done writing and pull out the thumb drive the picture will be saved as a bitmap in the root directory of your thumb drive there are a few things to consider when buying an oscilloscope and knowing what the term mean can help you read the specs the first is bandwidth this is the maximum frequency range that the scope can accurately measure the problem is that as you go up in frequencies the amplitude of your signal gets attenuated up to about 30% by the time you reach the listed max frequency to accurately represent a sine wave it's recommended that you choose a scope with a rated bandwidth at least five times the maximum frequency you intend to measure most entry-level scopes have a bandwidth of around 100 megahertz which means they can really only accurately show sine waves of about 20 megahertz which is about two percent amplitude attenuation if you're analyzing digital signals you'll probably be looking at sharp edges as the signal goes from low to high and high to low the rise time rating represents the scopes ability to draw details in fast transitions it's related to bandwidth and much like the bandwidth rating it's recommended that your expected signals rise time be five times that of the scope listed rise time digital scopes need to sample a signal many times per second in order to draw it on the display and the sample rate is the number of times per second a signal is read entry level scopes will generally have around one to two giga samples per second here we see the rule of five times again to get a good reading it's recommended that you pick a scope with a sampling rate of at least five times your expected highest frequency resolution refers to the number of bits used to measure and quantify each sample the more bits the more precise each sampling will be while there are other specifications you should consider when looking at scopes such as probe impedance and auto measuring capabilities these four should really get you started goats can be quite pricey but there are alternatives for example USB scopes are nice because they are portable and they get the job done about 90% of the time that being said if you want to make it look like you're doing something sciency whenever somebody walks by all you got to do is just measure a sine wave

Channel: SparkFun Electronics

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Length: 12min 32sec (752 seconds)

Published: Mon Feb 13 2017