Build this Spectrum Analyzer Protection Circuit

hi there in the last video I did which was the restoration of that DeForest Crossley receiver and a bunch of people request to see what's inside these little white boxes that I've built that protect the input and the output of my spectrum analyzer now I work on a lot of high voltage gear you know vacuum tube gear and the rough high voltage in of say on everyday vacuum tube receiver that has a transformer in it would be around 300 volts DC and that's called the B+ now the last thing I want to do is feed that into the input of my spectrum analyzer or into my tracking generator output so these little boxes are designed to block DC and they also very lightly couple my spectrum analyzer into what I'm aligning and I'll get into a little bit more of that at the white board right over there here's what's inside that little white box it really is quite simple we have eight capacitors here that are in series they're 500 volt 100 Pico farad capacitors so that gives me four kilovolts of isolation and since they're all in series I get 12.5 Pico farad of coupling now you've seen in the videos when I'm doing my alignment even 12.5 Pico farad is a little bit too much so what I do is I clip the alligator clip to the insulation on the wire in order to get the signal through and that's about maybe a Pico farad or more of coupling into the circuit and that's on the grid cap lead from the tracking generator in and that's also on the last I F output transformers wire that connects to the diode plate in the receiver that's about a Pico farad of coupling there also remember I'm using one of these on the tracking generator output and on the 1 Meg ohm input of my spectrum analyzer that's another thing I have to mention this circuit here is designed for a 1 mega ohm input system so if you put this onto your 50 or 75 ohm input on your spectrum analyzer your results will vary this actually needs to be modified a little bit to run on that system now if you have a 50 ohm tracking generator output that's going through here that's absolutely fine when you hook the 1 Meg ohm input of your spectrum analyzer through this box to the other box on the tracking generator output it's completely flat no problems there but again when you put this on to the 50 ohm input or 75 ohm input of your tracking generator it is not flat it actually looks pretty linear it's you know like this so that's unfortunately the the way that things go it's with this particular unit anyways again a little bit of a modification you could probably make this absolutely fine to work on the 50 ohm input it's just I didn't plan to do that because this is designed to work with high impedance circuitry so I have it on the high impedance input of my spectrum analyzer so now this is four thousand volts of isolation from this alligator clip here to this part of the stack I would never test that at 4000 volts because the box is so small and the area between the capacitors is very small so I imagine the breakdown would be a little bit less than 4,000 volts so I wouldn't test that for most receivers you're dealing with a 300 to 500 volt range inside the receivers so not a big deal for this whatsoever I still shudder when I hook the alligator clip up to high voltage though it's just a uh you know input of my spectrum analyzer that's a real touchy piece of equipment and that spectrum analyzer works absolutely flawless so I'm very careful with what I do now this is the kind of the glory of this little adapter here okay you have all that isolation right here up to say 4 kV if it was going to hold that up I imagine if you potted that whole area it would probably hold up the 4 kV I would never try and I don't suggest you do either especially if you're playing with a you know a really nice spectrum analyzer and of course again if you're going to be hooking this up to any high voltage circuitry or you're working on any high voltage circuitry you're doing so at your own risk so just take care and if you're gonna build this and hook this up to your spectrum analyzer you're also doing so at your own risk make sure that you put this together correctly and you know and test your circuit before you actually apply any voltage to it because you don't want to damage your input spectrum analyzer inputs are very expensive to fix ok so again the glory spot of this thing here is is that okay if there is supposedly some form of arc over in here we have diodes here on this side that will allow only a positive or a negative 5 volts to be at this particular point anything over 5 it's going to clamp it down so that's a little bit of extra protection right there this is a limiting network so that only you can get about 10 volts peak-to-peak through this whole system and that's going to be all that's going to travel through this this will start to clip at about 10 volts peak-to-peak so remember we have you know either positive or negative five volts is all that's going to be able to get through if there is a DC voltage present there that's all that's going to be able to get there and then this would clamp down and of course we have another capacitor here just as a little bit more protection right to the input of our spectrum analyzer this here's a little bit of a strain relief here any of you that have played with any kind of a BNC connector or used BNC s over the years BNC connectors with lots of mileage on them the center conductor sometimes gets a little bit loose and that tears up traces on circuit boards and stuff like that so this here is just a little little kink of wire there that allows a strain relief so if that does for some reason get loose it'll just break the wire and not wreck the little circuit inside and that's all that that is and I suggest that whenever you build anything with a BNC connector unless it's soldered directly to a piece of fr4 board or something like that if it's if it's right on the board and the spoon of the center's solder there usually it's pretty solid but there is a bit of a gap between the plastic box and the actual circuit board itself so this here is a little bit of a relief between that point there and this here will just break if something ever does happen so this is point zero one micro farad this is a 50 volt rated capacitor that's all that it is these again are 500 volt rated so they're very very safe the reason that I use these capacitors is because I have a lot of them have a whole roll of these capacitors and these are all C C 0 G capacitors you know don't need to be like that anyways it's just for testing you're feeding a signal through it and and all that kind of stuff but this is just what I had on hand right so I use what I have now I put a jumper adaptor here just in case I could put a little I think it was a no 603 or Oh 805 I forget the actual size of the pads there I can use a zero ohm jumper and I can jump out four of these capacitors if I want to up my coupling to 25 Pico farad if you up it to 25 Pico farad you are then losing to kilovolts of isolation and you only have 2,000 volts worth of isolation again I would never test that at that because the board is very small and it is not potted so that I can service it and if something ever happens or whatever I just I didn't pop the board and that's pretty much this these are just alligator clips here on the end this ground is connected to this ground of this ground that's a pretty thick trace on the board out to this alligator clip right here and this is that high-voltage flexible test lead that you see right here and that high voltage to flexible test lead is the same stuff that he used in that DeForest Crossley receiver that little loop that you saw into the center shaft of the of the band switch that's exactly what that stuff is is multi strand silicone you'll see it in the box when I show you the little boxer and that's pretty much it my spectrum analyzer puts out a 0 DBM which is about point two to four of a volt or 1 milliwatt worth of power that's dramatically lower than this diode stack here at this point 2 to 4 oval volt is gonna be coming out of here this will handle 10 volts peak-to-peak the reason that I did that so I can use these boxes on a signal generator or anything else I don't want to lightly couple into a circuit so that I'm not limited off if you wanted to limit this down a little bit more you just can remove some of these diodes now these are bav 99s which are two diodes and 1 SOT 23 packaged a device so there's six devices we have one two three four five six and you'll notice that one is up and one is down so you know that's creating a limit at 10 volts peak-to-peak all right now there's two diodes in one package these are very fast diodes you could get away with using one in 911 4s or one in four one five 2's or anything like that some in it some really fast gloss diode would be absolutely fine so let's head on over to the to the bench here and check out what's inside this little unit and then we'll look at it on the spectrum analyzer and I'll explain why it's important for these little units to be flat when you're doing an alignment on on any kind of receiver one other thing I will mention before I go again if you have watched any of my other videos you know the least amount of coupling is the most important thing to have whenever you're doing any form of alignment so if you can get away with running a Pico fret of coupling into the circuit and out of the circuit that's awesome because the more coupling you have you're adding more capacitance into the circuit that you're tuning and stray inductance and you don't want any of that because when you remove your test gear you've effectively tuned that inductance and capacitance out in your circuit so your response pattern is gonna change so the least amount of coupling to get the signal in and the least amount of coupling to get the signal out is what you want again you want your test gear to be invisible to the device under test you don't want them to see each other you want the radio to think that it's just operating normally and the signal is just coming from somewhere okay that's the whole trick of doing a proper alignment if you over couple onto the grid cap of say the mixer tube or if you over couple to the last if' transformer output when you're adjusting the slug on that last if' transformer or when you're adjusting that variable capacitor in the top like on the DeForest Crossley you'll be tuning out your you know you'll be working on that response pattern but what's also effecting that response pattern is the stray capacitance that's imposed by your test lead clipped into the circuit and also the stray inductance of that test lead so remember one to two Pico fret if you can get away with that that seems to work very very well with the 3585 be spectrum analyzer let's check out what's inside this little box so you can see I've removed one of the screws already and they're self tapping Phillips screws so the first time you run these screws into the box they tap a little extrusion that holds this lid on here and then after that they thread in and unthread really easily it's a really soft plastic so they go and relatively easy the first time anyways now the reason that I've chose a plastic box for this application is because this is dangling inside chassé T's a lot and there's a lot of high voltage circuitry that a metal box could short out on so by having a plastic box it kind of saves you from doing any kind of damage now when I've got this B and C on here and this is an exposed collar here I'll put a piece of black tape around it or you know slide a piece of heat shrink up over that BNC connector and then that way it's absolutely safe if this is in any kind of a you know real tight environment where I might touch something so these are real handy boxes they're made by Hammond and about these many many years ago and I've got the number written on another open box over there I'll go grab that in just a second here and I'll tell you what the boxes are they're really cheap they're about a dollar fifty or something like that and they're great little boxes for projects like this so this is what's inside the box here I'll zoom on in now this camera doesn't zoom anywhere near as close as the my old camera there but I think it gets the point across here so right about there now these are the capacitors here in the stack all right so about 12.5 Pico fret of coupling here and I've noticed that I've tapped it a little bit further down the stack than the 25 Pico fret so there's only three capacitors in and I didn't tap it right at the 25 mark I believe because I wanted just a little bit more coupling for a signal generator and I didn't draw the schematics out this is just all right off the top of my head from what I remember I didn't even take any notes on this I just put this together very quickly because I needed something from my spectrum analyzer here very fast so I didn't have any notes so there there's one little amendment to what you see on the board there it's time just a little bit further than the 25 Pico farad mark now I'm not even using that I never have because I've just used these for aligning if's and radios and and I find that 12.5 Pico farad is is enough coupling anyways for anything that at least that I'm doing so this little little jumper here isn't populated it's just left open and that's a looks like you know 603 part would fit on there so we have 12.5 Pico farad of coupling from this point here to this point here you can see that I've got this trace really close to the ground here so there's a you know a very very small amount of area there and that's if if something's gonna arc over I want it to arc over at this point here I don't want anything to get up through this stack and get to the diodes and that's the reason I did that there so if it can jump that gap at that point that's pretty dangerous and I would say that I would probably be able to get to about 1500 volts around there before that gap would start to jump so that's a little bit of extra protection now again I don't ever plan on taking this you know above the 3 to 500 volt range at maximum because this is basically used for tuning receivers and and stuff like that so a little bit of extra protection right there now this runs into the be a v99 diodes which are right here there's six of them here that's that diode stack and then there's that point zero one capacitor that runs to the center conductor of this BNC connector this here is that little strain relief that I've noted in the on the whiteboard there you can see there's a really stout connection from the ground lead all the way up to the shell of the B and C over here and I want that because I don't want the ground to ever break all right this is this high heat silicone test lead wire now this is very good stuff this stuff here is very flexible lots and lots of strands it's the same kind of stuff that I used in that DeForest Crossley receiver on that band switch there you can see it's got one black layer and then another white layer in the center now these holes that I've drilled in the box right here where these test lead wires run through are a little bit smaller than the silicone outer jacket here now I've tapered the hole so it doesn't cut the silicone but in order to get these silicone wires through you have to oil them up and then press them through and then of course that creates quite a friction fit at the plastic here and once you take the oil off here these are tight and you don't need to put any strain reliefs in now that's nice for easy replacement if I want to replace these wires down the road if one of them breaks all I have to do is click the inside clip the outside and just press the little centerpiece out and away I go just put another piece in with a little bit of oil this would never ever come out that is extremely solid in that box and that's exactly the way that I intended this to be now this is isn't not intended for high frequency use and anybody that's been in the RF game will know that already because of these long clips here we have a lot of lead inductance here so I primarily use this for if' alignment and this is for lower frequency use and we'll take a look at the frequency response on a spectrum analyzer right up to about 40 megahertz and that's the top of that 3585 be these two little screws here mount the circuit board to to smaller extrusions in this box here and this is the box right here and that's what it looks like with nothing in it so you find two small screws and then just thread them into there now when I got this box it didn't come with any smaller for these smaller extrusions so you'd need to find a smaller screw for that but they do come with these two large ones for the lid and you can see here it is a Hammond one 551 G that's the number of the box these aren't real handy little boxes I bought a whole bunch of these things quite a while back and I've used up a lot of them and in fact I've made a few of these for my friends as well so that they with their spectrum analyzers don't have any issues so that really is how the Box works and what's inside so off to the spectrum analyzer we go and let's check out this little items performance let's check out the performance of these little white boxes this is the first little box here that you see is attached to my tracking generator output now my tracking generator output is DC coupled so I need to be very very careful that I don't back feed any DC back into my machine or I'll ruin my tracking generator that's exactly what these little boxes are designed to protect me from doing these little boxes will block DC but only let the signal through alright this box and the other box are designed around a high impedance system but it's okay to drive this box from any DC couple 250 ohm tracking generator the second box is hooked to my one mega ohm input on my spectrum analyzer again this blocks DC as to protect the input of my spectrum analyzer and will only let the signal through this is designed to be used in vacuum tube gear that's why I designed this around the 1 mega ohm input on my machine most vacuum tube gear is pretty high impedance now what we want to do is we want to see how flat this is through the entire range now we can use the entire range of my spectrum analyzer to see this is a 20 Hertz to 40 megahertz spectrum analyzer so this is a lower frequency spectrum analyzer but it is very very good for sweeping if's and it's very good for a lot of RF use in fact this is a very very nice spectrum analyzer if you have a chance to ever get one of these things I strongly suggest that if you're doing the kind of stuff that I'm doing so since this starts out at 20 Hertz I'm not consider you know concerned about going down to 20 Hertz cuz that's audio frequency so I intended these boxes to really work in the RF range so let's see how flat these are from say we'll start frequency 100 kilohertz and we'll go stop frequency 40 megahertz so that's basically you know 40 megahertz is the top end of this machine that's where this machine stops so from 100 kilohertz to 40 megahertz now now what I'll do is I'll hook these boxes together and we'll see how flat they are through the entire range so I got to reach around my tripod here is kind of difficult this is uh reaching here there we go so they're put together here I'll just put these back down on the bench so I can free up my arms here a little bit so there we go so we have a start frequency of 100 kilohertz all the way to 40 megahertz up here and we're dealing with about a 6 DB difference from end to end that greatly exceeds what I had planned for these boxes now I basically do RF alignments you know on old receivers in the kilohertz range so as you've been seeing I've been doing RF alignments and you know roughly around 450 5 kilohertz now when I do an alignment at 455 kilohertz I usually want to see a span of about 20 kilohertz so if I want to do that I need I need to make sure that my machine is flat within that 20 kilohertz whenever you're gonna do an alignment you want to make sure that this is absolutely flat from start to finish so if I'm going to do an alignment at 455 kilohertz with a span of 20 kilohertz I need to hit start frequency 445 kilohertz and stop frequency 465 kilohertz we're completely flat within that range our center frequency will be at 455 you can see 455 at a span of 20,000 cycles or 20 kilohertz that's what we need out of these boxes and that's exactly what they're doing so for a span of 20 kilohertz absolutely no problems can we flat so I really doubt that I'm gonna use these boxes you know with it greater than because I use these for sweeping so I really doubt that I'm going to you know be using these for anything more than say no to megahertz or anything like that so let's just try say well start frequency nine megahertz stop frequency eleven megahertz we're absolutely flat within that range as well so we have we're completely flat within two megahertz of sweeping right here we can see that our Center would be at ten megahertz here in a span of two megahertz and that's really what I intend with this I'm not really trying to look at an entire you know the the tire 40 megahertz spectrum with this with these boxes they're designed for being tied into vacuum tube gear and for doing sweeping use and of course aligning if's and this is exactly what what I intended now keep in mind we're running right now at at nine megahertz all the way to 11 megahertz with a center of ten megahertz with open leads all right let's pick this back up again with open leads right here just like this and we're still absolutely flat and that's exactly what we want in any design because if we have a dip in this design like this we cannot do an alignment because if we want to do an alignment we're going to have this to compensate for and we're not going to know how to compensate for that so what we want to do is always when we're doing an alignment you always want to test your test gear to make sure that you're flat from beginning to end if at that point you're flat you can go through and do your alignment so all in all these boxes worked out very well in fact they worked out even a little bit better than I expected this video it went so fast it's like it was here and gone it's not like those Radio restoration videos anyways I hope you enjoyed the video if you did please give it a big thumbs up and hang around I'm pretty sure there'll be many more just like this coming up in the very near future so on to the next project see you next time you

Channel: Mr Carlson's Lab

Views: 43,173

Keywords: Spectrum Analyzer, Analyser, Creative Design Components, HP-3585B, 3585 spectrum analyzer, HP spectrum analyzer, sweep alignment, IF alignment, RF alignment, over coupling, light coupling, tracking generator, Receiver alignment, Communications receiver alignment

Id: ETKyKC2Zj-M

Length: 22min 27sec (1347 seconds)

Published: Mon Jul 06 2015