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Thread: HF-VHF Noise Sources

  1. #1

    Default HF-VHF Noise Sources

    Having no tracking generator on my spectrum analyzer, I wanted to make a broadband noise source for doing filters and such. After trying a few of the 2N3904/Zener or LED schematics online (the parts I have on hand), I could only get noise up to about 40MHz. If anyone could advise on how I might improve that using 3904's, I'd appreciate it. I have some KSP10's somewhere too.

    Where I'm thinking of going from here: I made an avalanche pulse generator for TDR measurements a while back that did about .75ns rise time and that thing puts out noise well into UHF, but its still a bit weak for getting a good picture of what a filter is doing. I was thinking of putting about 10 2N3904's on a board and have them all avalanching into the same load resistor (timed with different capacitances), perhaps through diodes if that don't cut down on the bandwidth. The goal is total RF chaos with junk box stuff, any thoughts?

  2. #2
    Super Moderator 5B4AJB's Avatar
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    Hmm, total RF chaos eh?

    Build yourself a simple ioniser, parallel a neon with a small capacitor and connect one end to ground via a small spring.
    Adjust the gap between the other end of the neon and the ioniser' emitter so it bounces around while creating a spark.

    That will generate plenty of random RF noise...

  3. #3

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    I will try that this weekend. I have a few neon bulbs laying around somewhere. I do need to feed this signal into my spectrum analyzer though so I will need to use a low value resistor between the ionizer and ground so I have a way to sample the voltage low enough to not cook the input of the analyzer.

    I started in on the avalanche board and have 5 avalanche circuits paralleled. I was going to do 10, but with only 5, the other side of the board has room for the HV supply circuitry. After I finish this avalanche thing I will try the neon thing. I will hopefully have results worthy of posting soon.

    Thanks!

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    Super Moderator 5B4AJB's Avatar
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    You might want to do your test when nothing's on TV, you neighbours are out and you're not expecting any mobile calls

  5. #5

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    I will ba taking precautions to prevent that. I live about 2 miles from the nearest full-time resident and plan to enclose this in a metal case.
    Being my picoscope only does fft spectrum analysis up to 200MHz, i dont do much above that. I could always LPF the output so that im not sending out any wavelengths short enough to find an unintentional radiator of any effectiveness. If i cant tame the beast, i would then be forced to use a dip meter till i can afford better equipment, but i dont think it will be an issue with the short duration tests i intend to use it on..

  6. #6

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    Update:
    The multiple avalanche transistor idea failed. Adding the extra circuits added length to the traces and slowed the rise time too much. Slower rise time, fewer harmonics. I have not got around to making a high enough voltage source to do as KB4AJB suggested with the spring, but I can't wait to try it! I will hopefully get to it within the week. Anyhow, although not perfectly flat across the screen, here is how a single 2N3904 avalanching transistor (rebuilt with shorter leads) did as a noise source.

    Analyzer in averaging mode...
    Noise test.jpg

    The board is a boost converter to make 120v out of 12v, the avalanche transistor is in the PL259 barrel on the BNC tee
    Noise source.jpg
    Last edited by brandon lind; Mon 12th Aug 2019 at 01:51.

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