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I experimented with a simple RF white noise generator using avalanche breakdown on an NPN BFR93A transistor. I've used a voltmeter and an ammeter to determine the breakdown voltage and associated spike in current.

Next, I want to verify the randomness of frequencies in the generated signal on my oscilloscope. I've attached a probe to a capacitor connected to the RF white noise generator. I've enabled FFT on the scope to view the frequency spectrum and flatness. The BFR93A is quite wideband, so I expect results up to about 1GHz, with good flatness from DC to about 200MHz. The image I've attached is what I saw in my testing and it looks reasonable, but I think there is an issue.

QUESTION: Should the yellow signal trace be the same tone of yellow instead of having a bright horizontal region? The bright horizontal region indicates amplitudes generated more often than the darker yellow tracings further away from the baseline. Right? I'd appreciate someone helping me understand what I see.

If there is an error, I'd appreciate hints about possible causes. Would using a larger or smaller capacitor help?

Thank you for your time and interest!

enter image description here enter image description here

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  • \$\begingroup\$ Would it be correct to say that the yellow trace is the signal and the orange trace the FFT? \$\endgroup\$ Commented Jan 17, 2025 at 5:37
  • \$\begingroup\$ There seems to be a repeating pattern in 1/3 of the noise envelope. \$\endgroup\$ Commented Jan 17, 2025 at 5:47
  • \$\begingroup\$ Hi @Frog, you are correct. :) \$\endgroup\$ Commented Jan 17, 2025 at 5:52
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    \$\begingroup\$ Sweep faster and look for anything synchronous \$\endgroup\$ Commented Jan 17, 2025 at 6:07
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    \$\begingroup\$ Different densities of yellow are just an artifact of the algorithm used to make the digital scope trace 'look like' an analogue scope trace, which would itself have to be interpreted anyway. For white gaussian noise, you would expect values near the middle to occur more often than large values. C1 is a bit big for 'RF'. Your transistor is probably impeccable. Your circuit may have resonances enhancing or depressing response at some frequencies. The narrow spikes at regular intervals on the spectrum are certainly interference from some external source. Show a photo of your complete setup. \$\endgroup\$ Commented Jan 17, 2025 at 9:38

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