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Hmmm, you are right.... I still don't think that summing the voltage noise from a bunch of unipolarized zeners is going to get rid of the voltage assymetry. But I'd be happy to be wrong too. Have you ever tried this? It would be simple enough to put 5 or 6 together and see what the output looks like. (As long as you don't mind my summing with an opamp)... Maybe I can find some 'fun' time on Friday.

George H.

The math says it must be so. Still, the sum would converge to Gaussian faster if half of the lopsided signals were inverted.

Zener noise gets more symmetric at higher currents. 10 mA is usually OK for a small 10-volt zener.

I sometimes generate Gaussian-distributed numbers by summing a bunch of RAN() calls, which are uniform on [0,1]. Six to ten works well, and the crest factor is finite and known.

John

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You can use two zeners on a single power supply, in bridge configuration; couple the output through a transformer to get the difference. Symmetry is guaranteed if you balance the bridge correctly.

I've seen zeners (on a curve tracer) that were so noisy that the breakover region was a blur.

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OK I'll just have to try it. I find the math more convincing if I can see it in some experimental result. (Shot noise is a good example, but it=92s hard to get the current low enough so that you could see a non-Gaussian distribution.)

Yeah I think that is just the result of the I-V curvature. I'll try running them down at low currents where the asymmetry is larger. This is an experiment to show the central limit theorem and not make a good Gaussian noise source.

George H.

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Ahh, more than one way to skin that cat.

Say speaking of noise sources, a colleague put together a digital noise source. A counter steps through a look up table that feeds a DAC. The lookup table holds a whole comb of sine waves equally spaced in frequency space up to 32kHz. (I don=92t recall the frequency spacing but I could find out.. a few Hz or so.) The phases of all the sine waves were chosen randomly. The DAC was 12 bit (an AD7541 I think). The whole thing was clocked several times lower than Nyquist limit (~128 kHz). Now the problem we observed, (and could never cure), was intermodulation distortion above the 32 kHz cutoff. The signals above the cutoff frequency were down by only 50 dB, and my colleague was expecting something closer to 70 dB down. (Is that right for 12 bit resolution on the DAC? ) I worked on all the layout and analog portions of the circuit but could never make it any better. There was talk about clock jitter on SED recently and I wondered if this could be the source of the problem? Or maybe you have some other idea.

Thanks, And If you think it might be clock jitter then can you tell me how to measure it also. (OK I can use google too.)

George H.

A pseudo random noise (PRN) generator (a shift register and a few XOR gates in the feedback path) will generate quite good white noise sequences at frequencies well below the clock frequency.

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Here is my elegant, peer-reviewed research paper on the subject:

ftp://jjlarkin.lmi.net/Zener_Noise.pdf

John

The DAC quantization, and any nonlinearity, will add harmonic distortion. Plus the sines may occasionally peak together and clip the dac. Plus it's not trivial to get -70 dB distortion at these frequencies.

We use random number generators and boxcar filters to generate Gaussian noise to feed into dacs. This little box does this, all in a Spartan3 FPGA...

Rob cleverly, somehow, allowed the user to program the noise bandwidth from mHz to 2 MHz without affecting the RMS amplitude.

John

For cheap random number generators, I prefer LCGs to LSFRs. It is very simple to compute something like:

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We might as well. They have no other value, aren't biodegradable and they need an EPA permit for disposal.

• Try the trick i mentioned; 80uA was about the max noise point, with the noise going toward zero at around 120uA.

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Yup, I've got that stored in my Zener noise folder. Someone (Bill Sloman?) directed me to a previous thread on Zeners and there are some nice references from Bell labs back in the '50s. One by McKay Phys. Rev. 94 pg.877 is a nice summary.

George H.

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Very nice, (Are prices listed on your website?) Y'all make stuff that's several orders of magnitude above what we're doing.

You know what I'd really like to make (and could probabbly sell too.) is a random pulse generator. Short little pulses maybe 10nS or less coming at an average rate of 1us or so. This would be a psuedo shot noise generator. With a pot on the output one could change the amplitude of the pulses and see how the noise scaled. ... Hmm it would be nicer if the average pulse rate could be changed too. So that one could keep the average 'current' the same, but made with bigger 'electrons'. Sounds like a digital circuit. (Which I find a bit boring) an analog 'something' would be more fun.

George H.

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You could start with any noise source, like a zener thing, drive a comparator to nab the peaks, and use a tach-like feedback to adjust the comparator trip point to servo the rate. Fire a one-shot, adjustable width if you like, and a pot to set amplitude. All analog, pretty simple. You could also do the random rate thing digitally.

John

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Yeah I had something like that in mind. I didn't quite know how to do the random rate part.. By tach-like I assume you mean a frequency to voltage converter.

"You could also do the random rate thing digitally."

Oh that sounds easier than the tach-servo idea. Divide by N before the one shot. I don't need fine resolution on the rate, just the ability to change it and then adjust the pulse height.

Are one shot's still 'kosher'? I once talked with an FPGA guy who had never heard of them. Anything 'better' than the 74AHC123 from NXP? This has a minimum pulse width of 5ns which would be fine. Other one shots looked to be a lot slower.

Ahh so many circuits to try and so little time.

George H.

How about avalanching a phototransistor? ;)

Cheers

Phil Hobbs

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I never heard of it. Google gets a lot of hits though, I'll have to explore tomorrow when I have higher bandwidth at work. Do I need a special phototransistor or will anyone work? I assume I have to reverse bias the BE junction.... But won't current flow out the collector?

Thanks for something fun to think about.

George H.

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point it at an Americium source from a smokedetector :) real random noise

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Something optical is nice because you can change light intensity with a knob.

George H.