Zener Noise (was: 1N4007 varactors)

It's not particularly flat over the whole band, and I haven't attempted to make it be. My interest has been mostly in relatively narrow band filters, or the shape of a main filter rolloff. The noise is adequately flat over the bandwidths I've been interested in. It's been a while since I've fooled with it, but as I recall, the shape of the noise spectral distribution changed all over the map as I changed the diode bias. I imagine that it changes with temperature and with individual diodes, too. So any circuit used to flatten it would only work for a particular diode, current, and probably temperature. The bottom line is that it's probably a lousy way to try to generate a flat broadband noise spectrum.

Roy Lewallen, W7EL

Good zener noise sources are carefully bred and tested, they do _not_ come naturally from garden-variety zener diodes. I suggest you go read the hundred or so messages in the famous zener oscillation thread a few years back here on s.e.d. In this you'll learn of my substantial investigations into the topic, some physics - and, very important, learn what a zener microplasma is. You'll see my ASCII waveform plots of actual bench measurements showing exactly what's going on. After all this you may decide to avoid using zener diodes for calibrated noise sources. Sorry about that!

What I did for some project which needed equal amplitude uncorrelated noise,is amplify the zener noise with a wide band video opamp,with high pass and lowpass filtering. Used the low pass as input for a zero cross detector, delayed the zerocrossing 10 microseconds,and used that for clock to a circulating bit in a shift register. each parallel output of that register controlled a sample/hold opamp,sampling the highpass signal. Voila!! 8 audio frequency, non-correlated noise sources. The zerocrossing clock was made this way,to avoid detectable clock tones int the output.(2 to 20 microsec between crossings) the 10 microsecond delay was used to get a voltage at the sample and hold opamp which was not correlated to the zerocrossing. If you need only one signal ,leave out the shift register, and just use a 10 and a 1 microsec. oneshot for the s/h opamp clock. The application? A wind and engine noise generator for a car simulator.

These folks will sell you serious noise diodes...

John

That's one thread, perhaps the first in a series. That thread doesn't have the waveforms I was referring to (although there are some waveforms in posts 51 and 66). Tony, Bill, Roy and I, and some others here wasted masses of time on this subject over a period of a few months, eight and a half years ago. We took bench measurements, did calculations, found the scientific literature (it was a subject that occupied physicists in the late 50s, see posts 72-76), and we did plenty of speculation.

All of which led Roy McCammon to remark (post 90), "I'd have to say that it is the best thread this year." He said that Aug 5th, after 3 weeks of posts, and yet the followup threads in Aug and Sept on the same topic were just as long, and perhaps even more interesting. Ah, those were that days!

OK, then. A zener makes a poor noise source according to what I'm reading. Noise.com used to sell off-spec diodes by the onesies for we poor peons to play with, but for whatever reason that doesn't seem to be the case any more.

Given that a zener (at whatever current) is a poor noise source, what is a good source of electronic broadband noise from low HF through high UHF -- say, 5 to 500 MHz.? (No smart remarks about spark gaps.)

Jim

We took

A hot resistor.

How about a thermistor or a lamp filament that was 50 ohms at some high temperature. You could heat it with DC, sense its resistance/temp, and let it make noise, all in a single part.

Old vintage noise figure meters used gas tubes. And I think there was a pencil tube that mounted in a waveguide and made shot noise.

And, of course, the old photomultiplier trick.

John

I have seen a small "gain of wheat" type light bulb used as a noise source...

Feed it with DC through a choke and AC couple the noise out ....

and you can vary it too!!

Mark

Immediately you say "choke" you have modified and peaked the bandwidth. I'll buy that you can feed it with a resistor for broadband, but in my humble opinion the construction of a grain of wheat bulb won't get up into the UHF region with noise. Do you have any idea of the output level of this circuit?

Jim

I still like the flashlight/photodiode trick. You can get a really good calibration just from the dc, and can calibrate the frequency response with a spark plug.

Cheers,

Phil Hobbs

ORIGINAL MESSAGE:

"Phil Hobbs" and can calibrate the frequency response with a spark plug.

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I generally use a pipe wrench, but I'll try anything once.

Bill, W6WRT

I've got a gaussian noise generator, some SS in the power supply, tubes everywhere else, found it on the curb and apparently works. Uses a pair of

Tim

-- Deep Fryer: a very philosophical monk. Website:

GR?

John

What's the light-flash waveform look like from a spark plug? What do you drive it with?

Don't you have gobs of femtosecond lasers around your place?

John

Try this

Come again?

Tim

-- Deep Fryer: a very philosophical monk. Website:

Jim wrote: "...what is a good source of electronic broadband noise from low HF through high UHF -- say, 5 to 500 MHz.?"

A linear feedback shift register. Small, repeatable. 500MHz should be no particular problem these days. (There's an idea for some IC manufacturer...32 bits clocked at 1G/sec repeats every 4 seconds, which would be OK, but I'd prefer 40 or more bits. Should fit nicely into a