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Noise Diode

M

Marceli Firlej

Jan 1, 1970
0
Which Zener diode could be used for white noise generator?
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Marceli Firlej <marceli.firlej@oze
mail.com.au> wrote (in said:
Which Zener diode could be used for white noise generator?
Any one rated substantially above 6 V. 12 V devices are often used. They
are really 'avalanche' diodes; true Zener operation occurs at breakdown
voltages below 6 V and is not noisy.

You get better Gaussian probability distribution if you use two diodes
from the same batch and couple them to the + and - inputs of an op-amp.
The output is the difference between the two noises and is good quality
band-limited Gaussian white noise.
 
J

John Larkin

Jan 1, 1970
0
Which Zener diode could be used for white noise generator?


The classic RF noise source is something like a 16-volt zener... I'm
not sure why. A 10 or 12-volt zener, biased at, say, 1 mA, will give
something like 350 nv per root Hertz noise with pretty good
statistical properties. Lower currents may do funny things. Low-power
(small junction) zeners give good stats and are wider band than bigger
ones.

350 nv/rtHz would give you about 50 uV over the audio range, so it'll
need some amplification!

So, any old 1/4 watt 12 volt zener should be pretty good. You can buy
selected "noise diodes" for big bucks if you need guaranteed wideband
precision; maybe NoiseCom has some appnotes.


John
 
R

Robert Baer

Jan 1, 1970
0
John said:
I read in sci.electronics.design that Marceli Firlej <marceli.firlej@oze

Any one rated substantially above 6 V. 12 V devices are often used. They
are really 'avalanche' diodes; true Zener operation occurs at breakdown
voltages below 6 V and is not noisy.

You get better Gaussian probability distribution if you use two diodes
from the same batch and couple them to the + and - inputs of an op-amp.
The output is the difference between the two noises and is good quality
band-limited Gaussian white noise.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

And drive them at "low" current (tens of microamps seem to be ideal).
Zeners tend to get quiet as the operating current approaches the rated
test current.
The drop can be rather fast, and every zener (same voltage and batch)
can have a different current VS noise dropoff characteristic.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Robert Baer
And drive them at "low" current (tens of microamps seem to be ideal).
Zeners tend to get quiet as the operating current approaches the rated
test current.
The drop can be rather fast, and every zener (same voltage and batch)
can have a different current VS noise dropoff characteristic.

I used to think that, but some experiments that Win reported a while
back indicated that a few milliamps would be a better choice. Anomalous
effects occurred at very low currents, like 'burst avalanching', IIRC.
 
W

Winfield Hill

Jan 1, 1970
0
John Woodgate wrote...
You get better Gaussian probability distribution if you use two diodes
from the same batch and couple them to the + and - inputs of an op-amp.
The output is the difference between the two noises and is good quality
band-limited Gaussian white noise.

Is there a reason to believe the difference should be any better
than the sum?
 
K

Ken Smith

Jan 1, 1970
0
John Woodgate wrote...

Is there a reason to believe the difference should be any better
than the sum?

If the low frequency noise modulates the amplitude of the high frequency
noise, the difference would be much better. I suspect that the ampltude
of the RF components do decrease when the low frequencies lower the
"average" voltage.
 
G

Guy Macon

Jan 1, 1970
0
Winfield Hill said:
John Woodgate wrote...

Is there a reason to believe the difference should be any better
than the sum?

If 60 Hz or your local AM radio station is leaking into the two
circuits, then the difference is better than the sum, but if there
is no such interference and the noise from the diodes is random,
it shouldn't matter whether yu take the difference or the sum.

Also, for those who are planning on using the noise from the
diodes to generate a random stream of bits, I refer you to the
"Recurring topics in comp.arch.embedded" web page at
[ http://my.execpc.com/~geezer/embed/cae.htm ], which says:

What's a Von Neuman compensator?

Take a biased stream of 1's and 0's two at a time.

If the pair is 00, do nothing.
If the pair is 01, output a 0.
If the pair is 10, output a 1.
If the pair is 11, do nothing.

(some implementations switch the outputs so that 01 = 1 on odd
cycles and 01 = 0 on even cycles. The hardware random number
generator on Intel's latest motherboards uses this method.)

a Von Neuman compensator will remove the biases caused by switching
threshholds that are not exactly at 50%, and other biases common to
the analog to digital stages in typical hardware random number
generators, and will also remove the biases caused by asymmetrical
duty cycles and other biases common to typical counter/timer based
random number generators.
 
J

John Larkin

Jan 1, 1970
0
John Woodgate wrote...

Is there a reason to believe the difference should be any better
than the sum?

Zener noise can be asymmetric in shape. I think that, given the choice
of summing ++ or +-, the +- might be more symmetric. That sort of
creeps up on the Central Limit Theorem a little quicker.

John
 
J

John Larkin

Jan 1, 1970
0
And drive them at "low" current (tens of microamps seem to be ideal).
Zeners tend to get quiet as the operating current approaches the rated
test current.
The drop can be rather fast, and every zener (same voltage and batch)
can have a different current VS noise dropoff characteristic.


I've played with regular 1N759-types a bit. At 1 mA, the noise is
usually pretty Gaussian and spectrally flat, maybe 350 nv/rthz. As
current goes down, one sometimes sees asymmetric noise, then either
geiger-counter-like spikes or very ratty sawtooth oscillations. Some
of the NoiseCom noise zeners do work at very low currents, 10s of uA,
but they are special parts, I suspect maybe ultra-small-area diodes.

Agreed that individual types can vary a lot.

A neon lamp makes lots of nice noise. Old GR noise generators used a
gas tube of some sort with an external magnetic field for some reason.
I have schematics somewhere...

You can just use native opamp noise, but the statistics and spectra
can be awful.


John
 
J

Jim Thompson

Jan 1, 1970
0
On Sun, 15 Aug 2004 11:48:01 -0700, John Larkin

[snip]
A neon lamp makes lots of nice noise. Old GR noise generators used a
gas tube of some sort with an external magnetic field for some reason.
I have schematics somewhere...

You can just use native opamp noise, but the statistics and spectra
can be awful.


John

I'd like to see schematics. The magnetic field might be to keep the
discharge from "dancing" around, perhaps adding a "popcorn" noise
element ?:)

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
On Sun, 15 Aug 2004 11:48:01 -0700, John Larkin

[snip]
A neon lamp makes lots of nice noise. Old GR noise generators used a
gas tube of some sort with an external magnetic field for some reason.
I have schematics somewhere...

You can just use native opamp noise, but the statistics and spectra
can be awful.


John

I'd like to see schematics. The magnetic field might be to keep the
discharge from "dancing" around, perhaps adding a "popcorn" noise
element ?:)

...Jim Thompson

The manual's at work; I can post pics to abse Monday. I think there
are some directions for adjusting the magnet position, too. I bought
an old GR toob-type noise gen on ebay for some jitter making, but it
hums a lot and I've got to find time to replace all the 'lytics.

John
 
M

Mike

Jan 1, 1970
0
Zener noise can be asymmetric in shape. I think that, given the choice
of summing ++ or +-, the +- might be more symmetric. That sort of
creeps up on the Central Limit Theorem a little quicker.

Assuming the diodes are independent (which certainly seems like a good
assumption), the density of the sum is the convolution of the densities.
For the difference, the result is the same.

I've been playing with various distributions here in MathCad this morning,
and while the sum and difference may result in different offsets, once the
offset is removed the distributions appear to be identical.

-- Mike --
 
J

John Larkin

Jan 1, 1970
0
Assuming the diodes are independent (which certainly seems like a good
assumption), the density of the sum is the convolution of the densities.
For the difference, the result is the same.

I've been playing with various distributions here in MathCad this morning,
and while the sum and difference may result in different offsets, once the
offset is removed the distributions appear to be identical.

-- Mike --


Are you saying that, given two basicly identical random signals having
lopsided probability distributions, the sum won't be lopsided? The
difference will certainly have a symmetric distribution.

If the signals were low-duty-cycle random positive pulses, there
certainly would be a difference in result in probability distribution
between the summing and differencing schemes. Zener noise can be
pos/neg asymmetric, getting very pulsey at lower currents and can
still be visually a little spikey at 1 mA.

John
 
M

Mike

Jan 1, 1970
0
Are you saying that, given two basicly identical random signals having
lopsided probability distributions, the sum won't be lopsided? The
difference will certainly have a symmetric distribution.

I was indeed saying that, but that was before I had my coffee and started
thinking. You're absolutely right.

-- Mike --
 
J

John Larkin

Jan 1, 1970
0
I was indeed saying that, but that was before I had my coffee and started
thinking. You're absolutely right.

-- Mike --

Well, I typed my query *after* I had my latte, so naturally I had
unfair advantage. But I did phrase it as a question in case I turned
out to be wrong.

John
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
Are you saying that, given two basicly identical random signals having
lopsided probability distributions, the sum won't be lopsided? The
difference will certainly have a symmetric distribution.

It's highly unlikely that two "nearly identical" zener diodes will
have basically identical random signals.
 
J

John Larkin

Jan 1, 1970
0
John Larkin wrote...

It's highly unlikely that two "nearly identical" zener diodes will
have basically identical random signals.

No two zeners will be identical, but their noise properties should be
close if they're the same types.

The issue was John W's observation that the difference between the
noise of two zeners has better stats than the noise of just one, or
the noise of the sum. Obviously, either the sum or the difference will
be better than the noise of just one, but I think he's right in that
the diff is better than the sum, since zener noise tends to be a bit
asymmetric, and always in the same direction.

John
 
K

Ken Smith

Jan 1, 1970
0
John Larkin said:
The issue was John W's observation that the difference between the
noise of two zeners has better stats than the noise of just one, or
the noise of the sum. Obviously, either the sum or the difference will
be better than the noise of just one, but I think he's right in that
the diff is better than the sum, since zener noise tends to be a bit
asymmetric, and always in the same direction.

Consider this:

If the low frequency components modulate the amplitude of the high
frequency components, the PSD will still look flat but the noise isn't as
random as one would like.
 
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