Old noise generator

[Tim Williams]

This is an Elgenco 331A gaussian noise generator. I know very little about
it, though I did see an ad in an old journal proclaiming it's good to four
sigmas. It seems to be roughly "audio frequency", by which I mean, the
circuits are audio quality, no particular attention paid to high or low
frequency bandwidth (AC coupled, no peaking coils). I would guess 10Hz to
200kHz or so.

Front view. Some old rackmount thing:
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise1.jpg
So I'm guessing Beloit got it from NASA surplus or something. I found this
thing on the curb one day outside of a house near the college, which I'm
assuming is the residence of someone interested in Beloit physics...

Other side of the faceplate:
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise2.jpg

Behind the faceplate, the noise generator and amplifiers:
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise3.jpg
On the far right, two 6D4 gas thyratron triodes are set in magnetic fields,
generating noise per the datasheet. They look to be connected like any old
gas tube, with coupling capacitors going to the rest of the circuit. I'm
going to guess they use two and mix them somewhere (I haven't traced the
circuit yet), to make it extra randomized or something.

Signal leaves through the coax in the top-right corner, which goes to the
output stage on the left (coupled through that big fat 1.0uF). I think it's
a 12AU6 cathode follower.

On the far left, two PCBs hold some germanium transistors (typical date
codes are 1963), the TO-39's are 2N398As, while the TO-1's are something
rather fancy (I forget the number, but they're epitaxial mesa germaniums
with almost 100MHz fT). I don't really know what to make of it, but it must
be in the signal path, why else use such fast transistors.

PCB closeups:
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise4.jpg
Gotta love that clear glass body RN60B!

The power supply hangs in the back of the rackmount chassis (which must be
annoying for the moment arm...)
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise5.jpg
Two potted transformers supply everything, I think one is for heaters, the
other for HV, and the thermal relay (bottom right) delays HV by a minute or
whatever. Rectifiers are all silicon (I guess).

A 6BX7 and associated tubes (12AX7, and the other one may be another 12AX7)
regulate high voltage. The two PCBs and heatsink appear to regulate 12.6V
for heaters. The TO-3's are all 30V, 1-5A germaniums. The two pots say
they're for + and - 156V, I guess one or both go to the tube regulator part.
Either that or some of the TO-39's are HV germaniums, I haven't checked.

Rear view:
http://myweb.msoe.edu/williamstm/Images/Gaussian_Noise6.jpg
Filament supply seems to be the bottom transformer, feeding a press-mounted
FWB and computer-grade cap.

Tim

 

[Tim Williams]

Yikes, that must have cost a fortune, back when.

The gas tubes tended to make asymmetric (not prefectly Gaussian)
noise, so people subtracted the output of two tubes. Nowadays,
pseudo-random is easier.

Ahh. Say, that reminds me: I once ran a zener at fairly low current, and
saw a jagged waveform, as if the junction (which is avalanching, since this
was like 12V or so) has a randomly variable threshold and hysteresis band.
Say, gas discharges are avalanche, too -- same physical phenomenon? I can
see subtraction having some use there.

Hey, we have clouds too.

Ah, snooping around my directory? Well, since I'm on a higher floor at
school than I've even lived on before (which still isn't much, this is
Wisconsin after all), I can get a good eye on the weather. Last month there
was a span of two solid weeks of that sort of inland moisture coming up off
the lake. Bleh. But you'd be no stranger to that in SF, I've seen your fog
before.
http://myweb.msoe.edu/williamstm/Images/Cloudy.jpg

Most of the time, it's average midwestern weather. West coast of the lake
doesn't get much lake effect.
http://myweb.msoe.edu/williamstm/Images/Panorama2sm.jpg

The animated GIF,
http://myweb.msoe.edu/williamstm/Images/Clouds.gif
I took each frame about 15 seconds apart. Really weird, I've never seen a
ribbon of cloud moving out to 'sea' before. Fairly straight edge, a few
thousand feet long, and moving at a pretty good clip. The edge was, ohh I'd
guess a hundred feet tall, and rolling back fairly well (in most frames, you
can see the edge curling over).

http://myweb.msoe.edu/williamstm/Images/Rainbow1.jpg

Milwaukee's three tits,
http://myweb.msoe.edu/williamstm/Images/Milwaukee_Domes.jpg

Tallest building in Wisconsin:
http://myweb.msoe.edu/williamstm/Images/US_Bank.jpg

http://myweb.msoe.edu/williamstm/Images/Moon.jpg

I took these around Beloit College,
http://myweb.msoe.edu/williamstm/Images/SkyFire.jpg

http://myweb.msoe.edu/williamstm/Images/MorningRain.jpg

The joys of winter,
http://myweb.msoe.edu/williamstm/Images/Icy1_lg.JPG

http://myweb.msoe.edu/williamstm/Images/Icy3.jpg

Tim

 

[Tim Williams]

I spent some time in Madison, mostly summer, and it was nice, drinking
beer in the student union on the lake. Lots of good coffeehouses and
restaurants and such. Great Farmers' Market outside the capital
building on Saturdays.

Yep, lots of good food, good beer and most of all, great cheese. ;-)

I helped this thing

http://www.imago.com/imago/

get started, a spinout of the ME department at UW.

Interesting. So, they put a big stinking pulse into it, which rips off and
ionizes exactly the surface atoms, which are then detected in time and
space, I suppose with microchannel plates?

Hmmm, it's almost time for pecan pie.

Ahh, we didn't have any. But we did have pumpkin chocolate chip cheesecake,
which is also quite delectable.

Tim

 

[Tim Williams]

Sorry. I thought this was a thread about our resident right wing nuts.

Never mind.

Ah, good point. They're a little slow for this application, but you might
tempt some cryptologists into downloading their entropy. Run the archives
through a hash function and you've got a lovely source of random numbers.

Tim

 

[Joop]

Yup, a regular zener like a 1N758 will make pretty good gausian noise
at several mA bias, ballpark 300 nV/rootHz. As the current drops, the
noise starts to get asymmetric and at low currents turns into a very
noisy sawtooth. Subtracting two diode signals would be good here, too,
if extreme noise quality were needed.

Would adding do something similar?
One example I found on the net suggest 24V zeners with a current
source of about 20mA. However I have plenty of 9.1V/500mW smd ones in
the cupboard that I could use in series.

Joop

 

[Tim Williams]

Would adding do something similar?
One example I found on the net suggest 24V zeners with a current
source of about 20mA. However I have plenty of 9.1V/500mW smd ones in
the cupboard that I could use in series.

They're fine at a few mA. 9.1V might be a little low; I forget how the
noise of zener effect compares to avalanche. Generally, the zener effect
ends around 6V (the crossover point having an interestingly low tempco right
around 6.2-6.8V), so >6V zeners are actually avalanche diodes.

Tim

 

[Joop]

The Central Limit Theorem says that summing a number of imperfect
noise sources trends towards Gaussian. I don't know if putting zeners
in series is equivalent to summing noise sources, or if three 9 volt
zeners in series behave like a single 27. I doubt it.

Just because something is on the net doesn't make it sensible. 9.1
volt zeners, biased to maybe 5 mA, make nice noise. Sum a few or
better yet sum and difference an even number of them... that's
probably a faster approach to good gaussian noise than straight
summing.

Summing does push the probability distribution towards gaussian, but
it doesn't much improve spectral flatness.

John

I think it was only mentioned that the 24V zener produced more noise
power than a 9V one. On flatness the different voltages were not
compared. The current had some influence on flatness though.
I guess I will just experiment and see what it brings.

Thanks for responding.

Joop

 

[Tim Williams]

How high in frequency does the noise extend with reasonable flatness,
John?

Offhand, I once hooked up a 1N4742 at around 5mA and 'counted' the
frequency. It was somewhere in the 160-240kHz range, so I guess bandwidth
won't be far past there. Zeners have pretty fat junctions, so you can't
expect much. Low wattage zeners might have some RF performance.

Tim

 

[krw]

I'm afraid not. The string "leftist weenies" keeps popping up
repeatedly.

I didn't know you were a string.

 

[Baron]

Photocurrent shot noise. Get a 2.5 Gb/s PD off eBay and shoot a
milliwatt or two of light into it. The photocurrent noise is given
by the shot noise formula, so you know it a priori.

Cheers

Phil Hobbs

Hey Phil, how do leds compare as noise sources ?

 

[GregS]

This is an Elgenco 331A gaussian noise generator. I know very little about
it, though I did see an ad in an old journal proclaiming it's good to four
sigmas. It seems to be roughly "audio frequency", by which I mean, the
circuits are audio quality, no particular attention paid to high or low
frequency bandwidth (AC coupled, no peaking coils). I would guess 10Hz to
200kHz or so.

I think I have a newer solid state unit. Glad. Mine has selectable BW.

greg

 

[Joop]

If you do get any data on zener noise for various diodes at different
currents, please post it.

John

Ok, I'll try and collect some.
It will not be in the next days though.

Joop