Some noise measurements for LEDs and zener diodes

[hjelm]

Thanks Christer
Sorry if i was too lazy to look it up myself.

So there is an actual gain in using more than one LED to have the same voltage as a Zener even though you add the noises (geometrically).

Ex. 5.6vzener @ 20mA ~1.6uV

5xIRleds@ 20mA sqrt(5)*0.24 ~0.536uV

9dB less noise

I tried previously to find any kind of information on zener noise and the only thing i found was the cryptographic people using it as random generators or trying to get it as broadband as possible. Thanks again!

[Christer]

Originally posted by hjelm
Thanks Christer
Sorry if i was too lazy to look it up myself.

So there is an actual gain in using more than one LED to have the same voltage as a Zener even though you add the noises (geometrically).

Ex. 5.6vzener @ 20mA ~1.6uV

5xIRleds@ 20mA sqrt(5)*0.24 ~0.536uV

9dB less noise

I tried previously to find any kind of information on zener noise and the only thing i found was the cryptographic people using it as random generators or trying to get it as broadband as possible. Thanks again!

In many cases it seems it would be better to use a string of
LEDs. However, note that higher voltage zeners seem to have
a low noise, so it wouldn't pay off to replace a 12V zener
with LEDs it seems. OTOH these measurements are on a too
small scale to draw any statistical conclusions. I am currently
measuring som old LEDs from the 70's which seem to show
horrible performance, so more LEDs should be tested. Any
volunteers?

[Christer]

I've just measured some old 1970's LEDs from my junkbox
and the results are very different from the brand new ones
I measured before. I don't know if it is that LEDs in general
has changed a lot over the years or if it can actually differ
this much between types/brands/batches?

Attaching measurements for these.

[hjelm]

Would there be any point in anyone else measuring something?

Without calibrating your setup anyone else's results would not be comparable to yours.

They would however be showing relative differences between the types the second measurer is measuring so maybe not a waste anyway.

I think there is a possibility that the LEDs of other colors than red had lots of problems in the 70's, seem to remember they were harder to make.

Assume that the noisy red ones are degrading due to old age and that the green and yellow ones are inferior due to worse manufacturing methods back then.

[Christer]

Originally posted by hjelm
Would there be any point in anyone else measuring something?

Without calibrating your setup anyone else's results would not be comparable to yours.

They would however be showing relative differences between the types the second measurer is measuring so maybe not a waste anyway.

That was the point, although I wouldn't say my setup is
entirely uncalibrated. The major uncertainty factors are
the bandwidth of the soundcard (and probably the ADC)
and that I measured the level of the calibration signal with
my scope which has never been calibrated. It is not entirely
off, though. I would think the scope is at most some 5% off,

I think there is a possibility that the LEDs of other colors than red had lots of problems in the 70's, seem to remember they were harder to make.

Assume that the noisy red ones are degrading due to old age and that the green and yellow ones are inferior due to worse manufacturing methods back then.

At least the manufacturing techniques has progessed. There
were nu blue LEDs at all then. There is also a risk that some
of my old LEDs have been treated badly by me, but hardly all
of them. Anyway, I have just come back from Elfa with some
more types of LEDs and zeners, so when I get time to do some
measurements I will post.


If someone else would be interested to contribute with
experiments, there are probably many things that could be
done. For instance, buying a larger number of the same
device, but at different times and/or from different suppliers
to check the variation between devices. It would also be
interesting if someone with access to other brands of LEDs
and zeners tried to repeat my experiments to see if the
results correlate with mine.

[Circlotron]

How about doing a test on a laser from both a CD and a DVD player? And eventually a Blu-Ray laser?

It probably wouldn't have any broadband noise, just a single tone.

[jackinnj]

Jim Williams at Linear Tech has done several application notes dealing with the measurement of noise -- he uses a "Danish Butter Cookie" tin to isolate the device under test from the "world". It's really worth while perusing the application notes at Linear for the varied discussions of noise measurement.

Williams and others also have schematics for very low noise amplifiers and the high pass and low pass filters to bandwidth limit the measurements.

I doubt that you will find it "used" in Europe, but some of the Tek Analog scopes can measure down to a few uV when equipped with the 7A22N or 5A22N differential amplifiers -- I have a 5A22 in a TEK 5103 and it really works well -- they practically give away these units in the US, but shipping to Europe would be a fortune.

Batteries have noise too!

[till]

may there be a reason why Nelson Pass always uses 9,1V Zehners? Are they least noisy?


I desperately search for one of those 7A22, they are really expensive here.

[peranders]

Originally posted by jackinnj
Jim Williams at Linear Tech has done several application notes dealing with the measurement of noise -- he uses a "Danish Butter Cookie" tin to isolate the device under test from the "world".

You could use "Marabou Kexchoklad" tin but they has gone over to plastic.

Now you can use "Fazer Geisha" tin, more tasty also....

Curious: Do you americans have Marabou chocolate over there? Daim, Japp, etc?

[sam9]

Christer,

Did you measure 1N4148? How does a sting of them compare to an LED of roughly equivalent voltage drop? I ask since 1N4148 is sort of the "default" for setting a voltage ref or biasing a CCS.