Some noise measurements for LEDs and zener diodes

[Christer]

There has been many discussions on the forum over the years
about how noisy various voltage references are. I can't really
remember anyone pointing to any measurements or satisfactory
theoretical arguments, but maybe it is my memory being bad.
It seems to be generally agreed (maybe) that LEDs are less
noisy then zener diodes, but I don't think anybody has tried
to put figures on the difference. There has also been discussions
about what LEDs are lesat noisy. The only claim I can remember
reading on the forum is that green LEDs are the least noisy,
but no argument was provided to back it up. Hence, I decided
to see it it was possible to get some answers to these questions
using only some simple DIY test equipment. I don't have the
time or money to do a proper scientific study, but maybe this
can be considered a somewhat systematic pilot study for
further investigations into this matter. For now I have done
some measurements on various types of LEDs and zener
diodes and some of the results were quite unexpected and
begging for further studies.

i didn't do this just because I had nothing better to do, because
I had, but since I like the forum and it members so much I
decided to put some time and money into doing this study
(OK, I admit I also did it to still my scientific curiosity ).

The study and the measurements so far are described in
the attached document. Please note that the study is too
small and uses too questionable equipment to be considered
properly scientific, so no hard claims or conclusions can be
drawn from it. There are hints at some interesting phenomea
though. Had I know beforehand what the results would be,
I would have used a slightly different selection of components
for study, but thats usually how it is.

[Sch3mat1c]

Very nice, I'll have to save this for future reference.

[Christer]

And here is a schematic of the test rig. Sorry for the bad
quality. I had to compress it heavily to be able to attach it.

[tool49]

Excellent work Christer, very informative (even though you claim it is not scientific, I believe it to be very good work).

So if I'm not mistaken, the Infrared Led at 20ma is nearly as good as it gets! Wow I never tought I'd use a voltage reference in a CCS to heat my dinner

Thanks for your devotion to this forum!
Sébastien

[Christer]

I'll try to briefly summarize the conclusions one might perhaps
try to draw from the results:

The least noisy choice is an IR LED running at close to maximum
current.

Blue LEDS share the property with IR LEDs that they get less
noisy with larger current, but they are so noisy compared to
other LEDs that they are best avoided.

All other LEDs seem to have an optimal current for low noise.
however, don't draw the conclusion that this optimum is at
5mA just because I happened to use that as one of the test
currents. Further investigation is needed to find the optimum
current for various LEDs. The differences between colour in
the red to green area or for differenc currents are small though
and nothing to loose sleep over. Red LEDs at optimum current
seem the least noisy, though (next to IR LEDs at high current,
that is).

Zener diodes seem an altoghether different and more confusing
story. Below approximately 6V they have true zener breakdown
and above this they have avalanche breakdown. It was thus
obvious to test zeners close to and to each side of this
"transition region" and also zeneres far into each region.
Surprisingly, the important distinction seems not to be which
type of breakdown they exhibit, but how close to or far away
from the transition they are. Low volt and high volt zeners
turned out to have moderate noise and the high voltage
avalanche type could even rival most LEDs. Those diodes
close to the transition, on the other hand, turned out to
be noisy as h**l (maybe the electrons are confused and
don't know if they are doing a zener breakdown or an
avalanche one? ).

Zener diodes generally share with IR and blue LEDs that
the get less noisy with larger currents.

For high power zeneres vs. low power zeners, the high power
ones seem more noise in the zener region and less noisy in
the avalanche region. Since these diodes could take higher
currents, the noise could probably be brought down by
and rival the LEDs if going far above 20mA.


Please note that the study is much too small and has to many
error sources to really claim any of the above conclusions.

[Christer]

Originally posted by tool49
Excellent work Christer, very informative (even though you claim it is not scientific, I believe it to be very good work).

So if I'm not mistaken, the Infrared Led at 20ma is nearly as good as it gets! Wow I never tought I'd use a voltage reference in a CCS to heat my dinner

Thanks for your devotion to this forum!
Sébastien

Thanks for the kind words.

An added bonus with the IR LED is that you will avoid stupid
questions why you have put LEDs inside the box where nobody
can see them.

[Nisbeth]

No, people will just ask why you have non-functioning LEDs in your amplifier

/U.

PS: Nice work and interesting reading

[Christer]

Regarding the interesting results for zeners, i just made a
quick check with an 8.2V zener and it had reasonable noise
values of 1.1 uV, which should be at least as good as stringing
up a corresponding number of LEDs. Hence, it seems there
are mainly the zeners just around 6V that are terribly noisy.
i suspect it has something to do with these diodes
having a mixture of zener and avalanche breakdown or
something like that.

[Christer]

Originally posted by Nisbeth
No, people will just ask why you have non-functioning LEDs in your amplifier

Well, but most people won't even realise it is a LED since there
is no visible light coming from it. For them it is just another
strange component they don't understand.

[mandat]

The same colour LEDs have got a different kind of semiconductor inside. Have you got such information for tested LEDs.