Re: Are LEDs a noise source

Neil Smith G4DBN

Perhaps adding a piezo element and lightweight front-silvered mirror on a low-mass flexure into the path would give sufficient bandwidth for modulation into the kHz range so you could drive it directly with soundcard output from WSJT-X.

Neil G4DBN

On 24/01/2021 19:09, KENT BRITAIN wrote:
We mechanically modulated ours.  Much easier!  
Simplest was to just run the beam though the blades of a muffin fan. 
Turning the fan at about a 45 deg angle greatly improved the on to off ratio.  
With a more controlled motor and disk with a pattern of holes,   
 precise modulation tones can be produced.

On Sunday, January 24, 2021, 12:16:44 PM CST, Robin Szemeti - G1YFG <robin@...> wrote:

It is *quite bright*, sets fire to the shed door ... pulls 30 amps from the mains, watercooled.  The usual lamp-pumped YAG laser with a ktp, so not a gas laser. This one is q switched to improve the efficiency of the ktp frequency double The Q-switch is fed with RF at about 100MHz, which can be gated with TTL logic.  I'll have to experiment with pulse rates one day.

On Sun, 24 Jan 2021 at 18:05, Andy G4JNT <andy.g4jnt@...> wrote:
Ah, didn't read the footnote.  It's not a SS laser :-)

80W Green !
Don't shine that at any aircraft

On Sun, 24 Jan 2021 at 18:03, Andy Talbot <andy.g4jnt@...> wrote:
You should be able to modulate a solid state laser at many tens of kHz,   More than enough to use with all the MFSK modes as a subcarrier

On Sun, 24 Jan 2021 at 17:52, Robin Szemeti - G1YFG <robin@...> wrote:
I have a small green laser [1] that can be pulsed by controlling the Q-switch, I would think tones of a few hundred hertz would be possible ... I've been trying to find a sensible use for it for years ...

[1] 80W

On Sun, 24 Jan 2021 at 17:02, Neil Smith G4DBN <neil@...> wrote:
Back on to the original topic for a second, there are some articles
showing what appears to a forward-biased LED used as a noise source. The
author in this piece says the noise was about 6dB down on a zener in the
same topology (two-transistor amplifier).

I did wonder if the LED was drawn incorrectly and it was actually being
used in reverse breakdown.  The diagram towards the end of the page
definitely shows it drawn in the forward conduction direction.

LEDs do show some rising levels of shot noise and 1/f noise below 10
kHz, but that is at low levels. However, at high currents, the authors
found a correlation between the LF noise and the brightness fluctuations
of the LED in that frequency range.

There is another article using a totally different topology where a LED
and photodiode are used to generate noise with 7dB ENR up to about 100
MHz, but it hits the usual 1/f noise issues.

I've done this at extremely low light levels to make a "Quantum" random
pulse generator, but that's well away from the OP's question.

Neil G4DBN

Robin Szemeti - G1YFG

Robin Szemeti - G1YFG

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