[Laser] Re : PWM of LED in QST

[f1avyopto at aol.com]

If the ampltitude to pulse width convertion stays in a linear fonction, it seems to me the full range of 1/99 to 99/1 is possible but the PWM response of an optical receiver depends of its design.
On a very sensitive K3PGP RX or a transimpedance RX with its very high feed back resistor and with its small added capacity for stability, it is the photodiode capacitance that integrates the pulses train.
This effect is not linear and asymmetric because the capacitance is fast loaded each pulse, but this capacitance unload very more slowly versus the equivalent resistor viewed from the photodiode.
This produce a charge amplifying effect that give a very high gain on the VLF signal from the modulation envelope.
This gives a big audio flickering and increases so much the signal scintillation that then appears saturate "click" noises and cut periods.
It is not really a PWM problem because this phenomenon can be the same with a 100 % direct linear amplitude modulation on the Led or laser current.
To reduce this phenomenon one can reduce the load resistor of the photodiode in the RX, also reduce the inter-stage capacitors to reduce the VLF amplification and to reject the PWM frequency modulation one can filter with a dedicated stage.
Because the price to pay is a sensitivity loss, the best way to cancel this phenomenon could be to simply reduce the optical signal ?



-----E-mail d'origine-----
De : Tim Toast <toasty256 at yahoo.com>
A : laser at mailman.qth.net
Envoyé le : Mardi, 28 Septembre 2010 12:54
Sujet : Re: [Laser] PWM of LED in QST



I had read somewhere while looking at the PWM circuits recently that you should 
only use half of the total pulse width range when generating AM. So you wouldn't 
use the full range of 1/99 to 99/1 as you do when controling motors - Is that 
true?

In other words, with the strongest modulation, the pulse width might vary from 
duty-cycle 25/75 to 75/25. But it could also be set to use 10/90 to 60/40. Or it 
might use 40/60 to 90/10 and still have full modulation?

Would using the full (or almost) range of duty cycles cause some distortion of 
the AM detected in a receiver?



      
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