[Laser] PWM of LED in QST

[Tom Becker]

  Equally obvious to me is the notion that an AM signal must exhibit 
variable power, lest there be no information conveyed by the mode.  If 
it is average LED power that is the medium (opposed to frequency or 
phase) then the power must change - and will be visible for the 
low-frequency signal components.  The LED power is, in this instance, 
changed by duty cycle.

Since this scheme uses duty cycle - the LED on:off ratio - as the medium 
to convey the instantaneous 10kHz-sampled input signal voltage, a signal 
that spends some time below zero, represented by a lower duty cycle, 
will appear momentarily dimmer - due to the retinal integration - during 
that period, and it will appear brighter for excursions above zero when 
the LED On period is greater, a higher duty cycle.  The eye's 
integration performs the same function as, I assume without seeing the 
schematic, the receiver's integration of the incoming variable-width 
light pulses, perhaps a simple a low-pass RC, to reconstruct the 
transmitted audio before amplification to a speaker.  This is the same 
effect as a motor, whose rotating mass integrates variable-width PWM 
pulses to produce some degree of instantaneous torque.

Can you build the circuit portion you are concerned with, to verify 
either your prediction or his reporting?  It does not sound complex and 
could be breadboarded easily, I suspect.

Tom
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