Experiments in Laser Comms DX Notebook

08/01/00: Soundcard to Laser Diode Module interface:

In order to carry out experiments with IFK, AFK and FDK modulation techniques I needed a way to drive the Laser Diode Module from the output of the Soundcard.  The LD module from Oatley Electronics (designated LM-2) runs from three series-connected 1.5V batteries.   The drive I use for the LD module is 5V.  The onboard LD module regulator should be able to handle the extra 500mV above 4.5V.   With a fixed swing of 5V p-p a square wave drive entails no penalty and is simple as well.   The circuit below shows the interface:-

In my installation the input is driven from the Line Out socket on the Soundcard.  This is because the Speaker Out socket is used for (you guessed it) speaker output.  If you drive the circuit from the Speaker Out socket then you might try replacing the 3K:3K miniature audio transformer with 3K:8ohm speaker audio transformer with the 8ohm winding being connected to the Soundcard Speaker Out.  In any case set up the drive to give about 1V p-p into the comparator.

The 1Kohm variable resistor is used to set the offset into the comparator such that the output of the comparator is high (+5V) when there is no signal going into the comparator.   This ensures the the LD module has zero volts across it (and therefore is off) under no-signal conditions.

This circuit with the Oatley Electronics module allows a drive signal of 320Hz to produce an approx. 50% optical output.    Higher frequencies result in an ever decreasing duty-cycle until there is no pulse output at about 2000Hz.   Look at the modifications I have done to extend the frequency range of the modulation signal that can be applied to the module.

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