Experiments in Laser Comms DX Notebook
Laser Diode Module modifications to improve frequency response:
After constructing the Soundcard to Laser Diode Module interface I did some measurements by illuminating the photodiode receiver with the Laser Diode Module optical output. What I found was an almost instantaneous switch-off (relatively speaking) but the turn-on behaviour was characterised by a fixed turn-on delay. This had the effect of decreasing the duty-cycle as the frequency was increased until, at about 2000Hz, the turn-on delay is longer than the driving turn-on time. Result - no output.
Measuring the current profile showed minimal current during this turn-on delay time. This prompted me to try connecting a resistor from the output of the comparator to ground to stop the LD module current to drop to zero during the "off period". This allowed the drive at 1000Hz to produce a 50% duty-cycle optical output.
I was unhappy with this because it would be necessary to juggle the effects of the better duty-cycle with the fact that the optical output doesn't drop to zero during the "off time".
I decided to be brave and tried to get rid of the turn-on delay by removing C1 (1uF chip cap) from the regulator side of the module PCB near the negative (-) spring terminal.
Note that the negative lead is not soldered to the battery spring as the connection from the spring goes through the unsupplied switch (SW1). Rather it is soldered to the pad next to the spring on the switch side of the PCB.
After removing C1 carefully, I re-tested the module for frequency response. The turn-on delay was eliminated and testing with a 20kHz drive signal from the Soundcard through the comparator interface showed a 50% duty-cycle (with no bias resistor).
NOTE: The modification shown here is done entirely at your own risk. If you don't want to risk damaging a module - don't attempt the modification.
The value of C1 is approx. 0.8uF. I determined this by measurement. I hope you appreciate this piece of information as it was obtained with great difficulty. Picture measuring the tiny chip cap with two leads made from resistor leads with the aid of 50 year old eyesight. The measurement endeavour involved 10 minutes searching (by touch) for the little chip cap in the carpet on the floor when a slip and the springiness of the leads catapulted the chip cap off the bench.
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