Optical receiver noise floor measurements

A PC with some simple free software makes a very useful piece of test gear.

For example, today I wanted to check the noise floor of my optical receiver head between 0-20kHz in total darkness and in daylight and compare this with the noisefloor of the PC with the head turned off. SM6LKM's excellent little software receiver designed to listen to SAQ's transmissions on 17.2kHz was ideal. This tunes from 0-22kHz. Here are the results (see plots) which show noise goes up by about 5dB in the daylight, although noise at 0-2kHz seems to go down in sunlight. By connecting the optical receiver head to the PC I was able to use this little package to see the noise floor changes.  G4JNT thinks the FET is possibly being biassed to a less optimum position, or it is saturating, in daylight. By changing the bias I may be able to get a lower noisefloor and better sensitivity. That I shall try tomorrow.