K3PGP receiver for 481THz

One of the simplest, yet highly sensitive, receivers for optical communications is one designed by John K3PGP. The G8CYW design in Practical Wireless in the March and April 2013 editions is based on this. This works really well in darkness, but is easily overloaded in any light. Many circuits are optimised for speech communications but this one works really well with digital modulation at very low frequencies where the detector sensitivity is highest.

http://k3pgp.org/Construction/Frontend/preamp.gif

Today I've been building a version and I am in the process of optimising it for subcarrier frequencies below 500Hz. I have added an extra transistor gain stage after the basic K3PGP design but want to see if lower noise FETs and transistors make a worthwhile improvement. When satisfied that it is working optimally I shall be trying this at the RX end of my over-the-horizon tests. I am quite excited about trying much lower frequencies in the next test using QRSS3 and continuous carrier.