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- this is what a typical PMT tube (41k) looks like without the base and resistor ladder network
- front view (125k) of an experimental photomultiplier detector/receiver for chopped and also linearly modulated lasers
- here is a rear view (123k) of the detector/receiver with BNC outputs and DC input connectors
- a spectral plot of a fairly weak chopper modulated laser carrier (3k) taken with R.S. Horne's nice software
- typical audio of backscattered 200 kHz SSB subcarrier (214k) signal on my 50 mW SSB/FM/CW 650 nm laser transmitter
- here you can listen to a much stronger (136k) SSB subcarrier reflected off some foliage about half a kilometer away.
- a spectrogram of an 940 Hz CW carrier (93k) being reflected off the same foliage (no wind, calm conditions !).
- this is what the signal sounds like (1509k) with no filtering and no audio compression (i.e. big file !)
- using the Datong FL2 tunable CW/SSB filter/notch improves the S/N ratio considerably !
The receiver has a built in regulator to supply the required +12 V, -12 V and +5 V voltages. The gain can be controlled from the front panel via a pot that fine adjusts the PMT HV supply voltage. A built in high fidelity amplifier allows the field use of headphones and also can be used to feed any suitable PC sound card FFT analysis software. There are also buffered broadband 50 ohm test outputs, one DC coupled and the other AC coupled. These are very useful for connecting a scope or FFT analyser to. Usually this is where I connect my Drake R7A receiver for tuning in the SSB/CW or FM subcarriers I experiment with, typically somewhere around 200 kHz. The complete receiver can be operated off any handy power supply that gives 8 ... 16 V DC. The front panel DC microampmeter gives an indication of ambient light level and helps prevent overdriving the PMT tube in normal use. This receiver can only be operated during very dark conditions at the moment. I am still trying to locate a suitable 630 - 650 nm optical filter (my lasers are red semiconductor types or HeNe types). If I lay hands on hundreds of IR LED's one day, I might comtemplate building a modulated IF array to experiment with.
