Thursday, 24 September 2015

Airband receiver kit complete - first thoughts

So, on arriving home I got out to the workshop and cracked on with the Chinese Airband radio. I had only intended doing the electrolytics and the connectors, but it took no time at all to solder those in, so I got on with the diodes, then the transistors and the 78L08 regulator. This done I tested it with 12v but without installing the ICs, and checked I had an 8v DC rail.

All was well, so I depowered and installed the ICs. The kit as supplied has a3.5mm jack socket audio output, so I tag soldered a loudspeaker across it for testing. Power applied, I was relieved to find that I had noise from the speaker, and that it stopped when I adjusted the squelch control.

With it connected to my Marconi 2955B and being fed a -100dBm 125MHz AM signal (1kHz at 70%), a bit of tweaking and I located the signal. This was quite poor, but much improved after adjusting the IF transformer. Ive pretty much left the band pass filter alone for now, as my to hand trimming tools dont fit.

It isnt quite as sensitive as i'd hoped, 12dB SINAD being around -105dBm, but its not too bad. After a lot of fiddling I managed to adjust the VFO coil to put 118MHz at the extreme of the tuning control, although I couldnt go any lower as the coils slug was all the way out. The tuning is varicap controlled by a 10k pot, and the range is so wide (top end at least 136MHz) that in its current form finding a signal is very tricky. Several ways might solve this, I could modify the circuit for a smaller bandspread, or swap the control for a 10 turn precision pot and a turns counter dial. Or, I could replace the VFO with a DDS synthesiser!

Even as is, mooching about with the control and with a 1m telescopic antenna, it pulled in several aircraft comms signals, ranging from late evening long haul ATC traffic, to my local field with a request that 'as were the last landing you can turn off the lights'!

Next step is to get the frequency counter on the VFO and see exactly what the bandspread is. If need be i'll modify the circuit to reduce this. Ideally, I want to modify the VFO anyway to get coverage down to 108MHz.

But, all in all, for about £12 its a fun little project.

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