Thursday, 30 January 2014

10m Wispy Receive chain started

This part of the wispy is pretty low on component count, which is just as well as I only have about four square inches of space to build it in.

Starting with the grounded gate RF amplifier and the Polyakov mixer, straight away I get hit with a small problem. The design specifies an MPF102 JFET, but I dont have any. I do have plenty of J310s and 2N3819s. Although I have asked for a confidence check on my decision from the members of G-QRP club, I went ahead anyway and selected a J310. Both the MPF102 and the J310 are listed as VHF devices, so hopefully all will be well here.

The RF amp and mixer occupy the left hand side of the receive chain space of the board. This allows the LO signal to be fed direct from the oscillator without any extended wiring. It does mean however that the antenna RF signal to the receiver now has to come a bit further! The toroid on the lower left and the trimmer cap on the right (next to the LPF toroid) form the front end filter, and somewhere along that line a pair of back to back silicon diodes perform the job of antenna switching. These will probably be located close to the trimmer cap.

The next trick is to turn my brain upside down, in order to be able to think my way around building the Rx audio pre-amp and power amps with their power rails to the lower edge of the PCB and their grounds towards the middle!.

I have stopped work for today though, as its getting bloody cold!

I have also realised that I really need a way to measure RF power down to a few tens of mW, and up to a watt or two, for testing these projects. So a QRP power meter looks to be on the bench for tomorrow, so long as I can find a suitable design. Yes, I know Sprat is full of them, but the only meter I have spare thats not the size of something you'd expect to see in a 1920s Frankenstein film, is a little 250uA movement, so I need a circuit that will work with that.

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