Monday, 25 May 2015

A transceiver for not a lot

    I've just built a transceiver for the 4M amateur band. QRP(low power) and AM, it's not going to be something I'll work DX with, or indeed given my rural location not something I'll work *anyone* with, but I've had a lot of fun making it.
    My starting point was a 2M QRP AM transceiver courtesy of G3XBM, the 'Fredbox'. The receiver is a FET super regenerative design with an RF amplifier to stop unwanted radiation, and the transmitter is a very simple series modulated PA giving <100mW. I used 2SJ310 FETs in the receiver instead of MPF102s, and 2N3904s throughout in the transmitter. Everything else came from my hoard of electronic bits. I can't claim any astounding electronic design innovations as I was following someone else's proven design with a few adaptions when it came to winding coils and a few minor tweaks to bias resistors to get the desired voltage.
    The only significant difference from the G3XBM design is the lack of a crystal. Instead I'm using a Raspberry Pi as my frequency generator, running Jan Panteltje's freq_pi software. This controls the Pi SoC's clock generator, and can produce anything from the low kHz to 250MHz. Quite happy working on 70.260MHz.
    So there we are. A home made transceiver costing not a lot, and for not a lot of construction time. Rather more satisfying than a twenty quid BaoFeng handheld, I think.