RECEIVE/TRANSMIT MIXER
RX/TX SCHEMATIC
The same diode ring mixer and post mixer
amp are used for transmitting and receiving. A pair of miniature realys are used
for TX/RX switching. I use a pair of National SPDT relays. These relays are
almost completely silent when changing over. Avoid using noisy relays,
especially for VOX operation. I use four standard silicon switching diodes
1N4148, 1N914 etc. I spent some time with a fistful of diodes and a multimeter
to select four that were closely matched for forward resistance. Use 8 turns
trifilar wound on a medium to high permeability toroid core, for the
transformers in the mixer. The mixer I.F. port is terminated by a 3dB
attenuator. The post mixer amp is a 2SC784. I intended to use a 2N3866 but I had
none left in the junk box. Use 8 turns bifilar wound for the output transformer.
Have a look at the 80M
Transceiver for more tips on making diode DBM's.
RX/TX MIXER
As you can see in the picture, the mixer is
built dead bug style, on a piece of circuit board. I was planning to etch a
proper PCB but the circuit works so well, I decided to use the rough looking
prototype.
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