Friday, 19 June 2009
Yet another HF superhet rig
I like doing unusual radios, so when I decided to produce yet another HF superhet, I felt compelled to give it a twist.
The current project uses two SA602 mixer/oscillators, and a DPDT relay to swap the signal path around to take the rig from receive to transmit. I used the fortuitous layout of the SA602's inputs and outputs, along with the equally serendipitous pinout of the miniature relay to form a very compact and efficient SSB 'modem'.
So far, I've managed to get it running as a receiver, and added an AGC circuit to soften the blow when a strong signal is encountered. The AGC is pretty standard; just a charge pump and capacitor driving an NPN transistor, but the control devices are unusual. I've put a pair of red LEDs in a stack from the +12V line down to the AGC transistor, and coupled them into the RF input to the first mixer. As well as smoothly attenuating the signal, I get a pair of signal-strength LEDs as a bonus. Two birds, one stone (well, two bits of silicon).
I've housed it in a pair of 2oz tobacco tins, mounting the circuitry in the lids, which I've soldered together. This makes it compact and lightweight; ideal for backpacking. The photo shows an early incarnation, before I boxed it. The two mixers, the relay and the three-crystal filter are at the top. The fourth crystal is for the BFO / carrier, and the 12MHz ceramic resonator at left is for the 4.915MHz / 7.1MHz conversion. This choice of IF meant I could terminate the filter directly to the 1500R ports of the SA602s, and still maintain great passband shape even with just three crystals.