Monday, October 25, 2010

20m & 40m Dual Band Digimode-TRX

The 2048kHz crystal appearently got some potential, more than I initially thought.
In my previous entry, I motivated digimode superhets for 40m and 20m. The 40m version would be build around a 5.000MHz filter, for the 20m version, a 10MHz filter will do the trick. As you may recall, the idea for 20m was to use a subharmonic mixer, as to have an effective local oscillator frequency of 4096kHz. Well, the same could be done for the BFO. The 40m version already employs a 5MHz BFO, which could be easily used for the 20m version, provided the product-detector is subharmonic too.

And here is the idea for the RX part:
  • one switchable front-end
  • one (super-)VXO for 2048kHz, maybe using an additional penned down one, lets say for 2035kHz
  • one BFO for 5000kHz, slightly pullable, as usual
  • two mixers, one for regular mixing, one for subharmonic mixing
  • two IF circuits having crystal (ladder) filters, one for 5.000MHz, one for 10.000MHz
  • two product-detectors, one for regular mixing, one for subharmonic mixing
  • one audio stage
At first sight, this looks like an expensive solution, however, I believe it actually is a cheap one, since crystals for 5.000 and 10.000MHz cost next to nothing. For that reason, one could consider building individual transmit filters too. So the TX part only need the following additional building blocks:
  • one input audio stage
  • two modulators, one for regular mixing, one for subharmonic mixing
  • two IF circuits having crystal (ladder) filters, one for 5.000MHz, one for 10.000MHz
  • two converter mixers, one for regular mixing, one for subharmonic mixing
  • one switchable linear amplifier
I would stronlgy recommend to use two crystals for the LO. One on the original 2048kHz, which allows for WSPR on 20m and DATA on 40m, and one that is penned down to about 2035kHz, that one will be good for WSPR on 40m and PSK31 on 20m.

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