09 March 2009

Design ideas for a QRSS/WSPR 30m TX/RTX

The presence of a strong local QRSS 30m beacon nulls out my chances of successful DX in that portion of the band. So to enjoy these narrowband techniques, including WSPR, I started thinking of a homebrew transmitter.

WSPR support requires:
  • SSB-like modulation
  • linear RF amplification
  • stability
but the design is simplified by the fact that:
  • output sits in max 300 Hz (threehundred hertz)
  • no need for >0.5 W for satisfactory results
Given the above, combined with the fact that I have a 40.5504 MHz canned oscillator (10.1375 MHz * 4), I imagined three solution as depicted in the following picture:

Option 1 should be an SDR transmitter. Not much different in the TX chain from a simpler NE602 Gilbert cell mixer, but has an intrinsic bidirectionality that allows building a transceiver without duplicating stuff. The single XTAL filter removes the unwanted sideband (both on option 1 and 3).
Option 2 should generate an USB signal but requires a I+Q WSPR signal, that should be possible to produce software-wise.

Ah, one more thing: driving a QRPp transmitter with a QRO computer is complete nonsense. I would record WSJT signal in a WAVe file and send it to the TX with a simple mp3 player.