Hence, I grabbed issue 6/2009 of the "Funkamateur" and was looking into the 455kHz mod of the SDR receiver.
This is what I have done so far:
- replaced the 14.000Mhz oscillator by a 1.843MHz oscillator
- replaced the tank coil with a 4k7 resistor
This results is no pre-selection at all. OK, a test anyway.
The official mod also replaces sampling capacitors and all inductors for improved performance.
Wired up to the BB6W clone (7m wire + 9:1 balun) and spectrumlab tweak a bit, this is a first snapshot. Not only does this show two WSPR signals, the faint drifting signal at about 502740Hz is the exciter operating at the ground floor of my house, with 30cm of wire at the 7493 Q2 pin. As assumed, a ceramic resonator drifts, and will even spoil the fun when the frequency is divided by 8. (see image below)
Next step, something selective in the front-end, maybe replacing the sampling capacitors...