SDR using 4x the qrg. Here, we a got crystal from Nick, 28.188MHz, resulting in an SDR center frequency of 7.047MHz. Alternatively the canned 28.322MHz-oscillators provide a center frequency of 7.0805MHz.
SDR using half the receive frequency, phase shifting of 90 degrees done by an RC circuit. A crystal is available: 3.535MHz (from Rich). This results in an SDR center frequency of 7.070MHz.
And finally, there is a crystal available, from box73.de, having a frequency of 3.530MHz. This is too close for SDR, but allow for a subharmonic direct conversion receiver, more or less in the same way as I use it for 30m. The only drawback here, there is not 7.060MHz crystal yet, hence, no easy way for a sideband filter. However, there are crystals for 7.055MHz (Rich, box73.de) and 7.058MHz (Rich), which could serve as a notch filter, just like in Gene Marcus 30m WSPR transceiver.
