Monday, January 4, 2010

30m Subharmonic I/Q-SDR Receiver

As promised, some detail about the subharmonic I/Q-SDR receiver. The RX has got a preliminary front end, just for testing, therefore not worth showing. Anyway, some pre-amplification is not that hard to design.

The trick with this design is, to shift the phase by exactly 45 degrees. This is accomplished by the RC combination. Theoretically, a phase shift of 45 degrees is reached when the condition R=XC is fulfilled. I chose a 1k resistor, thus, the capacitor should be 15.6994...pF  32pF (see comments below). Ah well, let's add a trimmer...
For the I/Q-stuff functioning, the levels of both LOs need to be exactly the same, therefore, I added a voltage dividing trimmer to the In-phase path. This trimmer is 2k in order to balance the RC phase shifter.
The total level is adjusted by the trimmer between the oscillator and the filter crystal. This trimmer seems to have a tiny influence on the phase shift...

So, here you got it, the first sneak preview of the 30m I/Q-SDR stuff. See my previous posting for other possible frequencies, most will not have the convenience of a canned oscillator however.