Idea Box: Single Band SDR Receiver With Old SDR Kits.

 

I have some old "single XO" SDR kits from Germany. I think I have some Softrock kits somewhere, too, they use discrete crystal oscillators.

As indicated they have a canned (DIP/DIL) crystal oscillator running on 4x the RX operating frequency, and the quadrature signals are generated using a 74AC74 dual flip-flop as divider/phasing, providing quadrature local oscillator signals on the receiver boards. 

The receivers cover only one small frequency range around the LO frequency, so with a 48kHz sound card interface the max bandwidth covered is a bit less than 48kHz. Sound cards with higher sampling frequencies can, of course provide more coverage, but the 48kHz sampling frequency is now ubuquitous in computer sound systems.

Here is the idea. Not particularly complicated or original, but this could make better use of those old kits.

My intention is to replace the fixed XO with a Si5351 module controlled by Arduino. This could be programmed in, say 10kHz steps for the LO, or indeed for a single frequency. There are several Arduino programs for making a VFO with the Si5351, so I should be able to modify the program to suit this purpose. 

I have one finished kit of this kind, and a few (2 or 3) not yet assembled, so I should be able to make simple SDR system for rarely changed frequencies, such as beacon frequencies or digital mode frequencies. All for monitoring purposes.

In addition, if all the wanted frequencies are within 45kHz, the system could be used as-is for a few frequencies, branching out the "stereo" I/Q outputs to more than one sound card/computer for dual monitoring, or alternatively, use a local webSDR server. 

I think that the splitting of I/Q signals is the simplest solution to this, though.

The single frequency can be generated by a suitable XO, if that cannot be found, the Si5351/Arduino can be used to generate a single frequency.

I do have a finished kit for 80m, and I will check if I have a suitable XO for covering the FT8, JS8, WSPR/QRSS frequencies. It does look like a 14318kHz XO is usable for covering these frequencies with the 48kHz sampling frequency, although JS8 could be a bit tricky.