I have been taking a look at the information on Linrad and other developments in software defined radio (SDR) and wondered if anyone has experimented with producing data suiitable for DSP using a combination of an analog/digital converter and digital down converter.
Leif's QEX articles mention a couple of devices and I was thinking of putting together the following:
1. 14 Mhz analog IF input from 144 - 14Mhz transverter
2. Analog to digital conversion using an AD6644 or similar sampling at 65Mhz
3. Digital down converter (HSP50016, GC4016 etc.) to shift from 14Mhz down to 0Hz and produce complex (I and Q) data. Local oscillator in DDC provides frequency coverage e.g. 14.00 - 14.500Mhz. The DDC decimation rate determines the output bandwidth - for example 100kHz for monitoring and down to 20Hz for CW.
4. Complex (I and Q) data used as input to Linrad - maybe using USB to move the data from the DDC into the PC.
5. DDC control from PC based application.
There seem to be a number of commercial boards available that follow this approach but they are all very expensive. It may be a bit daunting for an amateur to produce the PCBs for this type of system but I don't think it is impossible.
I would be happy to discuss this with anyone.
73s Niels, G8RWG