[hpsdr] I didn't think it would happen again "PING"

[Murray Lang]

At 10:04 PM 22/08/2006, jeff millar wrote:
>>BTW, I realised that the phase shifting can't be done at I/Q level 
>>because that's just the modulation.
>Not so fast. With most sampling schemes, anything can be I/Q. Another way 
>to say it, "meet the Nyquist rate and nothing is lost, all possibilities 
>remain".

OK. I can see that if all txs/rxs are fed from the same oscillator then the 
fact that any modulation is possible, including phase, suggests that it can 
be done.
I'd need to understand how a constant "DC" phase shift is represented in I/Q.

>>Would shifting the LO arbitrarily be any easier than the outboard scheme 
>>that I hinted at? Anyone?
>>
>With direct sampling, the LO doesn't exist in analog, it moves into the 
>DSP. In general, SDR uses digital because it make all forms of signal 
>manipulation easier.

I guess that means a PLL with a phase adjustment input. I seem to remember 
this has already been mooted for the synchronisation of multiple 
geographically separated HPSDRs using GPS-locked clocks. Easier than 
A/D->memory->D/A and without the same speed limitation I suppose. How would 
that impact the noise performance though? If you can do it at the sampled 
I/Q stage then you avoid the extra noise source.

I think an outboard A/D->memory->D/A unit would have some buyers though. 
Nothing to do with SDR but it uses the knowledge gained.

>The really cool trick comes when the DSP simultaneously form beams and 
>null to optimize reception of _all_ the signals. The receiver effectively 
>has multiple audio outputs, each optimized for a separate signal.

You could put a spin on the beam too for scanning.
Repeaters could use it to home in on the transmitting station.
etc...etc...etc

Calibration would be an interesting exercise.

Murray




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