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

[jeff millar]

Murray Lang wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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.
>   
Imagine a local oscillator (LO) mixing with a carrier (RF), the output 
(IF) contains the difference between the two frequencies.  If the LO and 
RF happen to have the same frequency, then the IF comes out at 0 Hz, or 
DC.  The actual DC voltage depends on the signal amplitudes and phase of 
the LO vs RF.  When exactly in phase the output has the maximum positive 
value, when exactly out of phase the maximum negative value, and when at 
90 degrees the output has zero value.  Measuring the output voltage 
provides a hint of the RF phase, but two different phases can produce 
the same output (ambiguity).

Now route the RF to two mixers and use two LOs 90 degrees out of phase.  
The two IFs become "I" and "Q", each the same IF but now they provide 
enough information to determine the RF phase without ambiguity.

In general, I and Q provide two copies of the same IF, but 90 degrees 
out of phase.  This enables a number of signal processing tricks such as 
canceling one sideband to create SSB.  It also enables "zero IF" 
receivers commonly used in SDR.
>>> 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.
>   
What do you mean by ADC-memory-DAC?
>> 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.
>   
The arrays may not need calibration.  Some papers on "blind signal 
separation" and "principle component analysis" discuss how to separate 
signals using randomly placed, uncalibrated antenna arrays.  You don't 
know which direction the signals come from, just how to separate them 
from each other.

jeff, wa1hco
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