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

[Murray Lang]

At 10:32 PM 23/08/2006, jeff millar wrote:

>>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.

I have a rudimentary understanding of QSD. My question was really a more 
specific one: what is the mathematical function applied to I and Q in 
software that will produce a constant phase shift on the carrier that it 
modulates? I wasn't really expecting anyone to answer. When I have a better 
understanding of the math (I'm working on it) then I'll be able to answer 
it myself. Something doesn't smell right though - something about frequency 
response of the I/Q output. Anyway, I'll get over it.


>>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?

Sample the RF, put it into memory, wait, take it out of memory, then D to A 
(eg LT2208/FPGA/DDR RAM/Some D/A). There's going to be a minimum shift 
caused by the  processing delay but that will cancel out if all of the 
signals have the same circuit. I can't see this working at very high 
frequencies due to the amount of memory required and its speed. Could be 
feasible at HF though, where antenna gain means unmanageable size.

>>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.

Interesting. You need to know the direction for transmitting though. Also 
if you are looking for a signal from a specific direction (eg a beacon) and 
no others.

Murray VK6HL
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