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Murray Lang wrote:
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<pre wrap="">***** High Performance Software Defined Radio Discussion List *****
At 10:04 PM 22/08/2006, jeff millar wrote:
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<pre wrap="">BTW, I realised that the phase shifting can't be done at I/Q level
because that's just the modulation.
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<pre wrap="">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".
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<pre wrap=""><!---->
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.
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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).<br>
<br>
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.<br>
<br>
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.<br>
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<pre wrap="">Would shifting the LO arbitrarily be any easier than the outboard scheme
that I hinted at? Anyone?
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<pre wrap="">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.
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<pre wrap=""><!---->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.
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What do you mean by ADC-memory-DAC?<br>
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<pre wrap="">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.
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<pre wrap=""><!---->
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.
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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.<br>
<br>
jeff, wa1hco<br>
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