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[Linrad] polarization measurement with digital downconversion receivers
- Subject: [Linrad] polarization measurement with digital downconversion receivers
- From: Ken <cs.com; ke2n@xxxxxxxxxxxxxxxx>
- Date: Sun, 22 Feb 2009 07:52:39 -0500
Leif - just a followup to your last
"Signals near the center frequency will then get the correct
polarisation readout. A signal 75 kHz away may get the correct
phase (=time) but it may also be shifted by up to 5 us.
At 75kHz a full period is 13 us so the phase error could become
as large as 140 degrees. We want the phase error to be less than
45 degrees to be sure what tx polarisation to use and therefore
the useful bandwidth would be +/- 25 kHz.
Forgive me - I do not know how Linrad figures the polarization (is it described on your web site? or is it Joe Taylor's?). It seems to me that Linrad has two signals: one that is known to be vertical and one that is known to be horizontal. I expect that the only phase angle you could measure reliably would be one versus the other - i.e., one is leading or lagging the other. So you might determine that the received signal is (for example) 23 degrees clockwise from the vertical towards the horizontal.
This 140 degree phase error (or shift) that you describe would apply to both signals - so you would still find that the received signal was "23 degrees clockwise from the vertical towards the horizontal" even with the 140 degree shift - no?
BTW - I believe that we can sync the CIC filters etc. in the Mercury boards, so I am hoping to have something that works in that way eventually. (Travel in Europe and Asia this month has me away from the shack for an extended amount of time). I am thinking that the first experiment I would do would be to take the 96 kHz baseband from the audio outputs on the Mercury boards and feed them into a sound card like the Delta44. That way (I hope) no changes would be required on the Linrad MAP65 side of things.
That would completely remove the issue of making the twin USB ports work (for now anyway).
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