• Surely the answer to the reflection elimination thing is that when you move the source up by lambda/4 the phase of the wanted signal increases by 90 degrees, the reflection goes from the source to, e.g., the window, back to the source and then into the receiver, so it increases by 270 degrees. If you subtract 90 degrees from the phase of the second data set and vector average you get the wanted signal and the reflection cancel. If you add another 90 degrees and average the wanted signal cancels and you should get the reflection. We should try this with a suitable data set. -- Best Richard

  • Agreed. It would be interesting to look at the reflection, after cancelling the wanted signal. Presumably, for example, if it's a slightly off-axis reflection then there should be some clues. Cheers, Darrel.

  • Richard, I've now done that in this file. The sheet "AmpZ1+iZ2" should show the reflection, and the sheet "AmpZ1-iZ2" should be the wanted signal. This was done using the separate nearfield listings for beams 1 and 2 for scan 103. -Josh (Date: Fri, 05 Sep 2008 11:23:00 -0400)

Date: Fri, 05 Sep 2008 20:54:21 +0000
From: Richard Hills <rhills@alma.cl>
Subject: [Alma-feic] Sums and differences

OK. This seems to show the reflection is very spiky and at about -40dB peak - mostly -50dB, while the wanted signal is at -18dB. This is not a lot of margin and it would certainly be worth trying to improve it. With luck the open ended waveguide will do that.

I was puzzled by a couple of things:

On sheet 1 column F seems to be wrong. I hope this doesn't get used for anything important.

I could not understand why column Y is the average of Q and X. Surely it should be Q and U to give the average of Z1 and Z2? I am not clear what you are after here anyway - the 90 degree phase difference will make this a strange object.

Best Richard
Date: Mon, 08 Sep 2008 08:44:15 -0400
From: Josh Crabtree <jcrabtre@nrao.edu>
Subject: Re: [Alma-feic] Sums and differences

Hi Richard, I corrected the formula error in column F, and I don't think we are using it for anything at the moment. I modified column Y to be the average of Q and U, and plotted the amplitudes again. The reason I was averaging Q and X was because I wanted to average beam 1 and beam 2 after shifting the phase of beam 2 (I may have gone about it incorrectly).

-Josh

http://www.cv.nrao.edu/~jcrabtre/Scan103Investigation/ZListingsNearfield.xls

-- ToddHunter - 13 Sep 2008

This topic: ALMA > WebHome > FrontEndIntegrationCenter > FEICBeamScanningResults > FEICBeamScanningMeeting2008sep04 > ReflectionCancellation
Topic revision: 2008-09-13, ToddHunter
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