Date: Wed, 03 Sep 2008 13:00:22 -0400
From: Josh Crabtree <jcrabtre@nrao.edu>
Subject: [Alma-feic] NSI "beam averaging" issue

Hi All, this is the latest of my correspondence with Karl from NSI, regarding the average beam issue. He said the NSI software assumes that the Z distance of second beam has been increased/ /by 1/4 wavelength, resulting in a -90 degree phase shift. We have typically decreased the Z-distance of the second beam by 1/4 wavelength. Unfortunately the software doesn't look at the actual Z distances for each beam, it just assumes the beam 2 has a -90 degree phase shift. We intend to use the longer Z-distance for beam 2 from this point on. As soon as we are able to do another scan, I'll check to see if the NSI-calculated average beam looks the way it should.

-Josh

Karl Wrote:

Josh, The position phase correction is doing exactly what it is supposed to do. The key to why it is not working for your case is the fact that in your measurements increasing the Z (probe to AUT) distance results in a positive phase change. NSI2000 expects a negative phase change. All the Agilent/HP analyzers produce a negative phase change as the distance between probe and AUT increases. See section 5.2.4.1 of the manual that I sent you.

Since I assume you are down converting you may want to change the LO/RF relationship.

Karl

Date: Wed, 03 Sep 2008 14:46:49 -0400
From: Josh Crabtree <jcrabtre@nrao.edu>
Subject: [Alma-feic] More information regarding position correction

This is a quick correction to the last email I sent. Geoff informed me that we are actually pulling Beam 2 back 1/4 wavelength further than beam 1, but that the resulting phase change is +90 degrees because of the sideband we are looking at. The nsi software assumes that beam 2 has a -90 degree shift, as I mentioned earlier. It turns out that we have done a few scans looking at the other sideband, for which beam 2 is shifted -90 degrees from beam 1. I chose one scan (Band 3, 2008 Scan 17), and analyzed it. Curiously, the nsi software appears to apply a -20 degree shift to beam 2 when correction is turned on, shifting beam2 even further away from beam1. I show the results in the link below. I don't know it this feature of the software doesn't work like it should, or if maybe there is just something else we are overlooking. I'm still corresponding with NSI to get this figured out. Thanks.

-Josh

-- ToddHunter - 13 Sep 2008

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