[Difx-users] Correlating Overlapping Bands {External}

[Jan Florian Wagner]

Hi Joe,

for a VEX-derived plot of frequency bands, you can use a DiFX utility under
trunk /sites/MPIfR/vex/plotVexChannels.py

The band plot could help you to visualize where the bands are, how they
overlap, and what the correct zoom band definitions could be, that fall
entirely in a recorded band at all stations.

If the zoom definitions look okay and fall inside recorded bands, then
another cause for the "Cannot find a parent freq" error can an unsuitable
spectral resolution. The error is somewhat misleading in that case.

For correlating experiments with ALMA for example, where recorded bands are
like

chan_def = &W : 87210.453125 MHz : L : 62.5 MHz : &CH01 : &BBC07 : &cp;
chan_def = &W : 87151.859375 MHz : L : 62.5 MHz : &CH02 : &BBC07 : &cp;
...

a correspondingly fine v2d  FFTSpecRes setting is required, as below,
otherwise ALMA bands are not considered for Zoom correlation, despite the
zoom band definition nominally falling into a ALMA recorded band

SETUP setupDefault
{
  tInt = 0.4
  FFTSpecRes =  0.0156250
  specRes = 0.5
}

regards,
Jan

On Tue, Feb 22, 2022 at 6:52 PM Joe Skeens via Difx-users <
difx-users at listmgr.nrao.edu> wrote:

> Hi all,
>
> I'm attempting to correlate data coming from two different types of
> antennas with different (but overlapping) frequency bands, and I'm having
> some trouble with setting up a zoom section in the v2d file to do this.
>
> As it stands, when running vex2difx, I get a warning saying, "Cannot find
> a parent freq for zoom band X of datastream Y for antenna Z," and the zoom
> band is dropped, leading to invalid records when the correlation proceeds.
> I've tried the ANTENNA parameters addZoomFreq and zoom, and both seem to
> lead to this outcome. I see in the freqId parameter in the SETUP section a
> blurb about selecting a parent band, but I'm not sure how this is
> applied--after selecting the freqIds of interest, there's no change in the
> warning message. (As a note, I have to select the freqIds of both antennas
> here because I'm unsure of how to apply different setups for different
> antennas to a single source if that is possible).
>
> Am I committing an obvious blunder here or missing something in my setup?
> I should also note that I'm attempting to correlate complex data with real
> data, is this something that is supported in the current version of DiFX,
> or do I need to convert one of my datasets for compatibility? Any
> advice/expertise would be greatly appreciated!
>
> Best,
> Joe
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