[evlatests] EA02 Stokes LL spectral comb also in Ka-band

[Emmanuel Momjian]

On July 9, 17, and 20, 2010, three sets of Ka-band spectral line
observations were carried out at around 36.17 GHz using a bandwidth of 4 MHz
with 256 spectral channels. In all these sessions, baselines to EA02, Stokes
LL, are always showing a spectral comb with spikes separated by 0.125 MHz
(see the attached EA02_LL_Ka-band.png). This is not seen on any other
antenna, or in Stokes RR.

However, this problem is not unique to Ka-band. This was also seen in L-band
with identical frequency separation between the spikes (i.e. 0.125 MHz). I
have reported the L-band case on June 14 in the JIRA ticket ECDATA-261. The
L-band observations were carried out on June 10, 2010, using 1 MHz bandwidth
and 256 spectral channels. A plot is attached for this one as well
(EA02_SPEC_L-band.png). Note that the left-hand-side panels are of Stokes RR
(data look fine), and the right-hand-side ones are of Stokes LL. The top
plots are of baseline EA01-EA02, and the bottom ones are of baseline
EA02-EA05.

The Ka-band observations were taken using the IF pair AC. I believe the
L-band observations also used IFs AC. If so, the problem seems to be unique
to IF C on EA02 regardless of the frequency band (or at least in two
frequency bands: L and Ka).

Emmanuel
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