[evlatests] Frequency Offset Tests

[Vivek Dhawan]

Array Frequency Offset Tests are summarized. The situation is still
confusing, though only C band has been tested. The goal is to have no
phase change for moderate retuning (a few MHz?) e.g. Doppler steering.


A. Small changes in F_sky, at fixed Widar offset, i.e. each antenna
   offset by 25.6 KHz*N_ant, this LO offset is removed in Widar.

1. delta F_sky 0.1, 1, 10 KHz : All antennas behave, no change of
   relative phase between antennas vs. F_sky. (A couple antennas
   18 and 23 had phase jumps, believed to be unrelated).


2. delta F_sky upto 200 KHz : most antennas behave, a few show small
   changes of phase, proportional to delta F_sky, ascribed to a few %
   error in the fiber lengths (3-5us) used to correct the LO phases.

    Ant 1 thru 7 and 27 show phase discontinuities. AC and BD may not
    behave the same on an antenna (though they have the same fiber),
    but the phase always returns when re-tuned to the same F_sky.
    A-B and C-D is always ~zero for all antennas (barring the two
    jumpy ones).

   Several repeats have shown consistent results.

3. delta F_sky 1 MHz, 10 MHz : all antennas show a phase change.


B. Barry suggested that we re-test the phase stability to changes in
   Widar offset frequency, at fixed F_sky. Odd and even *100 Hz were
   tried, 25.6=current default, 25.5, 25.4, 25.7, and 12.8 KHz.

1. A first try used an attenuator Set&Remember which caused large gain
   changes, even though the S&R caused no gain change when used in the
   previous test (constant W_off, variable F_sky)

2. When the S&R was disabled, the amplitudes are stable, phases too are
   stable for the first 3 settings, but the 12.8 KHz makes a large phase
   change (i.e. wraps) at most antennas.


C. Rob Long et al. are trying to set up a bench test to look at the
   beat note between two L302's, and watch the relative phase when
   they are commanded (in synch) to change by the same delta_f.

   L302's in use on the array are believed to be identical in their
   phase behavior, as I understand it there are minor versions of MIB
   software but it 'should not matter' and anyway does not match the
   antenna list above.

Any other suggestions?

Vivek.