[evlatests] Stability Tests, on Friday afternoon

[Rick Perley]

    Ken and I ran a 1-hour test early this afternoon to test for amp & 
phase stability,
and to see if phases return correctly after having changed bands. 

    Antennas:  13, 14, 16, and 18.    Antenna 24 was not available. 

    Bands:  X and C, alternating on a one-minute time scale.

    Integration:  0.41 seconds. 

    Correlator:  Continuum. 

    Weather:  Not great -- thunderstorms in all directions.  Phase 
stability was
pretty bad, with radian-size excursions on a few minutes of timescale. 

    Results:

    A)  Phase.

    Antenna 13 phase connects correctly on all four IFs.  There are no 
odd phases
visible. 
    Antennas 14 and 16 do not connect.  For each, A and C have identical 
non-connections,
B and D also (but different than A and C). 

    Antenna 18:  A and C do not connect.  B and D do connect correctly. 

    From this, it seems the L302s in antenna 13, and in 18 B and D, 
behave correctly. 

    B) Amplitude. 

    The situation here is not so good. 

    Antennas 13 and 18 B and D show the return of the fast dropout 
problem.  The
characteristics are the same as what we had many months ago:

    - Amplitude drops are large -- the most common are by 2/3 and 1/4.  
The dropped
amplitudes are not random in amplitude -- only certain values are seen. 
    - The drops only occur 'on the 1s' -- i.e. at 11, 21, 31, 41, or 51 
seconds in time. 
    - Not every time 'on the ones' has a dropped amplitude.  Most do. 
    - Drops are usually a single 410 ms record, but some are double.  
When double,
they are of different amplitude. 
    - There is no phase perturbation visible -- this is purely an 
amplitude effect. 

    Antenna 18, IFs A and C (at X-band only -- the c-band system is not 
working) is
a mess, with fast (but not random) variations in amplitude. 

    Antenna 14, IFs A, B, and C, is fine. 
    Antenna 14, IF 'D' shows a smooth change of gain at the beginning of 
every
scan.  The gain change timescale is 30 seconds, and the gain change scale is
large -- by 30% at X-band, and by 10% at C-band.  It sounds like the system
recovering from a significant change in power level -- except that for both
X and C bands, the gain change is in the same sense (the amplitude is 
too high
at the beginning of the scan, and decreases to the stable level after ~ 
30 seconds). 
If due to different power levels associated with X and C bands, i'd 
expect the
change to be positive for one band, and negative for the other.  Read on ...

    Antenna 16 is very good on all IFs, except for a small gain 
recovery, lasting
about 10 or 20 seconds, for all four IFs.  In this case, the amplitude 
is the same
for both bands (~3%), and the slope is oppostive -- fringe amplitude 
rises at the
beginning of an X-band scan, and drops at the beginning of a C-band scan.