[evlatests] Intriguing pulsed emission in S-band

[Rick Perley]

     Trolling through a number of S-band datasets have revealed 
worrisome issues with the switched power (although the visibilities in 
nearly all cases look fine).  We suspect that the strong RFI found in 
this band is somehow to blame, so Ken ran a short test yesterday to help 
diagnosis. 
    We didn't find any of the switched power troubles we were looking 
for.  But this relative small database has allowed a better look into an 
issue which I've seen in all S-band data taken so far. 

    In subband 6 (frequencies 2.628 through 2.756 GHz), both the sum and 
difference powers from the switched power system show a regular pulse.  
The effect is about 10% of the total power in that subband.  More 
careful review reveals the following:

    1) It is a *broadband* phenomenon.  There is no sign of any extra 
narrow-band power in the spectral plots for this subband.  (resolution = 
2 MHz).   There is no sign of this pulsation in subband#5, but there is 
a weak detection of it in subband #7. 
    2) Although it is seen on (almost) all antennas, and on both 
polarizations, there is a clear spatial preference for antennas near the 
center and down the east arm.  The pulsation is nearly undetectable on 
the antennas at/near the ends of the north and west arms.
    3) The pulse period is 4.6 seconds.  The pulse is (almost) never 
seen in two adjacent time records (time resolution is 1.0 seconds). 
    4) The pulse has a width comparable to our PDif switching period 
(100 msec, I think).  This is seen in the PDif values, with have a 
smoothly sinusoidal variation (from positive to negative), with a period 
of about 45 seconds.  (A little more thought, using the two 
periodicities listed above, should allow specification of the pulse 
duration). 

    I cannot see this pulsation in the visibilities -- presumably due to 
the wideband nature, and the (presumably) large delay.  I haven't yet 
found it in the autocorrelation spectra, but am going to look more 
carefully. 

    Any ideas as to the origin?