[evlatests] P-band spectrum

[Walter Brisken]

I've looked throught the P-band spectrum from antenna 14 -- the entire 
1024 MHz bandwidth captured by the samplers.  I find the following 
features:

Band		Sky Freq	Freq sampled
A, C		297-344 MHz	1245-1198 MHz	P pass band
A, C		27-103 MHz	1515-1439 MHz
A, C		74 MHz		1468 MHz
A, C		12 MHz		1530 MHz
A, C		0 MHz		1542 MHz 
A, C		-384 MHz	1158 MHz
A, C		-506 MHz	2048 MHz

B, D		297-342 MHz	1239-1194 MHz	P pass band
B, D		27-103 MHz	1509-1433 MHz
B, D		0 MHz           1542 MHz
B, D		-384 MHz	1152 MHz
B, D		-512 MHz	2028 MHz



Signals that appear at the same sky frequency must contaminate the signal 
chain before the second LO gets used.  The one at 0 MHz sky freq is likely
the 1024 MHz oscillator in the 4P converter getting aliased.  The feature
at 2048 MHz sampled freq (most negative sky freq) is likely a sampler 
offset.  The biggest unknowns are the line at -384 MHz sky freq and the band
between 27 and 103 MHz.  The latter may be FM, etc.  -384 MHz could 
correspond to a sky freq of 1664 MHz, but would have to be very strong
to evade various filters.

Level setting for P-band will be a bit trickier because more power comes 
from outside the 100 MHz band presented to the VLA equipment than from 
within.  A software filter can be used when determining the RMS sampler 
statistics appropritate within that band.  This should work as the P-band 
tuning range is quite limited.

-W