[evlatests] C-Band Update

[Robert Hayward]

On Wednesday Rick Perley and I measured Y-factors on the C-Band (S/N 1) 
in Antenna 14 at the output of the receiver. It was found to be working 
with the proper sensitivity (Trx < 15K) so the small amount of moisture 
we found in its feed a couple of months ago is no longer a problem.

We also measured Y-Factors at the output of the LSC Converter as well as 
the output of the T304 Downconverter. They were nearly identical to that 
measured at the receiver output which indicates little or no degradation 
in the signal path up up to the input to the Samplers. We didn't have 
time to check the Y-factors at the output of the T5 module at the D-Rack 
in the Control Room (which would tell us about possible sensitivity loss 
through the DTS chain).

On Antenna 16, Jim Jackson and I did a hot/cold Y-factor directly on the 
C-Band (S/N 2) receiver. This type of measurement removes the effect of 
the feed and is similar to what we do in the lab. We found that the 
receiver had a Trx about twice that of the C-Band on Ant 14. For good 
measure we checked the feed for moisture and found that the cavities in 
its ring load section were nice and dry (this feed was found to be full 
of water a while back).

So the poor performance at C-Band on Ant 16 is due, at least at this 
moment in time, to the receiver and not the feed. While it is no where 
near as sensitive as Ant 14, it should still be slightly better than a 
VLA system. Alas we do not have a spare C-Band that we can use to swap 
it out and allow it to be returned to the lab for evaluation. I would 
suggest we use the receiver (S/N 3) planned for Ant 13. It is currently 
undergoing tests in the lab with what we hope is the final revision of 
the new EVLA Card Cage design. We are about 2 to 3 weeks away from this 
goal. Once S/N 2 is returned to good health, we'd put it on Ant 13.

Finally, the C-Band on Ant 16 now has its Tcals turned on. So back-end 
switched power should be available.

Off topic, the M&C cables associated with the F14's on Ant 14 have been 
configured properly and the monitored data now reflects the correct 
receiver status (L, Q & K had been all mixed up).

-Bob