[evlatests] L-Band sensitivity on 13, 14 and 16

[Ken Sowinski]

On Tue, 25 Apr 2006, Walter Brisken wrote:

>
> On Tue, 25 Apr 2006, Rick Perley wrote:
>
>>   So where is the problem?   Bob Hayward and I did efficiency and Tsys
>> tests at the front end on antenna 13 which clearly showed that G/T should
>> be about the same as VLA -- perhaps at most reduced by 10%.
>>   I list some possible explanations:  a) High Tsys.  b) Low efficiency.
>>   c) Bad pointing.  d) Defocussed.  e) Subreflector seriously out of 
>> position.
>
> I'd add to the list two other possibilities consistent with having a good 
> receiver: f) out-of-subband power (RFI or internal) either causing clipping 
> or inappropriate level setting, and g) low or unstable LO power at chosen 
> tuning.  Could g have an affect in the LSC converter that would not show up 
> at C band?

Rick has two consistent lines of evidence for his conclusion:
analysis of the correlation coefficient and sync detector
deflection between blank sky and known calibrator.  It is hard
to see how g) would affect the SD deflection.  f) is certainly
a possibility.  I was going to add a similar statement to Rick's
list.  I think a case can be made that the sampler/transition
module can degrade both the correlation coefficent and the SD.
Perhaps we need to try a bandpass filter in front of the sampler
again to see what happens to L band when the input to the sampler
is band limited to make it more well behaved.