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

[Rob Long]

I don't believe that there any differences at L vs C band through the 
LSC converter.  We could look at this again on the antennas more 
closely, but I thought that Rick and Bob already checked Tsys through 
the T304 downconverter output.(?)

The other thing I have noticed lately is a fair amount of RFI at L-band. 
  When I use the maint script to set up L-band, I see badly distorted or 
completely indiscernible switched power at the T5 detector (using an 
o'scope.)  It is not usually all 4 IFs, but it sometimes shows in either 
one or the other polarization.  I have had to retune the L302s (most 
typically the L302-2) about 50 MHz to reduce the peak power, stop 
saturating the digitizers, and see reasonable switched power at the T5.

Rob

Ken Sowinski wrote:
> 
> 
> 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.
> 
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