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

[Dan Mertely3]

Rob:  Do you know what frequency you had to tune
50 MHz off from to make the RFI go away?

-Mert


Rob Long wrote:
> 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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> 
> 
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