[evlatests] 4band, interference and ea14

[Keith Morris]

There are no intentional 60MHz signals in the API.

Dan -- have you sniffed the API shack lately?



On 6/23/2014 1:56 PM, Rick Perley wrote:
>       Mike Revnell just showed me VLITE spectra from ea14.  There's a
> super-strong resonant feature ('birdie') right around 60 MHz.  Looks
> like external RFI.
>       We note the API is nearby ...  any chance it's from there?
>
>
> On 06/23/2014 11:33 AM, Frazer Owen wrote:
>> Details of 60 MHz interference:
>>
>> 1) My test was looking at an 8minute average.
>>
>> 2) The test that shows the strongest cross-correlation at 60MHz is
>> looking at the NCP (North Celestial Pole).
>>
>> 3) The antennas are ea12 at N9 , ea14 at E8, ea19 at W1
>>
>> 4) Except for the NCP, the signals when averaged over 8 minutes
>> decrease in amplitude relative to 2 second averaging.
>>
>> 5) The strength of the correlated 60MHz signal changes with different
>> pointing positions (Cas A, 3C48, NCP). Especially the NCP strength is
>> very different from the other two.
>>
>> 6) ea14 at E8 has no internal 60 MHz interference based on the total
>> power and is near the end of the east arm. However, ea14-ea19 appears
>> to have stronger 60MHz interference than ea12-ea19. These signal have
>> been through the requantizer step but it seems clear that whatever
>> signal ea14 is seeing that correlates with ea19 is not coming from
>> ea14 itself.
>>
>> ---Frazer
>>
>>
>> On 06/23/2014 09:51 AM, Rick Perley wrote:
>>> In the (distant) past, a similar effect was commonly noted at
>>> 4-band.  The accepted explanation was either that:
>>>
>>>      1) An exceptionally bad antenna is radiating to the others. This
>>> would be most prominent in D configuration.
>>>      or
>>>      2) The supposedly incoherent LOs are sufficiently close in
>>> frequency that, for periods of seconds to tens of seconds, there is
>>> insufficient phase slip between them to destroy the cross-correlation.
>>>
>>>      I recall that #2 was in fact the theory I came to embrace, as it
>>> was usually noted that the baselines with strong combs were
>>> independent of antenna location.
>>>
>>>      You could test this by looking at the phase of the line
>>> cross-correlation.  If from a single source (i.e., theory #1), the
>>> phase should slip with the fringe rate.  But if from different LOs
>>> with slightly variable phases, the phase relation will be
>>> 'different'.  (I'm not prepared to define 'different', other than it
>>> won't be strictly the same as the fringe phase).
>>>
>>>      Rick
>>>
>>>
>>> Interestingly the 5MHz comb frequencies correlate on baselines to 14
>>> (as they do on 12-19). Since the 5MHz clock is supposed to be
>>> incoherent from antenna to antenna, there must be a general source of
>>> the comb frequencies, especially 60MHz, which is being broadcast
>>> around the site. Could this be one very bad antenna ? ---Frazer
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>
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