[evlatests] 4band, interference and ea14

[Frazer Owen]

     The autocorrelation spectrum for antenna 14 shows no narrowband 
feature at 60 MHz. The correlated interference is at 60 MHz.

---Frazer

On 06/23/2014 02:22 PM, Keith Morris wrote:
> 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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