[evlatests] A curious new birdie at X-band

[Rick Perley]

    Good thing there are people reading this with memories better than 
mine ...

    So I can now answer Keith's query:

    The 10750 MHz tone is stronger towards the center of the array, and 
weaker (indeed, absent) on the east arm. 

Bryan Butler wrote:
>
> from an email from keith morris on may 10th of this year on this topic:
>
> "Rick:
>
> The first LO for the API (both old and new) is 10750MHz.  Do you notice
> the birdie stronger towards the center of the array, or on the east arm?
>
> Shielded chamber tests of the new API LNBs didn't show any emissions at
> the LO frequency, though there was radiation at certain subharmonics,
> which we have found a way to suppress."
>
>
>
> Rick Perley wrote, On 7/23/13 9:50 :
>>      As part of the 'receiver rotation' experiment, I've taken wideband
>> data covering all of L, S, C, and X bands.
>>
>>      Looking at the higher half of X-band (10 to 12 GHz), one, and only
>> one, piece of RFI is seen.  It's a pure tone, at
>>
>>      10750 MHz
>>
>>      This is not a harmonic of 128 MHz (but is close -- the 84th 
>> harmonic
>> is 10752 MHz).  No other birdies are seen, so this tone is definitely
>> special.
>>
>>      And it's only seen on antennas near the center of the array --
>> strongly arguing for a local origin.
>>
>>      The autocorrelations are useful for diagnosis -- unfortunately, 
>> only
>> half the antennas provide this (what do we have to do to get them all?)
>>
>>      Antennas with strong 10750 MHz tone are:
>>
>>      ea13 at W02
>>      ea15 at E02
>>      ea22 at N02
>>
>>      Antennas with detectable tone (but weaker than above list) are:
>>
>>      ea05 at W08
>>      ea09 at W10
>>      ea11 at W06
>>      ea24 at W04
>>      ea 26 at W12
>>
>>      Antennas with no visible tone (in autocorrrelations) are:
>>
>>      ea01 at N14
>>      ea03 at E10
>>      ea07 at E12
>>      ea17 at E14
>>      ea19 at W16
>>      ea28 at E16
>>
>>      All other antennas provided no autocorrelations.
>>      The cross-correlation data are consistent with a local origin, near
>> the array center.
>>
>>
>>
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