[evlatests] Strange X-band birdie

[Keith Morris]

Another possibility, given the vintage of these antennas and their 
T304s, is that 8192MHz is coupling within the T304.  ea16, in 
particular, has some of the oldest extant T304s in the array.

X-band might preferentially affected, since most other bands use a 
higher portion of the T304 input frequency range (?) and the 8192MHz 
would likely be filtered out.  This would make perfect sense if the 
symptom were exclusively in IFs A and C (B and D having been recently 
retrofitted).  But as Rick mentioned, the effect is seen in all four IFs.


Dan Mertely wrote:
> The real question then is whether the 4092 clock
> harmonic is conductively leaking into the signal
> path, or radiating out from the digitizer & finding
> it's way through the feeds into the RF signal path.
> 
> We've taken great pains to hold-in these harmonics. Mike
> & Kerry have applied careful PCB design rules to try to
> avoid radiation and cross talk.  Then, the digitizer
> boards are installed in shielded modules.  The D30x modules
> are the tightest we have, and are installed in an (open) bin
> lined with absorbing foam.  Finally, the FE/Utility rack
> was selected for it's shielding properties, and also
> includes absorbing foam on the walls.  The total system
> shielding should be in the order of 100 dB!  On the outside
> chance that the strongest harmonics are escaping above the
> EVLA detrimental levels, it shouldn't be strong enough to
> bother any but the few frequency bins at 4096*N.  We hate
> to lose any bins, especially during spectral line sleuthing,
> but that may be the cost of having such strong signals in
> the processing chain so close to our observing ranges, and
> so close to the inputs to our ultra-sensitive front ends.
> 
> -Mert
> 
> 
> 
> Rick Perley wrote:
>>     And Ken is right!  It is indeed at 8192 MHz (apparently I can't do 
>> simple arithmetic). 
>>
>>     I reviewed all 351 cross-power spectra, and confirmed that the 
>> birdie is seen only on baselines amongst the antennas listed below, and 
>> that it is strongest *by far* on baselines with antenna 16.  Start 
>> investigating with that one!
>>
>>
>>
>> Ken Sowinski wrote:
>>> On Wed, 15 Sep 2010, Rick Perley wrote:
>>>
>>>>    The X-band HercA data have shown an odd birdie, not previously known
>>>> to me.
>>>>
>>>>    All four IFs show a strong, unresolved (2MHz wide channel) birdie at
>>>> 8188 MHz.  But it's visible only on antennas 13, 16, 19, and 26.  Of
>>>> these, it is, by far, strongest on antenna 16.
>>> 8192 is a multicple of 4096 and all its sub-harmonics.
>>> Any chance you are off by two channels?
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-- 
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)