[evlatests] Birdies in 14a

[Vivek Dhawan]

Hi Jim,

I may not have the full picture, but it seems not to be a big deal -
the signals do not appear in cross-correlation. The effect on Tsys
measurement is small at the wider bandwidths, and, amending what I
said earlier (below): If the BW is dropped below 780KHz, the birdies
are excluded anyway.

I still think a longer imaging-type observation should be done
as a sanity check.

Vivek.

Jim Jackson wrote:
> As has been discussed in the past, I'm not sure we are going to completely
> get rid of these birdies without some significant change to the
> deformatter hardware.  The birdies should only be a transition issue as
> they seem to get into the analog output from the DAC.  There is virtually
> no way for them to affect the digital output once the cards are installed
> in the WIDAR correlator.
> 
> If this is a serious problem for the transistion, it could be a
> considerable setback and result in some delays.
> 
> Cheers,
> Jim
> 
> 
> 
> 
>>I looked at data taken today - dec29, UT 19-20:13 - after Mike's
>>fix. The 700KHz spikes are indeed reduced in 14A, but still visible.
>>
>>They are somewhat variable from scan to scan and at different RF band
>>settings, 3-30% of Tsys in a single 5KHz channel, on 14 A and B.
>>and also 16 A, B, at  0-3%. Thus the Tsys measurements would be
>>unaffected at the 1% level, until the total BW is well below ~1MHz.
>>
>>Occasionally the 2nd harmonic of 700KHz was seen, weakly. There was also
>>a spike at about 2.8MHz (4th harmonic?), only at Q-band, only on 16B.
>>The 700KHz was visible simultaneously.
>>
>>They are not present in cross-correlations (about 1-2min integrations.
>>Ken suggested a longer integration be done - e.g. as part of one of the
>>science tests.)
>>
>>Vivek.
>>
>>
>>
>>
>>Mike Revnell wrote:
>>
>>>I went out this morning to look into the problem.
>>>
>>>The deformatter in 14a was borrowed from the antenna 13 set to replace
>>>one that was having optical connector problems. There is a jumper at the
>>>line driver output that goes to the vla baseband system that wasn't
>>>installed. I installed the jumper and the birdie went down by over 30dB.
>>
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> 
> 
> 
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