[evlatests] Birdies in 14a

[Jim Jackson]

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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