[evlatests] Birdies in 14a

[Vivek Dhawan]

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.