[evlatests] Birdies in 14a

[Jim Jackson]

Sorry about that.  I had assumed the jumpers were installed on all of the
deformatters at the site and didn't think to check that one when I put it
into 14A a couple of weeks ago.

Jim

>
> 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.
>
> Some background on this jumper:
>
> The line to the VLA baseband system is driven directly by the high speed
> DAC output. This output is a differential current output and is coupled to
> the line by a transformer. It is difficult to know beforehand how the
> output side of the transformer should be treated so I put a component
> there which could be left open, shorted or connected with a resistor or
> capacitor as determined in tests. The boards are delivered from the board
> manufacturer without the parts installed.
>
> In our system they seem to want to be installed. When assembling the
> boards I don't always remember to install them.
>
> I installed the jumpers in the rest of the ant 13 board set.
>
> Here are the results of measurements I made at the ant 14 T5 test points.
>
> Observed with the Agilent spectrum analyzer in the central elec. room.
> At 100 Hz RBW the broadband noise level was -70dBm.
>
> BEFORE INSTALLING JUMPER
> IF  100HzRBW                     10HzRBW
> A   707KHz -50dBm 695KHz -50dBm  706 KHz -40dBm 694KHz -40dBm
> B   Not seen                     695 KHz -70dBm
> C   687KHz -60dBm 697KHz -70dBm  Didn't look
> D   Not seen                     Not seen
>
> After installing the jumper in IF A the lines went down to about -73 dBm
> at 10Hz RBW.
>
> The decrease in power at narrower bandwidth indicated to me the lines are
> beginning to be resolved. The frequency and frequency variation are
> consistent with the switching regulators used to reduce 48v to the
> internal operating voltages.
>
> Mike Revnell
> National Radio Astronomy Observatory
> Socorro, New Mexico
> mrevnell at nrao.edu
> revnell at ieee.org
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