[evlatests] Antenna birdies

[Mike Revnell]

The easiest way to deal with this is to up the level of the signal going 
into the digital to analog converter on the deformatter board and pad down 
to output. We haven't made an effor to optimize this setting. So far we 
are just using a more or less random number that seems to work.

The GAIN2 parameter in my user interface does this. Presently this number 
defaults to 150. In some tests I ran in the lab using a single frequency 
source from a synthesizer at about -40 dBm at the module input I was able 
to make this number about 750 without overflowing the DAC. This may take 
some effort to find the optimum value but I have no idea how to turn the 
power level of a single frequency into a broadband number from the 
downconverter output.

Changing this gain setting to 750 gained about 15 dB in the ratio of 
signal line to birdie level. 

In the board we looked at in the lab there are two birdies a little over 
400 KHz and a little under. 

The high levels in two of the IFs are probably due to the missing jumpers 
mentioned by Rob.

On Mon, 29 Aug 2005, Rob Long wrote:

> We just looked for the birdies here on the test bench and found that 
> they are coming directly from the two switching power supplies on the 
> deformatter boards.  IFs A and D may be worse because they may not have 
> the ground jumper installed on the deformatter boards.  We will check 
> this tomorrow and fix it if necessary.
> 
> We need to talk today about the effect this has on observing.  Are the 
> IFs with the lower levels causing a problem or just A and D with the 
> higher amplitudes?
> 
> This is, of course, only a transition issue I believe.  Once we are 
> using the digital signal straight into the correlator, we won't see 
> these power supplies.
> 
> Rob
> 
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-- 

Mike Revnell
National Radio Astronomy Observatory
Socorro, New Mexico
mrevnell at nrao.edu
revnell at ieee.org