[evlatests] LBR noise cal filter tests

[Paul Demorest]

hi all,

This is an update on the effect of the low-band noise cal (aka switched 
power) system on L- and S-bands (see various old emails from me to 
evlatests about this).  Note that currently the low-band cals are turned 
off during all non-low-band observations, so this is not an ongoing 
problem.

Last week, Wes and co. installed some extra filters into the low-band 
systems on ea09 and ea11 to see if this effect could be reduced via a HW 
fix.  To test this, I took some new data both before (2017/03/30) and 
after (2017/04/24) the filters were installed.

The short answer is that the filters do not really seem to have helped.  
The pattern of power vs freq does not look exactly the same as it did 
before, but the effect is clearly still there.  ea09 may even look a bit 
worse after this work.  I don't know what (if anything) this tells us 
about the nature of the problem.  Any ideas?

For those who want to see the data, the attached PDF shows the results 
from the two S-band tests for all antennas where data were taken.  The 
quantity shown is the ratio of "on-off" power at 10 Hz when P-band cals 
are switching to when they are off, as a function of frequency.  S-band 
cals were switching the entire time, so ideally we want to see a flat 
line with a value of 1.0 (ie like ea04 which for some reason is not 
showing this effect currently).  This was measured using binning-mode 
widar autocorrelations.

Most antennas show nearly identical results in the two tests, so this 
effect is apparently stable over few-week timescales.  The power vs freq 
pattern clearly changed for ea09 and ea11, presumably as a result of the 
filter work.  It also changed for ea01 and ea16.  Maybe there was some 
other work on the low-band systems on these antennas during this time 
range?

I also took L-band data but it turned out not to be very useful due to a 
bug in the data recording software.  ea09 and ea11 still do show a 
strong effect at L-bang though.

Cheers,
Paul
-------------- next part --------------
A non-text attachment was scrubbed...
Name: LBR_swpow_comp_S.pdf
Type: application/pdf
Size: 1264259 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20170425/607fa0c3/attachment-0001.pdf>