[evlatests] Pdiff compression, hardware

[Keith Morris]

Rick's "Cygnus A Pdiff Compression Test" showed that certain antennas
(notably 17, 27, and 28), when presented with a strong source, reacted
almost perfectly linearly, while others (especially 25) showed as much
as 2:1 signal compression.

These data did not correspond to hardware differences within either the
T302 LSC Converter or the T304 baseband converter; both "good" and "bad"
antennas shared a mix of new and old hardware in these IF modules.  The
hardware difference that followed the pattern seems to be the build of
the L-band receiver.  The three "most linear" antennas all have the
older revision L-band receivers, while the 16 most compressed antennas
have the new EVLA-style receivers.

The EVLA-style L-band receivers have high-compression point post-amps,
in order to accommodate strong levels of interference without
saturating.  As a result, these receivers have an overall higher output
level than the older style receivers.  Yesterday I set up the two
extreme cases -- ea17 and ea25 -- for the same sky frequency and LO
tunings that Rick had used, and I measured receiver output levels for
these two antennas.  ea25 rcp was 13dB higher than ea17 rcp, and ea25
lcp was 16dB higher than ea17 lcp, on cold sky.

Overall, ea25 L-band receiver was ~15dB higher than the signal power
expected at the inputs of the T302 and T304, and Cygnus A adds about 6dB
additional power to this, meaning the IF modules are seeing about 20dB
higher signal level than they are designed for, and are likely being
driven into compression.  The variable attenuators in the T304 are
capable in most cases of compensating for the higher power, but the
damage is already done.  The results of Rick's most recent test, where
the LSC solar attenuators were placed in the signal path, seems to
indicate that the signal is not compressed when it leaves the receiver.
  It is therefore either in the output stage of the T302, after the
solar attenuator, or the input stage of the T304, before the input
attenuator, that the signal compression is occurring.

I would like to run a test on these two antennas that involves making an
independent power measurement before and after the LSC Converter while
the Cygnus test is repeated.  This will allow us to (a) confirm that the
receiver is not compressing the signal, and (b) separate the T302 output
from the T304 input.

If the receiver output is linear, then attenuators installed at the
receiver outputs will bring the levels back to their nominal operating
points, and ensure that the IF modules remain linear.


-- 
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)