[evlatests] [Fwd: Effectiveness of New Switches] -- now readable ...
Rick Perley
rperley at nrao.edu
Tue Jan 22 18:53:29 EST 2013
-------- Original Message --------
Subject: [evlatests] Effectiveness of New Switches
Date: Tue, 22 Jan 2013 16:37:09 -0700
From: Rick Perley <rperley at nrao.edu>
To: evlatests at aoc.nrao.edu
We're a bit more than half way through replacement of the VLA's
switches. Time for a check.
To do this, I used a test, run last week, where the four high
frequency bands were observed in sequence, about 3 minutes each. Four
cycles around were completed -- a bit less than one hour.
Many curious phenomena are seen -- nearly all are very likely
connected with real, short-term (tens of seconds) gain changes in the
receivers or IFs. Nearly all of these short term variations are small
-- less than 2%. These are very likely unconnected with switch issues.
To isolate issues which are more likely due to switch failures, I
tabulated all changes in PDif greater than 5%. These were divided into
two types -- (a) those which occur between scans -- very likely a switch
issue, and (b) those occuring within a scan -- could be a bad switch or
a receiver gain change. A simple table -- shown below was drawn up,
color-coded by severity. There are four classes:
A) Changes greater than 40%
B) Changes between 20 and 40%
C) Changes between 10 and 20%
D) Changes between 5 and 10%.
The table is broken into two halves -- those with new switches, and
those with old. The numerals in the matrix give the number of
occurances. The maximum for any one antenna is 16 (four IFs * four
bands).
I) Antenna with new switches
Antenna Class A Class B Class C Class D
-----------------------------------------------------------
3 0 0 0 1
5 0 0 2 5
7 0 0 2 2
9 0 0 0 0 Perfect!
10 0 1 2 2 IF 'D' is bad
12 0 0 1 3
13 0 0 0 3
14 0 0 3 1
16 0 0 0 0
19 0 0 5 0
21 0 0 0 0
22 0 0 0 0
24 0 0 0 0
26 0 0 1 2
27 0 0 0 0
-----------------------------------------------------------
Note that 6 of the 15 antennas have no noted variations.
II) Antenna with Old Switches
---------------------------------------------------------
1 0 1 3 0
2 0 1 1 2
4 0 0 0 0 (No Ka-band rcvr)
6 0 0 3 0
11 0 0 0 2
15 0 0 0 2 (No Ku-band rcvr)
17 0 1 1 0
18 0 0 0 0
20 3 3 1 0 The Worst by far!
23 0 0 0 0
25 0 3 1 1
28 0 0 2 0
---------------------------------------------------------
Two antennas out of 12 are 'perfect'.
There is a clear difference in the number of severe (> 20%) changes
in gain, but very little difference in the rate of 'small' gain
changes. A possible conclusion is that the new switches are much better
in preventing large gain changes.
It seems likely that the smaller gain changes are not related to
switch issues, but rather to short-term gain changes in the signal chain.
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