[evlatests] WIDAR-0: fast (Hz-ish) oscillations
Michael Rupen
mrupen at nrao.edu
Wed Jun 17 11:04:20 EDT 2009
Fast Amplitude/Phase Wobbles
----------------------------
mpr 17jun09
Most of the results below are based on several data sets taken on Tuesday
16jun09, each a few seconds long, with time resolutions of 10 or 20 msec.
The sources with 3C84 (mostly) or 3C273 (towards the end).
Note that these observations are NOT sensitive to the phase oscillations
seen by Rick on baseline 2-18 in the 13jun09 X-band data. Those +/-3
degree oscillations had much longer periods, of order a few minutes.
These data were taken AFTER the antenna moves, and AFTER the fiber switch
between antennas 25 and 28. The antenna/WIDAR-0 setup was therefore
as follows (the * indicates the fiber swap; "m" indicates an antenna which
has been moved recently):
Slot Rack 1 Rack 2 Rack 3
0 sb0005 = ea19 = W 4 sb000a = ea25*= N 2m sb0004 = ea18 = N16
1 sb0010 = ea01 = W16 sb0009 = ea28*= N 8 sb0006 = ea23 = E18
2 sb000c = ea02 = E 2 sb000b = ea24 = W14 --------
3 sb0002 = ea09 = E 6 sb000e = ea03 = E16 --------
ea25 used to be on pad N24.
The 16jun09 observations reported here used ONLY THE EIGHT ANTENNAS IN THE
FIRST TWO RACKS: {1 2 3 18 24 25 28}.
1. Rapid oscillations (in either amp or phase) for subbands > 0 are seen on
only two baselines on 16jun09: 2-19 and 19-25.
a. On 21may09 we checked antennas {1 3 9 18 19 24 25 28}, and
found amp oscillations on baselines
18-19 and 19-28,
with one instance of
1-25
as well. There may also be a slow oscillation on
19-24.
The possible "solidly bad" baselines observable on both 21may09 and
16jun09 are thus
{19-25 19-28}
Both were observed on both days; the oscillation has moved from
19-28 to 19-25, showing that this oscillation the StB or StB
location (slot) rather than the antenna (see also d, below).
18-19, seen on 21may09, was NOT seen on 16jun09.
b. Subbands {1 2 3 4} show identical behavior.
- On 21may09 we found that subband 6 showed a slightly different
frequency from subband 2.
c. The same bad baselines (and no others) are present in both C and
X band data.
- This was true on 21may09 as well.
d. The frequencies of the oscillations, as measured in observations
within a few minutes of each other, are as follows:
2-19 19-25
C-amp ------ +/- 3.3% 2 Hz
C-phase +/- 5 d 3 Hz +/- 2.5d 4 Hz
X-amp +/- 1.8% 5 Hz +/- 3.8% 4 Hz
X-phase +/- 3.5d 5 Hz +/- 2.5d 7.5 Hz
- Here the oscillation frequency scales with observing/fringe
frequency (roughly a factor 2).
**This was NOT true on 21may09.** We did not check this as
carefully as on 16jun09, but did check the number of oscillations
per unit time on the screen, and didn't notice any difference.
- Note that the phases vary roughly a factor 2 more rapidly
than the amplitudes on 19-25. This doubling was seen for 19-28
on 21may09, confirming that the oscillation tracks StB or StB
location rather than antenna.
A re-analysis of one set of 21may09 data (4sec with 10 msec dumps)
gives:
1-25 18-19 19-28
X-amp +/- 1.4% 10.75 Hz +/- 3.3% 4.25 Hz +/- 2.4% 0.8 Hz
X-phase +/- 5 d 10.75 Hz +/- 2.5d 8.25 Hz +/- 2.5d 1.8 Hz
[Note that the 1-25 oscillation was not seen in other 21may09
observations.]
So on 21may09 both 18-19 and 19-28 showed the phase oscillating
twice as fast as the amplitude.
I attach plots of all these baselines.
e. As on 21may09, the oscillation frequencies are at least 10x the natural
fringe rate.
f. Only one autocorrelation shows any sign of a fast wobble:
antenna 2 shows an ~1 Hz wobble of +/- 0.3%. The other antennas show
no wobble to similar levels. There is no sign of any fast wobble
in the autocorrelations at the few % level seen in the
cross-correlations -- any such wobble would be *very* obvious.
g. We tried turning off fringe rotation and frequency shifting (i.e.,
fshift= 0.0 and PhEN off on all correlator chips). While the
lagfan results are still confusing, showing phase jumps and bizarre
slow amplitude drifts, there is no sign of the sorts of fast
oscillations seen in normal observations. BRENT will have to comment
on whether this is meaningful.
h. COMMENT: Slot 0 seems to be a bad place -- the offending baselines
all involve Station Boards in slot 0, and for the worst baselines --
and those with a phase oscillation twice as fast as the amplitude
oscillation on 16jun09 -- both antennas occupy slot 0.
The correlator setup uses a different Baseline Board for
each subband, with a given chip location corresponding to a given
baseline on each board. We have occasionally tried correlating a
"problem" baseline on a different chip, and didn't see any change
in the resulting correlation. If this is interesting we could do some
more work here.
The StB in Slot 0 provides DUMPTRIG as well as the reference clock (for the
different antenna inputs) for the Baseline Board. We could change
the origin of DUMPTRIG if that sounds interesting; the reference clock
is hard-wired.
2. I have not finished analyzing the 16jun09 subband 0 data, but a quick
look shows the following.
a. 21may09
This repeats the matrix from my 21may09 e-mail, showing
the variation frequencies (typically 10% in amp) measured on
each baseline: numbers mean that many full cycles over 4 seconds;
"--" means there was no obvious variation over that time.
1 3 9 18 19 24 25 28
W16 1 10 4.5 --- --- --- 7 ---
E16 3 6 --- --- --- 16 ---
E24 9 --- --- --- 11 ---
N16 18 5 3.25 --- 3
W 4 19 1 --- 0.5-1
W14 24 --- ---
N24 25 ---
N 8 28
b. 16jun09 -- the same matrix, now in cycles per 7 seconds. Note that
we had antenna 2 instead of antenna 18, and exchanged the fibers
for antennas 25 and 28. Also this is C rather than X band (we observed
X band on 21may09).
1 3 9 2 19 24 25 28
1 -- 0.5 --- --- --- -- 1
3 1 --- --- --- -- 2
9 --- --- --- -- 0.7
2 ? 0.3? --- --
19 --- --- 2.5
24 --- ---
25 ---
28
c. COMMENTS
- The two matrices look quite similar -- except that 28 now shows
oscillations and 25 does not. This says that the subband 0
oscillations are also associated with StB/StB location rather than
antenna.
- The oscillations are much slower on 16jun09. This may partly be
because we observed at C band; we have X band data as well but
I've not looked at that yet.
- The antennas occur in two groups, associated with ODD and EVEN
slot numbers: even slot-even slot pairs show oscillations, while
odd slots do not. This is highly reminiscent of the 180d phase flips,
which are also sorted into these two groups.
Comments? Inspirations?
Michael
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