[evlatests] More mysterious results from stepped power 3-bit tests
Rick Perley
rperley at nrao.edu
Thu Sep 9 13:26:22 EDT 2010
Late yesterday afternoon, Ken repeated the 'stepped power' test with
the 3-bit samplers. The idea is to step the power levels to the sampler
in 3-db steps, for each of which he observed the calibrator 3C286, and
an adjacent noise field.
The power levels chosen were -38 dBm through -26 dBm -- the output
analog power meter in the T304 modules was used to measure the power,
from which the appropriate output attenuator level was chosen. The
input attenuator in the T304 was kept fixed. (This is my understanding
of the experiment -- Ken should correct me if I am wrong). All
antenna/IFs were treated identically -- we should then expect (or at
leat hope) that 'downstream' measure of the power should be quite similar.
I give below some curious results from this test.
A) Power increments.
The output attenuators were adjusted in 3 dB steps. If all is
linear, I should see 3 dB steps in the output cross-correlation power,
and in the the 'autocorrelation' spectra. But this doesn't happen --
it's not even close. I give below a table of the power increments, in
dB, as judged from the CALIB solutions (essentially, the step in the
fringe power observed, factored out by antenna/IF). There are five
power levels, so we have four steps.
Power levels 12R 12L 15R 15L
22R 22L 28R 28L
---------------------------------------------------------------------------------------------------------------------------------
-38 -> -35 dBm 1.7 dB 2.7 3.2
4.4 2.4 3.4 4.0 2.8
-35 -> -32 2.7 2.5 2.4
2.2 2.6 2.6 3.1 2.8
-32 -> -29 2.4 1.8 1.8
1.9 2.3 1.7 1.7 2.0
-29 -> -26 0.5 0.9 1.3
1.6 1.3 1.0 1.9 1.3
----------------------------------------------------------------------------------------------------------------------------------
It's clear that as the power rises by steps of 3dB (as judged by the
analog power meter), the increment in fringe power becomes less, and in
some cases much less, than the expected step. All antenna/IFs share this.
Ken explains that this is likely due to the 4-bit requantizer gain
following the station board -- there is no attempt to adjust its levels
for the change in input power level. He also notes that at some point
the output attenuator reaches 0 dB, and can go no lower, so we will
(eventually) hit a ceiling. I personally think the first explanation
better explains what we see in the table above.
I checked the power increments as judged by the autocorrelation
spectra. They agree with the general trends noted above -- power
increments as judged by the correlator roll off as the input power rises.
B) Power Levels.
If the T304 power meters are good and reflect real power, then we
should expect to see about the same spectral power levels as judged by
the autocorrelation spectra, and see the same CALIB gains needed to
change the cross-power amplitudes to units of real Jy. (This argument
assumes that the spectral window we are actually using reflects the
overall spectrum -- i.e. there are no major slopes in the 2 GHz-wide
spectral power, and (for the CALIB gains) that all antennas have about
the same basic sensitivity).
The range in spectral power seen in the autocorrelation spectra
varies over quite a wide range:
12A 12C 15A 15C
22A 22C 28A 28C
Spec. Pwr 0.75 1.8 1.8 1.4
1.5 2.6 1.3 2.0
The maximum range is for 22C/12A -- about 5.4 dB. This is
conceivably a spectral slope issue -- I'll check this later.
But more confusion comes from analysis of the AIPS gains needed to
convert the cross-power to actual Jy. If all antennas have about the
same SNR, then the higher power antennas listed above should have the
smallest corrections needed to return the cross-power to real
amplitudes. But this is certainly not the case ...
12A 12C 15A 15C
22A 22C 28A 28C
Correction 6.2 1.7 2.3 2.6
10.8 2.5 7.0 3.5
There should be an inverse relation between the correction values
listed above and the power levels listed higher up. I see no relation
at all! The weakest antennas as judged by corrrelation power (22A and
28A) have average spectral autocorrelation powers. The strongest
antenna as judged by correlation power (12C) also has a normal
autocorrelation power...
C) Noise changes as a function of power input.
I noted yesterday a small improvement in rms noise as the input
power rose. I give below the values for all 12 correlators for three
of the 5 power levels -- the lowest, the middle, and the highest.
Baseline/Pol -38 dBm -32 dBm -26 dBm
----------------------------------------------------------------------------------
12 x 15 RCP .280 Jy .230 .220
12 x 22 .540
.540 .440
12 x 28 .400
.34 .30
15 x 22 .54
.42 .36
15 x 28 .37
.28 .25
22 x 28 .77
.62 .48
12 x 15 LCP .23 .17 .17
12 x 22 .27
.23 .23
12 x 28 .31
.26 .25
15 x 22 .32
.24 .23
15 x 28 .33
.26 .24
22 x 28 .43
.35 .30
-----------------------------------------------------------------------------
Despite the evident saturation of the requantizer, there is notable
improvement in nearly all noise levels. Only one correlator -- 12 x 15
in LCP, is even close to the expected level of 0.15 Jy.
D) Autocorrelation Curiosities
The 'parallel' autocorrelations (RR and LL) in general look
reasonable, and change as expected as power levels change (albeit not
with the 3 dB step we expected ...). But there is one notable effect
seen in early all of the spectra -- when the input power reached a
certain level, all spectra became negative! The threshold is the same
for all antenna/IFs. The shape of the spectrum remains more or less
correct as this threshold is passed. Evidently an overflow problem.
The CW 'tones' are visible in two of these 12 antenna-IF spectra:
15R and 28L. (This doesn't mean these are absent in the other
antenna-IFs -- we have only one spectral window in these tests).
One other item which is less hard for me to understand is the
'cross-power' autocorrelations (RL and LR). For three of the four
antennas, the 'cross auto' spectra are pure sinusoids of very high
amplitude and zero mean. Clearly a single lag has a very high
amplitude. The spectral frequency of the sinusoid is the same for RL
and LR, and different for the three antennas which show this. The
antenna which does *not* show this is 15. The 'cross-power' spectram
displayed is enormously negative and shows no imprint of the bandpass
(i.e. power goes up at the edges, rather than down ...). Barry, Ken,
and Michael all claim to sort-of-understand all this. I'm not too
embarrassed to admit that I do not ...
This is about all I can squeeze out of this ...
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