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I was unable to attend Bryan's meeting yesterday, dealing with RQ
adjustments. Based on some feedback from those in attendance, I
think there may be some misunderstanding remaining on the issue of
RQ settings. I write below (following some discussion with Ken to
ensure my understanding is correct), some comments on the need, or
not, for adjusting RQ settings.
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1) The RQ adjustment attempt to optimize the digital power
within each spectral window. Optimize here means to maximize the
SNR, following the prescriptions found in TMS (for example). In
practical terms, this results in the 4 bits (16 levels) of the
output requantizers being nearly fully populated, so that a small
fraction of the noise samples fall outside the 16-level range.
These over and under range values are not ignored -- they are
counted as maximum or minimum, leading to underestimates of the
total power. <br>
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2) Without SPW dependent RQ adjustments, a single value is used
for all spectral windows. Spectral windows at the edge of the
passband normally have lower than average power, while some in the
middle will be higher.
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3) Because the optimum SNR setting fully populates the 16
levels, it is far better to be low in power than high. Each bit in
the sampler is a factor of four in power -- losing 1, or even 2,
bits is hardly critical. However, being too 'hot' by a factor of 4
in power is bad, and will lead to large correlation errors, due to
overflow. (This is easily seen in my P-band tests, using Cygnus
A).
Being too low is not entirely without consequence either -- the 'van
vleck' correction will change depending on how many sampler levels
are actually used.<br>
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4) So, by how much does the SPW power vary? There are three
cases to consider:
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a) Variation between sources or sky positions. The strongest
isolated sources -- Cas A and Cygnus A, each cause the total power
to rise by about a factor of up to 5 at P, L, and S bands. This is
slightly more than one bit -- and hence of modest concern. (It is
why, when I observe these sources as part of a run, I set up the
system attenuators and RQ on Cygnus A, arguing that being low on the
colder objects is better than being too high on Cygnus). More
significant is the variation in sky background power (temperature).
At low frequencies, the sky dominates system power. The variation
in power easily exceeds an order of magnitude between the galactic
center and the coldest sky. Factors of two between 'ordinary'
sources, or between calibrators, will be standard. This is of
significant concern.
<br>
<br>
b) Variation across the passband. To judge this, I looked at
some real data. Note that examination of the BP table is not
sufficient to judge this issue -- the SNR can vary dramatically
across the bandpass (especially at P and L bands),and the BP
solution sees only the correlated power. I used the digital power
(PSum) monitor for this (as, unfortunately, the autocorrelations had
been removed by my processing).
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At LBand, the maximum variation I could find, between SPW
8 and 16, was about a factor of 5. Most antennas had the range
within about a factor of two. All these ratios are for the same
source -- this does not include the on-off Cygnus A problem -- which
boosts the maximum range to about a factor of nearly 100. Thus, if
you're not observing Cas or Cygnus, the current arrangement of not
applying SPW-dependent RQ corrections should be safe.
<br>
But at P-band, the situation is much worse. The <b
class="moz-txt-star"><span class="moz-txt-tag">*</span>typical<span
class="moz-txt-tag">*</span></b> range between SPWs for a single
antenna was a factor of 10. The extreme between antennas was a
factor of 100 -- for the same cold sky. At this band, (and at
4-band) we really must employ RQ corrections for every separate
observation.
<br>
<br>
c) Variation in the BP, due to strong RFI. A SPW with strong
RFI will certainly overfill the 4-bit RQ window. For some SPWs,
notably SPW3 in S-band, it does more than overfill the 16 levels --
it can overflow the accumulators of the correlator, causing complete
rubbish in the spectrum on ~ 50% of the scans. Implementing the RQ
correction (scan by scan here is recommended, since the RFI is so
dynamically variant, due to the sidelobes) <b class="moz-txt-star"><span
class="moz-txt-tag">*</span>should<span class="moz-txt-tag">*</span></b>
help both overflows (but I'm not promising it will make everything
better).
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5) Finally -- a note on my experience with utilizing RQ
corrections. For the P-band holography, we employed a single RQ
setting at the beginning. This works fine, as we were not changing
source or tuning. (Note that any change to the correlator will
'lose' the RQ settings).
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For my calibrator model/flux density work, RQ resetting was
employed for <b class="moz-txt-star"><span class="moz-txt-tag">*</span>every<span
class="moz-txt-tag">*</span></b> scan at P, L, and S bands. It
was arranged that this be done 20 seconds after the first antenna
reached the source. (There's apparently a python command for
this). The 20 seconds was to allow the system to setup, primarily
to move the subreflector. This procedure worked beautifully well.
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