[evlatests] [Fwd: ph-slope]
Jim Jackson
jjackson at nrao.edu
Fri Jan 5 10:17:51 EST 2007
Can you please clarify what you are referring to as IF 1 and IF 2. Does
this refer to individual antenna IF's (A,B,C or D), the AC and BD pairs,
or RCP and LCP polarizations?
Jim
At 06:26 AM 1/5/2007, Jim Ulvestad wrote:
>Just for completeness, I looked carefully at the difference between
>IF 1 and IF 2 for my AG730 X band data from November 24-26.
>On a given day, the difference has the same very slow drift or
>scatter on EVLA antennas as VLA antennas, a peak-to-peak range
>of about 10 degrees over 10 hours.
>
>The only difference is that the EVLA antennas appear to jump
>40-60 deg. betweeen Day 1 and Day 2, whereas the VLA antennas
>stay constant. I assume that this is because a VLA antenna
>was the reference antenna, and the relative EVLA-VLA phase
>doesn't come back to the same place on the second day that it
>was on the first day. Since who knows what happened in between
>the two runs (14 hours between the end of one and the start
>of the next), this is hardly surprising.
>
>Jim
>
> > Phase jumps and Phase slopes / 2007 Jan 4.
> >
> > A one-hour test file was run at L-band, for reasons below.
> > First some minor notes:
> >
> > A. No jumps of the global sort were seen on the EVLA, but
> > EA23 alone did jump by 170+_2 deg. The jump scales with
> > frequency, comparing IF1 and IF2.
> >
> > B. Many VLA antennas (7,8,10,11,12,19,22,25,27,28) had small
> > jumps of ~10deg, at mostly unrelated times. Only a
> > small fraction of data was bad - the jumps were mostly
> > very short. This seems unusual to me, but unless it
> > persists I'll skip it for now.
> >
> > C. The main purpose of the test was to poke at an un-
> > explained feature that persists: The two IFs on the
> > EVLA have, SOMETIMES, a phase slope with respect to
> > one another (when referenced to a VLA antenna).
> > This is a 'global' phenomenon, i.e., the phase of
> > (IF1-IF2), on any VLA to EVLA baseline, has the same
> > slope. It could be on either array, but my money is
> > on the EVLA.
> >
> > I have mentioned this before, but Jim Ulvestad's
> > imaging result prompted me to write it up in a bit
> > more detail. I don't think it explains what he found,
> > i.e., the EVLA added in makes a 4 sigma VLA detection
> > 20 microJy, to degrade to 2-3 sigma (I forget the exact
> > number).
> >
> > The facts so far:
> >
> > o IF1 phase drifts at 28deg per hour compared to IF2.
> >
> > The exact number may be 25-30 deg. It is close to
> > 2 turns per day, or 23 microHz. Half that value has
> > been seen, just once.
> >
> > o At L band, it is always present at the default settings
> > of 1465 and 1385 MHz.
> >
> > It reverses sign when the IFs are interchanged in
> > frequency.
> >
> > It disappears (unmeasurable, < 1deg/hr) when the IFs
> > are close together (1421.46 & 1420.28 MHz).
> >
> > It is unchanged (still ~28deg/hr) when the IF's are
> > set wide apart 1341 and 1666 MHz.
> >
> > o At C band it was
> > ~0 (i.e. <1deg/hr) on June 10th.
> > ~25 deg/hr on July 13th.
> > ~12 deg/hr on Oct 4th.
> > ~0 deg/hr in recent December data.
> > All at the standard settings 4885 and 4835MHz.
> >
> > o At X-band it is nearly always zero at the standard
> > settings 8435 and 8385 MHz. Except on Oct 4th,
> > when it was 12 deg/hr like C band on the same day.
> >
> >
> > I found the slopes while looking at the round-trip
> > fiber delay change, which causes phase slopes of the
> > same order of magnitude on individual IF channels, but
> > which should scale with frequency difference. The fiber
> > slope varies with temperature, and changes sign every
> > day, whereas the IF differential slope is much more
> > linear, at any time of day.
> >
> > I do not know if the IF differential reveals a roundoff
> > or truncation in a frequency calculation. In normal use
> > it is removed if the IF's are calibrated separately; even
> > if the IF are combined, it is a 15 deg maximum error (for
> > calibration every 30 minutes, typical at L-band).
> >
> > Vivek.
> >
> >
> > _______________________________________________
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> >
>
>
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