[evlatests] [Fwd: ph-slope]
Jim Ulvestad
julvesta at nrao.edu
Sat Jan 6 16:27:27 EST 2007
Yes, as Bryan said separately, IF 1 means AC, IF 2 means BD.
I thought that was very common usage; sorry for creating
any confusion.
Jim
> 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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