[evlatests] [Fwd: ph-slope]

[Jim Ulvestad]

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.
>
>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>