[evlatests] [Fwd: ph-slope]

[Bryan Butler]

it usually means the AC and BD pairs.  "IF" typicaly refers to the frequency 
tuning of an RL polarization pair.  see, e.g., 
http://www.vla.nrao.edu/astro/guides/sline/current/node4.html

i'm assuming this is what vivek and jim are talking about.

	-bryan


On 1/5/07 08:17, Jim Jackson wrote:
> 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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