[evlatests] [Fwd: ph-slope]

Fri Jan 5 10:42:15 EST 2007

Thanks Bryan, I was guessing it referred to AC/BD pairs but I just wanted 
to make sure.

I suspect this points to the DDS chips and/or MIB firmware in the L302's. 
Or possibly the higher level software configuring them?  Unless I'm missing 
something, it also only seems like a transition problem.  Would it affect 
VLBI observations with the EVLA in the future? Is this really something we 
need to spend a lot of time looking for at this time?  Matt and Pete 
probably have plenty of other things to do right now.

Jim

At 08:22 AM 1/5/2007, Bryan Butler wrote:

>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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