[evlatests] AM889 and phase slope

[Vivek Dhawan]

Hi Ed,

This is a mystery feature of the EVLA. here is a previous
report (edited a bit).

when the IFs are separated by more than a few MHz, where
few is not well known but believed to be in the single
digits, the 2 IS have a slope w.r.t each other. it
DOES NOT scale with frequency, to my knowledge.

V.

-------- Original Message --------
Subject: [evlatests] [Fwd: ph-slope]
Date: Thu, 4 Jan 2007 23:54:07 -0700 (MST)
From: Vivek Dhawan <vdhawan at nrao.edu>
Reply-To: vdhawan at nrao.edu
To: evlatests at nrao.edu

       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.

	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