[evlatests] [Fwd: ph-slope]

[Rob Long]

Explain the ant 23 jumps.  Did we see these jumps between the IF pairs 
on ant 23, or did we see similar jumps between IFs 1 & 2 compared to the 
rest of the EVLA antennas?

Rob

Vivek Dhawan wrote:
> 		         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