[evlatests] Modcomp-Free tests, Friday afternoon (25 may)

[Ken Sowinski]

On Mon, 28 May 2007, Rick Perley wrote:

Some comments on Rick's 'common' problems.

>    1) The EVLA x VLA incoherence in IF2, noted at L-band in prior tests
> is shown here to be present at all frequencies.  Ken has shown this to
> be an incorrect frequency setup for the VLA antennas.
>
>    2) The phase in IF 'D' (LCP in IF 2) *only* is slowly rotating at a
> rate, typically one turn in a minute, which is clearly proportional to
> frequency and to baseline length.  It looks like a miscalculation of the
> fringe rate.  But also note that this slip is not seen in any other IF.

The IF D Fluke is misset at (at least) 12.5 MHz BW; this can
explain both complaints.

>    3) Antenna 14, in IF 2 (IFs B and D) has two different phase states,
> separated by about 50 degrees.   The antenna spends about 75% of the
> time in one, 25% in the other.  The transition between them takes very
> nearly exactly 0.8 seconds, as a single intermediate phase value is
> *always* seen midway between the two states.  The transition is always
> identical in both polarizations.

Is this the L305 to L302 timing problem fixed by adjusting cable
lengths?

>    4) Antenna 24, all IFs, show a phase which is 'all over the place'.
> The antenna stays in any one phase state for only a few seconds, then
> abruptly transitions to another.  The transition seems to take 0.8
> seconds, as an intermediate phase point is always seen between the
> adjacent stable states.  Unlike antenna 14 (above), the phase changes
> appear random.  However, all four IFs 'jump' at the same time with an
> identical phase change.

Vivek reports that the RTP meter is misbehaving.  I disabled the
SW connection to the RT phase correction this morning.

>    5) There were no fringes from IF#2 in Antenna 19.  (I think this is
> a known problem).

Yes.