Description
Early 2008 it was reported that when calibrator and target were
observed at slightly different frequencies, phase transfer from
calibrator to target did not work - the resulting images were highly
corrupted.
VLA and EVLA antennas all showed a change of phase between 2 settings
with small frequency offset d_f, of
d_phi = 2pi * L * d_f / c,
where L is the electrical path length in LO transmission to the
antenna pad. A baseline formed by two antennas equidistant from the
LO distribution point has no phase jump. Celestial geometry (w term)
is not relevant, but L is of the same order of magnitude.
So, for example, for a 4kHz difference between calibrator scan and
target scan (e.g. on the same source, geometry unchanged), the phase
difference is 50deg at the end of B array (EVLA, glass fiber) or
~30deg for VLA (air).
How the VLA and EVLA differ
The EVLA does the fine tuning at the antenna (L302) using a reference
that has been distributed over fiber. The VLA does the Doppler tuning
in the Flukes, not in the antenna, and the Fluke reference signal is
not subject to the distribution path length. In the MODCOMP days, the
round-trip phase correction for changes in the LO path was done by
carefully excluding the Fluke part. Now it includes the Fluke and so
the VLA has the same behaviour as the EVLA, and is worse than the old
VLA system. (If cal and target are at the same frequency, this problem
goes away.)
Solution
The cause of the problems was traced back to the executor, which was
fixed. The new executor was deployed on 9 April 2008, at 0900 MDT,
and tests showed the problem had been successfully solved.
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National Radio
