Frequency offsets and phase


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.)


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.

Modified on Tuesday, 29-Jan-2013 13:57:47 MST by Gustaaf van Moorsel