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Round Trip Phase Monitors (RTPM)

  The original design of the RTPM was developed as part of the VLBA project. This description is just a summary of the information available about the design and performance of the RTPM. See Weimer [6] for more information.

The purpose of the RTPM is to measure the changes in the delay of the signals transmitted from the timing center to the telescope and the equipment room. The delay must be known so that the phase differences in the signals can be accounted for. The measurement is made by reflecting some of the 500 MHz signal back to the timing center with sidebands 1.953 KHz away, which is mixed with a 500MHz + 2.083 KHz signal at the timing center. This results in a 130.2 Hz signal that is compared with a locally generated reference, and exhibits the same phase change as the original 500.001953 MHz signal. The measurement is repeated and logged by the M&C system every 10 seconds.

John Ford
Fri Mar 13 16:06:38 EST 1998