Phase Cal

The phase cal provides very narrow pulses at the antenna at a 1 MHz rate which is equivalent to a comb of frequencies with a frequency separation of 1 MHz. Energy therefore falls within the input passband of each baseband converter and, if the delays are all equal, the correlation spikes produced from each converter will be coincident, i.e. will add coherently. The phase cal thus provides a way to check that the baseband converters have equal delays and that the microwave front end/IF amplifier chain is linear in phase vs. frequency.

A 5 MHz signal is sent from the control room unit, through a long cable, to the antenna unit. At the antenna unit, a tunnel diode provides a 5 MHz pulse train. When commanded, four of every five pulses are attenuated, so the output of the antenna unit becomes a 1 Mhz pulse train.

The phase cal is also a delay cal; the time shift in the correlated output indicates the total delay through the RF/IF/BBC chain. Of course one must know the delay in the long cable used to drive the antenna unit of the cal, and the changes in this delay due to temperture changes, cable wrap, etc. For this purpose, the cal includes a calibration system: A hybrid in the antenna unit is slightly mismatched to produce a reflection coefficient of 10%. At the control room unit, the phase of the reflected 5 MHz signal is measured with respect to the 5 MHz input signal. This phase measurement is improved by mixing both signals down from 5MHz to 25 Hz, so that a given phase difference, as a fraction of the period, becomes a much longer time difference, which can be measured more precisely.

In a further refinement, the reflected signal is first cleaned up (bandpass filtered) by a phase-lock loop. Note that this is a "true PLL" in that the phase detector error signal drives an electronically- controled phase shifter rather than a VCO. (This is possible because the reference and the "signal" are always at precisely the same frequency.

In another refinement, the reflected signal is phased modulated by a 5 KHz square wave (0.1 ms period). The reflection coefficient is (47-50)/(47+50) = -.03 for 0.2 ms, then (47+4.7- 50)/(47+4.7+50) = .017 for 0.2ms, etc.

Note that the cable carries several signals: 5 MHz to the antenna unit, a 5kHz modulation square wave to the antenna unit, a dc signal to slect 1MHz or 5 MHz, and a modulated 5MHz reflection back to the control room.