The PCG consist of electronic that accepts a fiberoptic delivered
100MHz stable reference from the DSCC Frequency and Timing Subsystem (FTS)
and produces calibration signals at S- and X-band and produces a 20-MHz
phase signal in its host feedcone. The PCG removes phase variations due
to unstable electronic components in a receiving system. In order to optimize
the calibration signal is injected as far forward in the receiving system
as possible. Calibration tones , produced by the PCG, travel together with
the incoming signal through the entire receiving system. Hence, the calibration
tones are affected by the same phase shifts as the incoming signal. For
example, in Very Long Baseline Interferometry (VLBI), the incoming signal
could be from an extragalactic radio source (a quasar). The signal from
the quasar is recorded together with the calibration tones on tape at the
end of the receiving system. By generating a signal at baseband and correlating
it with the recorded PCG signal, the phase shifts that occurred in the
receiving system can be subtracted out of the signal data from the quasar.
In addition, the PCG in conjunction with the Digital Tone Extractor (DTE)
provides a real-time VLBI system monitor. This monitor checks the entire
data path in real-time as a safeguard against instrumental breakdowns and
operator errors.
Frequency
Standards Laboratory
VLBI Science
Phase
Calibration Generator (PCG)
i
Frequency and Timing Advanced Development Group