There are 16 pulse cal detectors in the VLBA formatters, each with the ability to detect signals from 2 bit streams. Each can be set to detect a tone at a different frequency at any multiple of 10 kHz in the baseband. PCALFR1 and PCALFR2 are arrays of 16 frequencies in kHz for the 16 detectors. ACTUALLY, UNTIL A FORMATTER UPGRADE IS COMPLETE, THERE ARE ONLY 8 DETECTORS. If a frequency of 0 is given, the detector will be used to count bits in each state. PCALFR1 applies to the first channel of the detector, while PCALFR2 applies to the second; they need not be the same. SCHED is capable of setting PCALFR1, PCALFR2, PCALXB1, and PCALXB2 automatically, and this is probably the option that most users should take. SCHED will use the automatic settings if PCALXB1(1) is not set. If the PCALFR and PCALXB values are defaulted, they will be adjusted on a scan by scan basis for any changes in observing frequency, including those requested by FREQ or DOPPLER in the SCHED keyin file.
The default will be to set up all available channels to detect, using sign bits only, first the tone near the lower edge of the band in each channel, then a second tone near the high edge of the band in each channel, and then state counts (sign and magnitude for 2 bit data) for each channel, then a third tone which is (usually) 1 MHz away from the first tone being detected. As many of these tones will be detected as possible up to a total number of detected signals that does not exceed the number of available detectors.
There is usually no problem with setting the PCALFR and PCALXB parameters when PCAL is off, so there is usually no need to turn off this default. The one exception would be if the frequencies are being switched rapidly in such a way that the PCALFR frequencies are changing. Each time they are changed, the formatter is reconfigured, which blocks data to the recording for about 8 seconds and throws the VLBA correlator out of sync. Total data loss is at least 16 seconds and can be more if there is a speedup factor greater than one or if the correlator has a hard time resyncing, which happens maybe 10-20either specify the pulse cal detector parameters and they will be kept constant or make sure the default pulse cal setup doesn't change (keep constant the number of baseband channels, the channel bandwidth, and the pulse cal tone frequencies in the basebands (don't change the kHz part of the frequency).
Detection of Phase cal is not currently (Oct 2001) implemented on the PCFS, although the VEX file has a way of specifying the desired detection. On MkIV telescopes this requires the installation of additional hardware, but even on PCFS controlled telescopes with VLBA hardware this is not yet supported.