FREQREF is the LO sum to use for the baseband channel in MHz. FREQOFF will be added to it. The LO sum is the sum of the FIRSTLO plus or minus, depending on sideband, the BBC synthesizer setting BBSYN. A FREQ specification in the SCHED keyin file will override FREQREF plus FREQOFF, as long as the BBC settings stay within range and there are no sideband changes.
To set pulse cals properly and to deal with non-VLBA stations correctly, it is best to know all of the LO sum, the first LO, and the BBC settings, along with all of the required sidebands. Strictly speaking, valid VLBA schedules can be written without knowing all these, but SCHED can be made significantly simpler and more reliable if all this information is required. Not all of these values need to be specified since they are related. There are four possible complete combinations, and if more values are specified than necessary then a consistency check will be done. These complete combinations are:
1. FREQREF plus FREQOFF, FIRSTLO, NETSIDE
2. FREQREF plus FREQOFF, FIRSTLO, SIDEBAND
3. BBSYN, SIDEBAND, NETSIDE, FREQREF plus FREQOFF
4. BBSYN, SIDEBAND, NETSIDE, FIRSTLO