IFCHAN is used to specify the IF channel attached to each baseband channel for VLBA observations using VLBA data aquisition systems. For the VLBA, it should be A or C for 20, 13, or 6cm. For 90, 50, 4, 3, 2, or 1.3cm, it should be B or D. A and B are RCP; C and D are LCP. If DUALX is specified, both B and D are RCP. As an alternative, R or L can be specified. Then if RCHAN and LCHAN are specified in the setup files, IFCHAN='R' will be replaced with RCHAN and IFCHAN='L' will be replaced with LCHAN. If a bad RCHAN or LCHAN value is specified, SCHED will then complain about a bad IFCHAN specification.
1N, 1A, 2N, 2A are allowed values for MkIV telescopes. See BBC and the frequency catalog parameter IFNAME for more details.
The ``geodetic'' wired VLBA systems (all not controlled by VLBA software) have restrictions on their IF assignments because the IF distributors only provide enough signals from each IF to feed 8 BBCs while these systems typically have 14 BBCs. The systems are wired so that BBCs 1 and 2 can see all 4 IFs, BBCs 3-8 can see A and C, and BBCs 9-14 can see B and D. In addition, the racks can be switched so that they provide one bit samples from all 14 BBCs or 2 bit samples from the first 8. SCHED understands all this and should give the right default assignments. This is one of many reasons why, as noted below, it is best to let SCHED set many parameters including IFCHAN.
It should now be normal not to specify IFCHAN. SCHED will use the requested POL and the frequency catalog to determine the correct IFCHAN. If IFCHAN is specified as ``R'' or ``L'', and RCHAN, LCHAN, and POL are not specified, SCHED will get the obvious polarization channels, although use of POL is recommended.
At the VLA, there is now (as of July 97) a computer controlled patch pannel that routes the VLA IFs from the correlator sum ports or any of several single antennas to the VLB rack IFs. The mapping is now different than it used to be. Now VLA IF A goes to VLB IF A, B->B, C->C, and D->D. The VLA computer selects which antenna or sum port to use depending on the mode and on the reference antenna given by the operators. Nothing is required in the schedule files to specify the mapping explicitly. The switch is very general so other mappings are possible, but the above will be the normal setup.