Basebands

There are eight basebands, which must be operated in pairs, because there are only four copies of the second LO module. These pairs are usually to be used to select polarization pairs, but more exotic configurations are possible. Because of this pairing, it is convenient to describe basebands in terms of baseband pairs. Note that, in electronic terms, we are talking about pairing two baseband converters from different downconverter blocks. It's implementer's choice whether you want subscripts or a separate name for each baseband command. Issue bypassed here by writing, eg, BBselectXY, wherein X specifies the member of the pair, and Y specifies the pair. The above could mean BBselectA1 or BBselectA[1] or BBselect[1,1] or BBselect[0,0], depending on how you want to name or number things.

In particular, the BBselectXY command is a problem, in that not all combinations are possible (the switch structure is flexible, but not infinitely so), and not all possible combinations are useful (eg selecting different first LO sidebands for the two members of a pair). An alternative approach would be tied more closely to the structure of the switches, so that it would not be possible to specify an impossible or even a non-useful combination.

BBselectXY = <OFF | UV | UH | LV | LH>
   ! Selects the sideband and polarization of the signal fed to the named
   ! converter.  (I suppose R,L should be permitted as well as V,H, in case 
   ! somebody has a smart idea about a quarter wave plate.)

secondloY = <second LO frequency in GHz>

phasemod2Y = <PASS | WALSH | SINE | NONE>
   ! for some purposes it may be desirable to have the second LO remove
   ! the phase modulation of the first LO and impose a different one of
   ! its own.  (In particular, with the photonic LO scheme, it may well
   ! be convenient to use sines for sideband suppression and Walsh functions
   ! to cancel sampler offsets).  PASS lets the first LO modulation pass
   ! unchanged; NONE removes it, and does not impose a different one.

phasemodrate2Y = <modulation rate in Hz>

phaseoffsetY = <arbitrary phase in degrees to be added to 2nd LO phase>

secondlodopplerY = <OFF | SET | SETREF | TRACK | TRACKREF >
   ! SET = set LO at scan beginnings
   ! TRACK = track continuously during scan
   ! REF = set LO at mosaic center; otherwise at mosaic offsets
   ! If not OFF, secondloY frequency is barycentric
   ! dopsource (above) would also apply here
   ! if first LO doppler is not OFF, this only applies difference


phasecalibY = <OFF | AUTO | CALIB | POWER | WVR>
   ! source of phase calibration information
   ! AUTO indicates using current observations to phase up immediately
   ! (called Autophasing at the VLA) - principal use for VLBI
   ! CALIB extracts phases from last calibrator scan, again principally for
   ! VLBI
   ! POWER uses power level from receiver in use with a coefficient to convert
   ! to phase due to water vapor
   ! WVR uses a special water vapor radiometer being considered
   ! More codes, and combinations of the above, may be needed

deltaDelayXY = <Delay in nanoseconds to add to system offset value>

FIRfilterXY = <Code for set of filter tapweights>