2 IF connections

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Dual polarization (and possibly multi-frequency) observations are becoming more common in EVN sessions. The limitations MarkIII/IV racks and VLBA rack impose on multiple IF connections are slightly different.

A MarkIII/IV rack accept a maximum of two IFs. Any individual VC can be manually ``patched'' to select either of these. (There is an additional restriction that the patch panel IF output connectors for both IFs have been divided into two groups, ``low'' and ``high'' band, i.e.~depending on the IF patch panel connector, a given VC can be set to ``low'' or ``high'' frequencies only.)

VLBA racks have four IF input connectors. In the original 8 BBC configuration all eight BBCs have access to all four IFs and the selection can be performed remotely from within the FS. However, most EVN stations which have VLBA racks have the so-called ``geodetic VLBA'' variation of the rack with 14 slots for fourteen BBCs required by MarkIII mode A support. In these racks the original BBCs 1--8 have access to IF channels A and C and the ``extra'' BBCs 9--14 can use only B and D. BBCs 1 and 2 are a special case, since they can access all four IFs. (Please see the diagram attached as appendix to this document.)

There are many conflicting requirements for IF connections using a geodetic VLBA rack:

  1. MarkIII mode A requires the use of external power splitters to get a single IF to all fourteen converters, for example LCP to IFs A and B and RCP to IFs C and D. Dual polarization (or dual frequency) needs two power splitters and it uses all four IF inputs.
  2. Using up all four IF inputs prevents ``tpzero'' measurements using a terminated IF input as reference zero level.
  3. Measuring dual polarization in a VLBA/MarkIV mode using BBCs 1--8 only can be done using IF channels A and C much in the same way as with MarkIII/IV racks.
  4. MarkIII modes B, C, and E can be recorded in dual-pol either:
    1. With the two-power-splitter setup used for MarkIII mode A. This allows using the same VC/BBC numbers as with MarkIII/IV.
    2. Or alternatively, BBCs 1--7 can be used with formatter ``trackform'' set to route BBCs 1--7 as MarkIII VCs 1, 3, 5, 7, 9, 11, and 13. In this case IF inputs A and C can be used in the same way as with any VLBA/MarkIV mode.
  5. Using more than two IFs in a VLBA-compatible mode is in the general case impossible with geodetic rack wiring, since IF channels B and D go to BBCs 9--14 which do not have magnitude bit samplers connected. One-bit modes could be emulated if BBC numbers 9--14 are acceptable for third and fourth IF.

In summary, the IF flexibility offered by MarkIII/IV and VLBA racks is different:

Since the usefulness of more than two IF inputs in geodetic VLBA racks is limited (see above), we could decide to always use power splitters to connect IF number 1 to A and B and IF 2 to B and D. This will however preclude the use of an ``empty'' IF input as ``tpzero'' reference level and makes the need for a formula adjustment in FS more acute. (See section tpzero .) The net result would be that any BBC in a geodetic VLBA rack could choose between two IF inputs, much in the same way as in MarkII/IV racks.

There is still the final problem of manual patching of MarkIII/IV racks. We do not have sufficient experience with MarkIII to be able to suggest a set of suitable standard patching schemes. Obviously it needs to be made clear to the proposers what kind of IF patching will be available.

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