[evla-sw-discuss] supporting 75 MHz observations (fwd)

[Ken Sowinski]

Up until about a month ago the station boards used the net
sideband encoded in the delay packets sent by the executor
to configure the sideband flippers.  Station boards now
use the net sideband transmitted in the VCI document for
that purpose, ignoring thge information in the delay packet.
Now, in order to support 75 MHz observations we want to be
able to carry out an observation with a true SSLO less than
zero, thereby moving the 75 MHz bandpass from the subband nearest DC.  This 
will be impossible to do correctly until
we back out the encoding of net sideband in the SSLO of the
delay packet.

Here is a summary of what is done currently:
l302 is the nominal L302 frequency desired
sslo is the signed sum of the LOs desired
fshift is the Widar offset frequency for this antenna
netsideband is the net sideband for this baseband
rotatorside represents the sense of the signal that is
mixed with the L302 output.
The executor sends

(l302 + fshift) to the synthesizers; and sends the pair

(sslo*netsideband, fshift*netsideband*rotatorside)

to the station boards.

The station board takes fabs(sslo + fshift) to get the
frequency used to convert delay to phase.

Two changes are necessary.  First, the executor should
send (sslo, fshift*rotatorside) to the station boards.
Second, the station board should use (sslo + fshift),
leaving the sign intact.  The two changes need not be coordinated, but must be 
effected in the given order.

I write this note so I can be informed of silly lapses in logic, and to solicit 
suggestions for testing. Obviously, we must be sure that maps and frequency 
scales
are the right way around.  More subtle, is to be sure that fshift is added to 
the SSLO in the station board
with the right sign in all cases.

I understand that Steve wants to do some tests of the 75 MHz system during the 
move to B array.  What we are
doing now will be fine for his purposes.  We can put
the 75 MHz in subband zero and live with it, or use a
negative SSLO and accept the fact its magnitude  is used in the station board.