[evla-sw-discuss] station board phase calculations (supporting 75 MHz observations)

[Ken Sowinski]

Below is a fragment from an earlier note.  Some progress has been made
along the path described.  Allowing more resolution for f_shift is seen
to be part of this problem.

On Thu, 6 Jan 2011, Ken Sowinski wrote:

> 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.

The change to the executor has just been tested and commited.  After
the next executor update we may introduce the station board changes.

When introducing the term in phase to allow finer resolution of
f_shift I found an unexpected dependence on net side band caused
as a side effect of transmitting net sideband in the delay packet.
The changes to the station board software, when accomplished, will
correctly support both VHF observations with a negative signed sum
of LOs and the use of f_shift was 0.1 Hz resolution.