[evlatests] L302 update

[Vivek Dhawan]

The 28 uHz has changed - the acupuncture point has been located.

However, the old behaviour was that IF 2 twisted at a steady 28uHz
w.r.t IF 1, on EVLA antennas only, and only when compared to the VLA.
i.e. EVLA-EVLA was nice and flat. The twist could be made 0 by setting
the 2 L302's to values within 1-2 MHz of each other. For any wide
separation the twist appeared and was always the same.

Now, _both_ IF 1 & 2, on EVLA to VLA baselines, twist at the same rate.
(This is for IF's 500 MHz apart, I have not tried to chart the behaviour
with frequency difference). This is still invisible between EVLA
antennas, only seen w.r.t. VLA.

V.


Ken Sowinski wrote:
> Hichem found that the MIB does not always do floating point arithmetic
> correctly.  At least when computing the DDS frequency command it will
> sometimes get the bottom few bits wrong.  We have reformulated the
> computation so that this error is not triggered, at least not in the
> test cases that were examined.  With the hope that this may go some
> way to explaining the 20 microHz frequency offset that has been
> reported we have updated all the L302 MIBs with SW that avoids
> triggering this error.  By way of explanation I note that a
> somewhat non-standard technique was used to place the 48 bits
> required by the DDS in the right place of a double so that they
> can be easily sent out the SPI bus.  This technique was tested
> on other machines and has worked flawlessly.
> 
> Reports of frequency offsets, or other L302 related problems, or
> lack of these are welcome.
> 
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