[evlatests] antenna 24 AC - update

[Jim Jackson]

Sorry, I lied - we  don't have the spare L302 rev C needed to do this right 
now. We need to get one ready and will make the change when we can.  We 
could break antenna 17 to fix it but since that antenna is ready for first 
fringes as of today and 24 is still useable the way it is, Rob and I both 
decided we would rather not do that.

After checking the L302-1 in 24 and thinking about this a little, I think 
this could be a symptom of the AGC instability we have seen in some L302's 
- this particular module is still operating using the AGC based firmware 
(rev 0.201). The latest firmware (rev 0.21 and up)  eliminates the AGC and 
sets output level using a lookup table. The synthesizers we have done that 
to seem considerably more stable in output level stability  and lock 
reliability.  Unfortunately we can't just upload the MIB new firmware to 
the module - it has to come to the lab and be run through a calibration 
procedure to generate the lookup table.

We will advise when we make the change

Jim


At 11:03 AM 1/10/2007, Jim Jackson wrote:
>Rob and Mike are going to change out the L302-1 in Antenna 24 with a spare
>today. Hopefully, this will improve Antenna 24AC.  Per Ken's request, they
>will look at the SD variations before and after.
>
>This will also result in a swap of the L302-1 in Antenna 13 to get the
>proper revision B synthesizer back into it. The revision C spare needed for
>24 is currently running in 13 with an adapter cable for the power. We
>wanted to get that swapped back anyway so this is as good a time as any to
>do it.
>
>Jim
>
>At 10:18 AM 1/10/2007, Ken Sowinski wrote:
> >Antenna 24 has not been a very good pointer.  In examining
> >the pointing run from last night I found that many trials
> >were rejected because the estimated beam width was narrower
> >than all other antennas by about 8 percent.  This has happened
> >in previous pointing runs as well.
> >
> >Focus seems OK, in any case it should only make a broader beam.
> >However in examining focus I saw that the AC amplitudes are
> >not very stable.  There are changes of about 5 percent from
> >record to record in AC while BD is stable.  There are similar
> >variations in the IF A synch detector, but less clearly in IF C.
> >Again, both B and D synch detectors are stable.
> >
> >I don't see how to consistently make a narrower beam from this
> >behavior, but it certainly can make the pointing unreliable.
> >Given the symptoms: only A and C and at X band, it just about
> >has to be the AC L302.  If I had to guess, it might be related
> >to phase switching.
> >
> >If the SD variations are indeed linked to the amplitude
> >variations than it should be easy to see in real time whether
> >swapping L302s helps, even if we can't see correlated data.
> >
> >Further information might be gleaned by:
> >1.  Using 0.4 sec integration time to see the temporal behavior.
> >2.  Turn off phase switching.
> >3.  Convince the pointing solver to only use BD data.
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