[evlatests] 400 ms tests last night

[Walter Brisken]

Responses to Barry's questions:

On Fri, 12 May 2006, Barry Clark wrote:

> I guess I'm behind on these glitches; I wasn't aware of them.  How often
> do the dropouts occur?

Every IAT 10 second, synchronously with the reseting of phase "jags".  The 
amplitude drop is rather consistent, drop to drop.

> Do A/C and B/D glitch at the same time?

Yes

> Do they occur only on 10s boundaries?

Only.

> Is it really only an antenna 13 problem?

Only on antenna 13, to my level of sensitivity.  Given the band 
independence of the A/C case I can rule that out for 18 A/C, but if 18 has 
the B/D problem on A/C I might not be able to see it if it is weaker 
there.

> This looks like a delay rate error, about 12 ps in 0.4 sec.

This would imply a phase wind, right?  None such seen.

> This number is
> not easy to understand.  The natural fringe rate is much higher, about
> 700 ps in 0.4 sec.  But if it isn't related to the fringe rate, why only
> antenna 13?

I wish I knew!  We discussed some possibilities: L305 problem, 
synchronization over longer distances, particular walsh code (ie does 
it change at the 10 second tick?), waveguide blanking, ...

> Walter, are there phase glitches what one would expect from a rate error
> (ie 45d for an amplitude of 60% of full)?

No phase glitches.  Drops occurs simultaneously with the 'jag' resets. 
At the integration that we see a dropout the phase is jumping between the 
old and new jag state and the visibility phase is somewhere in-between 
the two.  This is true of all baselines.  There are no phase 
phenomena that appear to be related to this problem.

> Without any phase glitches, one
> would think in terms of a walsh function error for one cycle, but that
> should drop to 75%, not 60%.

We thought about that too.  Likewise with the B/D side, there isn't an 
easy way to lose 2% amplitude this way.

-W