[evlatests] ea09 Antenna Gain Pulsations

[Paul Demorest]

Rick,

I think I have noticed this same effect in some pulsar/FRB tests over 
the past few months, although I didn't realize it affected only a single 
antenna.  However, I just took a closer look at some 5ms time resolution 
autocorrelation data from the FRB project and it does indeed show up 
only in a single IF of antenna 9 and nowhere else.  I don't know what 
causes this, but here are a few more bits of info:

1. The periodicity is really 4 Hz, not 2 Hz, and there are just two 
states.  It probably looks like 2 Hz because you were using 100 ms time 
resolution for this (right?).

2. This has been present for a while, going back to at least Sept 2014.

3. It only seems to occur with the 8-bit sampler/signal path, not with 
3-bit.

-Paul

On 2015-03-19 16:21, Rick Perley wrote:
> I reported yesterday on a rapid gain variation in ea09, seen in (at
> least) L, C, X, and Ku bands.
> 
>     From the ACU/OTF tests done today, these rapid gain variations are
> still present.
> 
>     Ken asks if these are antenna-based, or are found only on some
> baselines.  The answer is:
> 
>     *The rapid variations are seen equally on all baselines*
> 
>     I attach a new plot to illustrate the problem.  This plot is in 
> power units.
> 
>     Key points:
> 
>     1) It's antenna based.
>     2) It's in one IF channel only.  (At X-band, it's IF 'D').
>     3) It has a *perfect* 0.5 second period.   Even after many
> minutes, the pattern seen in the attached plot remains.
>     4) There are four power states, each stable.
>     5) The Pk to Pk power variation is 20% -- about 1 dB.
>     6) There is no phase effect whatever.
> 
>     Fortunately (I suppose), the fact that the period is so close to
> 0.5 seconds explains why this has not been seen in the past -- nearly
> everybody uses a multiple of 1.0 second averaging.
> 
>     Any ideas out there?
> 
> 
> 
> 
> 
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