[evlatests] More on Switched Power 'Saturation'

[Rick Perley]

    ... but the trouble with this hypothesis (non-linear T304 somehow 
triggered by LO settings) is:

    1) We've shown pretty clearly that the apparent non-linearity in 
switched power is *not* seen in the visibilities.  This should direct 
attention at the mechanism for detecting the switched power,
          and
    2) Numerous tests done on the antenna with the same power meter (but 
admittedly at somewhat different frequencies, and at the input to the 
T304) show no linearity over a very wide range of input power
          and
    3) Specific tests done 2 weeks ago to isolate non-linearity within 
the T304 failed to show any such problem. 

    Now of course it's possible to find specific combinations of factors 
which could explain all these puzzling findings, but I (for one) haven't 
got a single solution to all that we've found so far.

    So, to quote Bob:

    'More tests are planned'

    Rick

Bob Hayward wrote:
> Rick Perley wrote:
>   
>>    Although the expected effect was seen, its magnitude is quite a bit 
>> less than expected -- the 'on/off  Cyg A' experiment -- which triples 
>> the cold-sky system temperature --  showed a considerably larger 
>> reduction in PDif than the ~8% reduction seen in the experiment.   No 
>> simple explanation for this comes to mind.
>>
>>     
>
> My guess is that it has something to do with the LO settings.
>
> In our experiment on Ant 24 we tapped off the signals at the input and 
> output of the T304 module and fed them into narrow 100 MHz filters 
> before being detected by an Agilent power meter. To ensure we were 
> looking at the exact same signal we forced the L301 & L302 synthesizers 
> to use different frequencies from what had been used on the Cygnus 
> observation. We still need to evaluate what sort of effect different LO 
> settings cause on the T304.
>
> Even more confusing was that we saw a *larger* Y-Factor at the output of 
> the T304 than what we saw on the input. We would normally expect it to 
> be exactly the same or slightly smaller (the T304 will add a little bit 
> of noise thus increasing Tsys and decreasing the Y-Factor). Either our 
> test setup was nonlinear or it's all thanks to some odd nonlinearity 
> within the T304.
>
> More tests are planned...
>
> -Bob
>
>
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