[evlatests] T302 Linearity Test

[Bob Hayward]

If the non-linearity is somehow coming from the LO used in the T302 
leaking into the T304 and compressing the first RF amplifier, then 
switching in the Solar 20 dB Attenuator wouldn't affect the strength of 
the LO birdy (since the Solar Attenuator is in front of the T302 mixer). 
Nor would changing the RF attenuator in the T304 affect this birdy, 
since they come after the first RF amplifier.

None of these scenarios affect the Tcal/Tsys ratio (at least to first 
order). This can only be done by increasing the actual noise being 
delivered to the receiver, as in the case of looking at your strong 
astronomical source (e.g., Cygnus).

Granted I have a hard time believing that enough of the LO from the T302 
can really be getting through the 7.5-12.5 GHz filter in front of the 
T304 to put it into compression, but this could explain why you are not 
seeing the non-linear effect when you muck with the various attenuators 
at your disposal.

-Bob


Rick Perley wrote:
>     The solar attenuators present in the T302 gives us another way of 
> checking system linearity. 
> 
>     In the most recent 'solar test', we observed 3C84 (calibrator) in 
> normal mode, then switched in the T302's 20 dB attenuator, while 
> continuing to observe the source.  The T304's logic adjusted its 
> internal attenuators to keep the power level to the samplers in the 
> 'happy zone'. 
>     If everything works as it should, the switched power system should 
> compensate nicely for this 100-fold change of power (seen only within 
> the T302, as the T304 has corrected itself).   The apparent 
> (post-correction) fringe power on 3C84 should not change after the 
> changes have taken place. 
>    
>     Result:
> 
>     Everything works as planned!  The apparent change of gain between 
> the 20db in and 20db out states is no more than 1%!  The system (from 
> the T302 through to the correlator) appears beautifully linear, as it 
> should be. 
> 
>     So where is the apparent PDif compression coming from? 
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