[time-nuts] 35601A as stand-in for 11848A ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jan 6 22:59:01 EST 2008


SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> I am pretty sure the Mixers' DC output is near zero when I am in lock,  since 
> it only needs to compensate for the Opamps' offset voltage (very  small).
>  
> While the PLL is working on lock, the Mixer is slowly "homing-in"  on the 
> lock voltage, and I don't really see that affecting the noise floor at  all (the 
> PLL loop bandwidth is about 1/2Hz, and thus below the analyzers' cut  off, so 
> I am actually seeing measurements before the PLL is fully locked  already).
>  
> Something else is a bit awry here...
>  
> bye,
> Said
>  
>  
Said

Yes the PLL will adjust the relative phases of the 2 inputs so that the
mixer output is near zero, however this doesn't necessarily mean that
the phase difference between the mixer inputs is 90 degrees.
The dc offset is the mixer output voltage when its inputs are in phase
quadrature.

I agree that something else is likely to be the problem however once
that is fixed...
Operating away from the ideal point will increase the AM noise
sensitivity particularly when the RF port isnt saturated.
This will raise the noise floor, but not by as much as you apparently
observed unless the oscillators connected to the RF port has a lot of AM
noise.

Bruce



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