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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jan 6 23:36:04 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

What is the apparent noise at the mixer output when both inputs are
driven in phase quadrature from the same source?
i.e. use a quadrature hybrid or a suitable length of coax plus
attenuators as required and disconnect the PLL.

If its not a calculation/calibration error then it may be:

1) faulty/noisy mixer

2) Noisy PLL (EL2125 noise isnt specified at low frequencies its 22uA
input bias current is fairly high for use as a PLL amplifier).

3) Noisy and/or oscillating preamp.

4) faulty preamp or PLL amplifier with output saturated so the power
supply noise appears on its output.


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