[time-nuts] EFC Input pin impedance for the HP 10544Aand10811-6011

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Oct 29 05:03:18 EDT 2007

Don Collie wrote:
> Surely the only impedance we`re concerned with is that at DC, since it`s 
> only an adjustment. The input would look like 200K, in series with a 
> constant voltage source  [zero resistance ] of 6.4 Volts.
> Cheers!,..........................................Don C.
> -
Wrong, at dc the resistors are in series with the reverse leakage
resistance of the diode.
The effective dc resistance could be as high as several megohms (NB
varicap is at the oven temp so its equivalent reverse shunt resistance
will be much lower than at room temperature.
The diode reverse characteristics are actually a little more complex
than that.

The EFC input can allso be used to frequency modulate the oscillator.
The modulation bandwidth of the EFC port depends on the associated shunt
C's of the crystal etc.
For a GPS disciplined oscillator the effective EFC update rate will
usually be less than once per second so the frequencies involved will be
less than 0.5Hz.


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