[time-nuts] Trimble 34310-T EFC Question

Charles Steinmetz csteinmetz at yandex.com
Thu Nov 7 06:09:31 EST 2013


Bob wrote:

>I think I may need to change the LPF feeding the EFC in my GPSDO to 
>get rid of dithering jitter.  Is there any point in adding a 
>resistor and cap in the EFC line, or do I need to go back and change 
>the values in the op-amp circuit feeding it?  Currently an op-amp 
>directly feeds the EFC pin.  I guess this boils down to something 
>like: is there any meaningful current flow in EFC circuit in this 
>oscillator?  Or should I add them but will I probably have to change 
>the existing filter, as well?

Generally, an EFC pin connects internally to one or more varicap 
diodes, which by definition must be reverse biased (thus, the only 
current flow is the reverse current (leakage) of the 
diode(s)).  There is often an internal series resistor of 50-100k 
between the EFC pin and the diode(s).  It is common to use an 
additional, external series resistor and shunt capacitor, generally 
10-50k and anywhere from 0.01uF to tens or even hundreds of uF, 
depending on the application.  Of course, this additional pole needs 
to be taken into account in the loop stability analysis of the PLL.

Best regards,

Charles





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