[time-nuts] Proper way to manually connect Vfc

Graham Haddock gh78731 at gmail.com
Sun Aug 17 13:44:28 EDT 2014

There are several things to think about that might create errors or noise.

1.) Is the voltage source feeding the "top" of your multi-turn pot stable
with temperature?  Similarly, is the low frequency noise on the voltage
source feeding the 'top' of your multi-turn pot quiet enough?  You are
sometimes concerned with milli-Volts or micro-Volts of noise or
variation.  Many of the common voltage regulator ICs are not stable
enough or quiet enough for precision frequency control.

2.) If the multiturn pot is some distance from the EFC input, is it possible
to introduce voltage drops in the shared ground connection (that might also
be serving a heater or something) that will show up as an EFC error.
Be sure to run a separate ground connection from the oscillator to the
pot ground that is not grounded anywhere except at the oscillator EFC

3.) What is the input impedance (resistance load) of the EFC input?
You would ideally want the output impedance of your multi-turn
pot to be low with respect to the EFC input impedance.

4.) RF bypassing at the EFC input is good hygiene.  But if you are
doing just DC control, then bypassing all the way down to a fraction
of a Hz can remove external noise.  Just make sure it is a very low
leakage capacitor.

--- Graham / KE9H


On Sun, Aug 17, 2014 at 7:15 AM, Ole Petter Ronningen <olepr01 at gmail.com>

> Hello, all
> I've started to build up a little collection of various OCXO's, and
> measuring various deviations and variances and whatnot. I'm putting them
> into proper enclosures, but up untill now I've adjusted them simply using
> the Vref through a reasonably low tempco (20-100ppm/C) multiturn pot, with
> just a .1uF cap on the Vfc pin to ground. Is this in fact the best way to
> do it? Or is there another (reasonably simple) way to improve on that
> setup?
> I presume the reference-voltage present on most OCXO's are "clean enough"
> to meet spec, but are there improvements to be had by using a separate low
> noise regulator, for instance?
> How about filtering, is there any reason to spend much effort on that, as
> the OCXO's are in separate enclosures, with coax soldered directly onto the
> 10Mhz output? Not sure where the noise would come from, but I stand to be
> corrected..
> Sorry if this has been repeatedly answered (as I have a feeling it must
> have been), but I failed to find it in the archives (perhaps poorly chosen
> search-terms on my part)
> Thank you
> Ole P
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