[time-nuts] Proper way to manually connect Vfc

[Bob Camp]

Hi

A good wire wound pot should have a decent “ratio” temperature coefficient. The whole winding may be 20 or 100 ppm / C but both “halves” of the winding (when set at mid point) track each other much better than that. The tracking can be messed up by this or that, but it’s usually a good thing to bet on. 

The *assumption* is that the OCXO will meet it’s specs with what ever it’s internal reference is and something like a pot dividing the voltage down. An RF bypass on each of the active pins of the OCXO is a pretty good idea. The output pin is the obvious exception to this rule. 

To see if there is a contribution from the Vref. Simply short the EFC to ground. If the “whatever” deviation does not change when you do this, there is no contribution from the Vref. 

Bob


On Aug 17, 2014, at 8:15 AM, Ole Petter Ronningen <olepr01 at gmail.com> wrote:

> 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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