[time-nuts] Re: Why do have OCXO a Vref output? (was: help reviving Trimble UCCM-LPS GPSDO)

[John Ackermann N8UR]

One thing I've seen on several OCXO and small Rbs is that the EFC pin is 
internally wired in the middle of a voltage divider using two fairly 
high value (>10K) resistors between Vref and ground.  That ties the 
varactor to the middle of the EFC range when no external tuning is 
required.  For external tuning, put the EFC pin on the wiper of a pot 
between the Vref pin and ground that has a much lower value than the 
internal divider.  That stiffer voltage will override the internal divider.

John
----

On 12/23/21 12:51 PM, Andy Talbot wrote:
> I've always assumed this is because they need to know the reference is
> clean and under the OCXO manufacturer's control if it's to meet specs.  If
> the user had to supply the reference there's no knowing how clean it is.
> 
> Andy
> www.g4jnt.com
> 
> 
> 
> On Thu, 23 Dec 2021 at 17:47, Attila Kinali <attila at kinali.ch> wrote:
> 
>> On Thu, 23 Dec 2021 14:42:27 +0100
>> Wilko Bulte <wilko.bulte at xs4all.nl> wrote:
>>
>>> A quick experiment learned that the OCXO freq responds to the EFC
>> voltage.
>>> So, looks like the Vref circuit in the OCXO has died.
>>
>> A stupid side question: Why do have OCXO a Vref output in the
>> first place?
>>
>> I can see that some form of reference might make stabilizing
>> the power in the crystal easier, but that still wouldn't make
>> it necessary to have an actual reference output.
>>
>> And related to that: Would supplying the voltage reference
>> externally, in case of a broken Vref output, work for whatever
>> is inside that needs this reference voltage?
>>
>>                          Attila Kinali
>>
>> --
>> The driving force behind research is the question: "Why?"
>> There are things we don't understand and things we always
>> wonder about. And that's why we do research.
>>                  -- Kobayashi Makoto
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