[time-nuts] TCVCXO Adjustment

[Bob kb8tq]

Hi

On a TCXO a proper 12 bit DAC should do fine. The “don’t use for DC” DAC’s
that are built in to a MCU …. who knows. At DC they probably aren’t 12 bits 
anyway. 

If the TCXO is rated to age < 1 ppm per year and has a 10 ppm EFC (yes you 
*could* wonder about that combo) the DAC would need to drift 10% per year to 
match the rating on the oscillator. That’s a pretty horrid DAC. 

If the TCXO is rated for 0.1 ppm in the first year, that’s a mighty tight TCXO. The
DAC now needs to do 1% per year. Even the MCU parts *should* get to that level.

The same basic logic applies to temperature. If your TCXO claims 0.1 ppm from 
-30 to +70, best to check it. Even so, the DAC is still at the 1% point and likely 
does an adequate job. 

Bottom line - TCXO’s aren’t OCXO’s. They are a bit easier to deal with unless you
happen to have one of the exotic ones. If you do have an exotic TCXO you likely
paid more for it than an OCXO and thus know why you did so ….

We are not talking about disciplining the oscillator. We’re just taking about putting
it on frequency after it has been built into a system. In the case of disciplining, a lot
of things come into play. In the case of one time set to frequency, once you get past 
the likely drift over a few weeks …. why bother? 

Bob

> On Apr 11, 2018, at 5:09 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> wayne.holder at gmail.com said:
>> Since the TCVCXO includes a voltage control input, my plan is to also add a
>> 12-Bit Digital-to-Analog Converter ...
> 
> What's the temperature stability of the D/A?
> 
> How does that compare with a 10 turn pot?  I remember a comment about pots 
> aging.  It was somewhat recently.  I don't remember the details.  I think it 
> was something like the wiper wore a small groove which made it hard to make 
> tiny adjustments.  I don't know how long it takes for the groove to form.
> 
> A possible option is to just measure the osc and do the adjustment in 
> software.
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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