[time-nuts] MCXO and dual mode

[Bob kb8tq]

Hi

Yes, you run both modes at the same time. You wire up two oscillator circuits to the
same crystal. One runs at the fundamental and the other runs at the third overtone. 
The two have a different temperature coefficient. (yes, that’s a bit weird, but it is true). 
The “offset” between the two modes lets you read out the temperature. 

If you build it properly, each oscillator will have a spur at the “other” 
frequency. That may or may not be an issue. If you use the fundamental output,
the third looks a lot like a third harmonic (but not quite ….).  Using the third 
overtone is a bit more problematic since the harmonic of the fundamental 
will create a close in spur. 

Bob

> On Jun 7, 2017, at 1:04 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> kb8tq at n1k.org said:
>> Let’s say both modes are running into a 32 pf load and it is a single
>> capacitor.  
> 
> I'm missing the big picture.
> 
> Can I run both modes at the same time?  Or do I switch between them?
> 
>> The beat frequency shifts since the two modes do not tune identically. 
> 
> That sounds like they are running at the same time.
> 
> What does the output look like?  I'd expect beats so the signal would drop 
> out for many cycles if I looked at the right place in time.  Is that sort of 
> signal good for anything other than being a thermometer?
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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