Re: [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  wrote:
> 
> 
> kb...@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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Re: [time-nuts] MCXO and dual mode

[Hal Murray]

kb...@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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Re: [time-nuts] MCXO and dual mode

[Bob kb8tq]

Hi

> On Jun 6, 2017, at 10:15 PM, Richard (Rick) Karlquist  
> wrote:
> 
> 
> 
> On 6/6/2017 3:16 PM, Bob kb8tq wrote:
>> Hi
>> If you do the classic MCXO with two oscillator circuits and one resonator, 
>> the issue is
>> pretty simple. You have a load capacitance on the fundamental. You have a 
>> load capacitance
>> on the third overtone. Even if it is the exact same capacitor, the tuning 
>> sensitivity on
>> the fundamental is different than the sensitivity on the third overtone. As 
>> the load impedance
>> changes (parts do drift) the delta between the two modes will show up as an 
>> offset between
>> them. If you run through the math, it gives you a delta temperature. How 
>> much? How fast? Obviously
>> that depends. When I brought this up at the time with the authors of the 
>> paper, the reply was that
>> a recalibration of the MCXO was provided for for this reason.
>> Bob
> 
> I don't understand what you are talking about here.  The tempco
> difference between modes is unrelated to load capacitance.  The
> dual mode idea would work just as well if the oscillators
> operated at series resonance.

The circuit that Stan Shadowski presented is a fundamental / third overtone 
dual. The example
below is based on that circuit. 

Let’s say both modes are running into a 32 pf load and it is a single 
capacitor. 

The capacitor changes due to aging by 1 pf, you now are at 33 pf load.

The fundamental changes frequency ~ 3X as much (in ppm) as the third overtone.

The beat frequency shifts since the two modes do not tune identically. 

Beat frequency shift = temperature error.

Yes the example is a little contrived. The real numbers would depend a bit on 
the design of
the crystal used. 

Bob


> 
> [I attended this talk in person ~25 years ago; it got a lot of
> interest].
> 
> The reason why the SC cut mode C and mode B dual mode patent
> from HP fell out of favor was the problem with activity dips
> in mode B.  Otherwise, it was a great idea.  It would still
> be fine for an OCXO, where you just avoid activity dips.
> However, the circuit design is very complicated.
> 
> Rick N6RK

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Re: [time-nuts] MCXO and dual mode

[Richard (Rick) Karlquist]

On 6/6/2017 3:16 PM, Bob kb8tq wrote:

Hi

If you do the classic MCXO with two oscillator circuits and one resonator, the 
issue is
pretty simple. You have a load capacitance on the fundamental. You have a load 
capacitance
on the third overtone. Even if it is the exact same capacitor, the tuning 
sensitivity on
the fundamental is different than the sensitivity on the third overtone. As the 
load impedance
changes (parts do drift) the delta between the two modes will show up as an 
offset between
them. If you run through the math, it gives you a delta temperature. How much? 
How fast? Obviously
that depends. When I brought this up at the time with the authors of the paper, 
the reply was that
a recalibration of the MCXO was provided for for this reason.

Bob



I don't understand what you are talking about here.  The tempco
difference between modes is unrelated to load capacitance.  The
dual mode idea would work just as well if the oscillators
operated at series resonance.

[I attended this talk in person ~25 years ago; it got a lot of
interest].

The reason why the SC cut mode C and mode B dual mode patent
from HP fell out of favor was the problem with activity dips
in mode B.  Otherwise, it was a great idea.  It would still
be fine for an OCXO, where you just avoid activity dips.
However, the circuit design is very complicated.

Rick N6RK
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Re: [time-nuts] MCXO and dual mode (was: Poor man's oven)

[Bob kb8tq]

Hi

If you do the classic MCXO with two oscillator circuits and one resonator, the 
issue is 
pretty simple. You have a load capacitance on the fundamental. You have a load 
capacitance
on the third overtone. Even if it is the exact same capacitor, the tuning 
sensitivity on
the fundamental is different than the sensitivity on the third overtone. As the 
load impedance 
changes (parts do drift) the delta between the two modes will show up as an 
offset between
them. If you run through the math, it gives you a delta temperature. How much? 
How fast? Obviously
that depends. When I brought this up at the time with the authors of the paper, 
the reply was that
a recalibration of the MCXO was provided for for this reason. 

Bob


> On Jun 6, 2017, at 5:40 PM, Attila Kinali  wrote:
> 
> On Mon, 5 Jun 2017 20:21:10 -0400
> Bob kb8tq  wrote:
> 
>> That paper is the basis for the MCXO. It is an interesting way to do a TCXO. 
>> The drift between the two modes makes it a difficult thing to master in an 
>> OCXO.
>> Plating a pair of electrodes (one pair per mode) is also an approach that 
>> has been
>> tried. 
> 
> That's the first time I hear of modes drifting respective to eachother.
> Do you have any references I could read on this?
> 
> I always wondered why the MCXO approach was not used more often.
> Or why none of the OCXOs used a dual mode approach to sense
> the temperature of the crystal directly instead of using a
> thermistor. 
> 
>   Attila Kinali
> -- 
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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[time-nuts] MCXO and dual mode (was: Poor man's oven)

[Attila Kinali]

On Mon, 5 Jun 2017 20:21:10 -0400
Bob kb8tq  wrote:

> That paper is the basis for the MCXO. It is an interesting way to do a TCXO. 
> The drift between the two modes makes it a difficult thing to master in an 
> OCXO.
> Plating a pair of electrodes (one pair per mode) is also an approach that has 
> been
> tried. 

That's the first time I hear of modes drifting respective to eachother.
Do you have any references I could read on this?

I always wondered why the MCXO approach was not used more often.
Or why none of the OCXOs used a dual mode approach to sense
the temperature of the crystal directly instead of using a
thermistor. 

Attila Kinali
-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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