[time-nuts] Designing and building an OCXO and GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Aug 11 10:09:46 EDT 2008


Philip Pemberton wrote:
> Bruce Griffiths wrote:
>  > If you are serious forget the fancy digital or semiconductor
>  > temperature sensors they aren't good enough.
>
> I was intending to use the slow Dallas chips as a calibration reference
> (out-of-box they're usually quite accurate) and for testing. Is there 
> any particular reason the analog-output (Microchip TC1047A) sensors are 
> no good?
>
>   
You need millidegree sensistivity and stability, no sensor of this type 
has the required stability.
Only good thermistors and RTDs approach this.
>  > For the best performance, unless you use a bridge oscillator circuit
>  > of some type, you will need to control the temperature of all the
>  > oscillator components as well.
>
> I did have a "plan B" -- a hollow metal box with a metal sheet soldered
> inside at the half-way point. The crystal and oscillator circuitry would
> be mounted in the bottom half, and the temperature control in the top
> half. The temperature sensor is a three-pin SOT23 (about the size of a
> grain of rice) and the ground is a single pin on one side. I was
> thinking about mounting the sensor directly on the copper, using a small
> piece of Kapton tape to stop the sensor's Vout or Vcc shorting against
> the grounded copper sheet.
>   

> That would leave two hollow air-filled cavities for the control
> circuitry and hold the temperature of that reasonably close to that of
> that of the crystal (minus a few degrees).
>
>   
The tempco contribution of oscillator components is such that there 
temperature should be stable to far better than a few degrees.
Thermal gradients within the oven need to be minimised.
>  > An analog bridge using an RTD or an NTC thermistor can have much
>  > better stability.
>
> That sounds about right.. I was going to use a Pt100 or Pt1000 RTD, but
> couldn't find any decent information on them other than the resistance
> being 100R or 1kR at 25C -- even the manufacturer's datasheets were
> somewhat thin on information.
>
>   
These sensors like most platinum resistance thermometers elements have a 
tempco of about 3900 ppm/K or so.
Copper resistors have also be used for this purpose and have a similar 
tempco.

Thermistors are much more sensitive to temperature and high quality ones 
are almost as stable.

The output from a bridge using RTDs is relatively low and precautions 
have to be taken to eliminate thermocouple effects as well as amplifier 
offsets.
An AC bridge excitation technique or periodically reversing the bridge 
excitation polarity can help.
>  > If you use an appropriate high resolution sigma delta ADC it can
>  > reverse the bridge excitation polarity as part of the measurement
>  > sequence and give you most of the benefits of an AC bridge with fewer
>  > devices and lower cost.
>
> IIRC the A/D on the PIC is a 10-bit successive-approximation type with a
> built-in sample-and-hold (though other types have 12-bit converters).
>
> That's a measurement range of 1024 counts, which with the 4.096V
> reference provides a resolution of 4mV, or 1/2.5 of a degree C per
> count. 4V is actually the minimum reference voltage the A/D can accept.
>
>   
Insufficient resolution you need a least count below 0.001C.
If you use that ADC it will need a suitable preamp.
However a linear range of a little less than 1C is sufficient.
A high resolution sigma delta ADC is probably a better choice as a low 
drift preamp isnt required.
The output DAC (of whatever type) needs to be monotonic and have high 
resolution (18 bits or more) to allow the temperature to be controlled 
to better than 0.001C.
There are several techniques for achieving more resolution from a low 
resolution DAC.
> Sensor output is ((degrees_c * 10) + 500) mV.
>
>  > I would build a room temperature version first for debugging.
>
> I'm planning to do that anyway. I've got a few 10MHz room-temp crystals
> of a similar spec to the oven crystals that I can use, and I can
> probably use the same parts in the prototype oven for testing.
>
>  > To minimise the phase noise contributed by the varicap the EFC range
>  > should be as small as is practical.
>
> That's the part that's going to need "a bit" of experimentation I think :)
>
>   
If you use a manual trimmer as well then a range of 1E-7 or so is 
usually sufficient.
If all frequency adjustment is via EFC then you need to accommodate at 
least 10 years worth of aging.
>  > A very low noise power supply is also required for good performance.
>  > A modified version (uses 2 transistors and larger capacitors) of
>  > Wenzels active supply filter can be used to reduce the power supply
>  > noise by 30-40dB for frequencies above 1Hz or so.
>  > http://www.wenzel.com/documents/finesse.html
>  >
>  > I can provide circuit schematics if you are interested.
>
> That would be great, if it's not too much trouble.
>
> Thanks,
> Phil.
>   
What are the crystal parameters:

Q?
ESR?
L?
Cshunt?
Cseries?

What crystal cut is used AT?, BT?, SC?
Is it a fundamental crystal?

Will post the modified P/S filter in a few hours together with the 
modified Wenzel low noise oscillator circuit.
If you are using an overtone crystal or an SC crystal then provisions 
need to be made to suppress unwanted modes.
In this case a somewhat different oscillator configuration is perhaps 
advisable.
Something like one of the Driscoll type oscillators which have a tuned 
tank for mode suppression can be used.
Otherwise a somewhat more complex oscillator with AGC may be necessary.

Bruce



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