[time-nuts] need example frequency vs temp equation

Ivan Cousins ijcousins at frontier.com
Sat Oct 15 14:50:37 UTC 2011


  Hi Jim,
When you mentioned
"I found a thesis from someone who was modeling this kind of thing 
(actually he was developing an set of tools to design it) and I can 
probably crib his matlab code.".

I was interested so I did a Google scholar search to find that reference.
I think I found the reference you referred to.
keywords used "thesis temperature compensated oscillator model tool"
http://scholar.google.com/
unclick patents
Look on the right for the available pdf files.

I found.
"Design Technique for Analog Temperature Compensation of Crystal 
Oscillators"
http://scholar.lib.vt.edu/theses/available/etd-11262001-111453/unrestricted/etd.pdf

Another interesting reference paper found in that search was:
"Performance analysis and architectures for INS-aided GPS tracking loops"
http://waas.stanford.edu/~wwu/papers/gps/PDF/AlbanIONNTM03.pdf
This has a clear explanation of architectures used in GPS tracking loops.

Thanks
John Cousins

On 11:59 AM, Jim Lux wrote:
> On 10/12/11 10:10 PM, Bernd Neubig wrote:
>> Hi Jim,
>>
>> There are different types of TCXO compensation techniques on the market.
>> Each of them generating a different style of f(T) characterisitcs.
>> Furthermore the f(T) response varies from unit to unit, because each 
>> TCXO is
>> usually uindividually compensated (or sometimes in groups of similar 
>> quartz
>> f(T) characteristics.
>> To name a few compensation types:
>> The classical types can be broken down in direct and indirect 
>> compensation:
>> 1. the first one using a thermistor/capacitor/resistor network 
>> connected in
>> series to the quartz crystal resonator. This network represents a 
>> temprature
>> dependenta load caopacitants to the crystal.
>> 2. the indirect ones using a thermistor/resistor network which 
>> generates a
>> temperature-dependent DC voltage, which is fed to a varactor diode (in
>> series to the crystal) and thus changing f over T. Sometimes the passive
>> network is combined with an op-amp to realize a higher voltage swing.
>> 3. The modern TCXO (all these small ceramic packaged SMD units) use 
>> IC-based
>> compensation techniques. There are different TCO on the market which 
>> differ
>> in their working principle slightly. But in general, most of those IC's
>> contain a temperature sensor, from which a DC voltage represented by
>> polynomial of 3rd or higher order is generated by analog techniqes:
>> The coefficients for the polynomial are
>> - a0 = reference voltage
>> - a1 = outoput from temperature sensor
>> - a2 = output from temperature sensor multiplied by the same with an
>> analogue multiplier
>> - a3 = output of a2 multiplied with temp sensor output  etc.
>> These components are fed into an analogue summing amplifier through 
>> analogue
>> potentiometers, which are setting the magnitude of each coefficient.
>> This summed-up voltage ploynomial feeds one or two varactor diodes in 
>> series
>> to the crystal.
>> In the (still individual, but highly automated)compensation process, the
>> coefficient potentiometers are set set through a serial data line 
>> such, that
>> the f(T) characteristic shows minimum deviation over temperature. This
>> process runs through the whole operating temperture range in small
>> temperature steps, mostly in both directíons to take into account 
>> some of
>> the hysteresis of the crystal's f(T) characteristic.
>> 4. Besides these techniques there are some other approaches, such 
>> like the
>> first generation of digitally compensated TCXO, which were using loo-up
>> tables for eacht temperature increment (bit), which contains the digital
>> word for the necessary compensation voltage. The disadvantage of this 
>> method
>> are the discontinuities between eacht temperature bit, causing small
>> frequency jumps and/or jitter
>>
>> To conclude: Because of the individual process, TCXO do not show any 
>> uniform
>> f(T) characteristic. You can fit it by a higher order polynomial, but 
>> the
>> responses are looking different for each individual unit.
>>
>>
>> Best regards
>>
>> Bernd, DK1AG
>>
>> AXTAL GmbH&  Co. KG
>> www.axtal.com
>>
>
>
>
>
>
> Thanks a lot.. This was quite informative.
>
> I think all I need for this purpose (it's an example of what you might 
> have to deal with) is any old scheme. Given the hidebound reactionary 
> conservatism of spaceflight electronics designers and their even more 
> conservative review board members, the first one (temperature 
> dependent resistor and capacitor, or maybe varactor) is probably the 
> most plausible.
>
> And for the non-temperature compensated case (e.g. the CPU clock), the 
> standard AT cut cubic will probably work just fine.
>
> I found a thesis from someone who was modeling this kind of thing 
> (actually he was developing an set of tools to design it) and I can 
> probably crib his matlab code.  (which is all I really need...)
>
> I'm trying to come up with some illustrative experimental scenarios 
> where you have multiple widgets varying in temperature (orbiting 
> around something, so they cycle every 90-100 minutes, with a bigger 
> cycle on a monthly/annual basis), and you want to do 
> comparisons/ensembles/doppler measurements.
>
> So it's not that it has to exactly match any particular scheme, just 
> be representative of some scheme in terms of overall magnitude and 
> number of wiggles in the curve of f(T).   I could arbitrarily just 
> pick something like a moderate order polynomial or spline that "looks 
> good", but hey, if someone has a model out there for a real device, I 
> might as well use that.
>
>
>





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