Re: [time-nuts] need example frequency vs temp equation

[Ivan Cousins]

 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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