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 directÃons 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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