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 -----Ursprüngliche Nachricht----- Von: [email protected] [mailto:[email protected]] Im Auftrag von Jim Lux Gesendet: Donnerstag, 13. Oktober 2011 02:13 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] need example frequency vs temp equation I'm putting together some examples of oscillator behavior vs temperature and I'm looking for some plausible coefficients and simple equations to use to generate nice looking curves. oscillators are TCXO and run of the mill whatever they use for computer clock oscillators (AT cut?). Something that gives me decent PPM vs degrees C for a range from cold (-50 to -40) to hot (say +70 to +80) I've got tons of measured data, but before I spent the time to try and do a curve fit (implying that I can actually read the data and not have to copy it by hand from a table or translate some oddball log file) I've got Vig's tutorial: If you look at page 2-7, it's those curves I want to generate. (or if you look at 4-43 in the tutorial) Maybe the Army book by Bennett? Even better if I can replicate the hysteresis with a simple model. I've got Frerking's equation delta F/F = A1* deltaT + A2*deltaT^2 + A3*deltaT^3 deltaT = T - 25C but what's some good numbers for A1, A2, and A3.. I found one reference that cites Gerber and Ballato A1 = -5.08E-6 * angle of cut relative to 35.25 degrees A2 = -0.45E-9 A3 = 108.6E-12 that gets me a generic AT cut.. But what about TCXOs (which have more lumps in the curve...) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
