Temperature compensated crystal oscillators typically employ a thermistor network to generate a correction voltage which reduces the frequency variation over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed. A TCXO that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when returned to normal room temperature. Usually this hysterisis or “retrace” error is temporary but a seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting. This hysterisis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult. The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature and SC-cuts and high overtone types cannot be tuned by a sufficient amount to compensate for the frequency excursion.
Several other considerations that often do not appear on data sheets should be kept in mind:
TCXOs may exhibit temporary frequency drift when the ambient temperature changes if the thermistor network does not have the same thermal time-constant as the crystal.
Adjusting the mechanical tuning (to compensate for aging) may change the electrical tuning sensitivity in some designs causing the thermistor network to under-correct or over-correct.
Warm-up may not be as fast as expected if internal heat sources slowly warm the thermistor network and crystal.
Phase noise may be inferior to oven designs and the noise may change over temperature.
TCXOs are preferred to oven oscillators in low power applications and when a warm-up period is not acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium and the total current consumption can be very low – often determined by the output signal power requirements.
Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency curve. Newer designs employ digital logic or a microprocessor to derive a correction voltage from values or coefficients stored in memory. Some designers are requesting non-compensated oscillators fully characterized over temperature so that a system microprocessor can provide the required correction voltage. These oscillators are designed so that the crystal has a long thermal time constant to reduce short-term instability and the electrical tuning is trimmed to a specified sensitivity. Each oscillator is given a “curve” number much the way crystals are graded and the external micro measures the ambient temperature calculates a correction voltage for the specific curve. The calculation may be an interpolation between points in a look-up table or a computation using a cubic equation with temperature and the oscillator’s “angle” as inputs.
