The achievable frequency vs. temperature performance is a function of how closely the compensating voltage curve generated by the polynomial generator matches the required voltage of the VCXO. Many variables affect this capability, including the tuning linearity of the VCXO; the quality of the crystal, i.e., how closely it follows the ideal AT curve; the temperature coefficient of other oscillator components; the inflection temperature of the crystal; and the stability of the voltage reference. Figure 3 shows the frequency vs. temperature performance that can be achieved.

The frequency excursions that occur during changing temperature conditions will vary depending on the direction and rate of the temperature change. An important feature is close thermal coupling between the crystal and the temperature sensor of the ASIC. This thermal path is inherently short with a miniature package since the crystal and ASIC are physically close. Because of this, most small TCXOs will perform well. Figure 4 shows a 20 MHz oscillator during a slewing temperature run. The red curve shows the chamber temperature on the right y-axis, and the blue curve is the oscillator frequency on the left y-axis. The x-axis plots time as normalized to the reading number (each reading takes 20 ms). Starting at +25 °C, the chamber is ramped up to +90 °C at a rate of 8 °C/minute. After stabilizing, it is ramped down to -60 °C at the same rate. Except for the peak at the hot end where the temperature exceeded the compensated range, it can be seen that the effect of the ramp is minimal, with little hysteresis evident from ramping in opposite directions.

TCXO crystals have historically been plagued with anomalies in their temperature performance caused by blank design or imperfections in the processing and manufacture of the crystal. Marginal blank geometry can lead to coupling of other modes of oscillation that may be close to the frequency of the desired mode. These modes can interfere with the oscillator frequency at various temperatures causing increases in the crystal resistance or “activity dips” and resulting frequency excursions. These perturbations typically occur over a narrow temperature band. It is possible that the circuit may cease to oscillate at these points, or may not start when power is applied.

Another inconsistency that may occur over temperature is a jump or step offset in frequency. These offsets are small and often are not observed under normal TCXO testing. Many times, TCXOs are only tested at six or eight points over the temperature range. Under these conditions, many perturbations and jumps will go undetected. In applications where this type of irregularity is critical to system performance, the oscillators should be tested over many more points. Testing at 2 intervals is a good compromise that will catch most perturbations without a great increase in test time.

For the greatest confidence, the frequency of each oscillator should be continuously monitored as the temperature is ramped from one extreme to the other and back. This type of test guarantees that any perturbation or micro-jump that is present will be captured. Figure 5 shows the screening results of a 20 MHz TCXO that was monitored during the 8 °C/minute ramp profile. For this entire time, the output frequency is continuously recorded 50 times per second with no dead time between the readings. The blue line is a plot of the difference between successive readings, which highlights any instantaneous jumps. This AT-strip crystal shows no perturbations and just a few small micro-jumps throughout the test, which indicates a TCXO with exceptional performance.

Figure 6 has zoomed in on the area around reading No. 3059. The y-axis is the frequency in Hertz indicating a step of around 10 ppb. These small steps are fairly repeatable, although they may not appear when the temperature is slewed in the opposite direction.

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