Introduction to Quartz Frequency Standards - Oscillator Categories

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A crystal unit's resonance frequency varies with temperature. Typical frequency vs. temperature (f vs. T) characteristics for crystals used in frequency standards are shown in Figure 11. The three categories of crystal oscillators, based on the method of dealing with the crystal unit's f vs. T characteristic are XO, TCXO, and OCXO, (see Figure 12). A simple XO does not contain means for reducing the crystal's f vs. T variation. A typical XO's f vs. T stability may be ±25 ppm for a temperature range of -55°C to +85°C.

Figure 11. Frequency versus temperature characteristics of AT-cut crystals, showing AT- and BT-cut plates in Y-bar quartz.

Figure 12. Crystal oscillator categories based on the crystal unit's frequency versus temperature characteristic.

In a TCXO, the output signal from a temperature sensor (a thermistor) is used to generate a correction voltage that is applied to a voltage-variable reactance (a varactor) in the crystal network [13]. The reactance variations produce frequency changes that are equal and opposite to the frequency changes resulting from temperature changes; in other words, the reactance variations compensate for the crystal's f vs. T variations. Analog TCXOs can provide about a 20-fold improvement over the crystal's f vs. T variation. A good TCXO may have an f vs. T stability of ±1 ppm for a temperature range of -55°C to +85°C.

In an OCXO, the crystal unit and other temperature sensitive components of the oscillator circuit are maintained at a constant temperature in an oven [13]. The crystal is manufactured to have an f vs. T characteristic which has zero slope at the oven temperature. To permit the maintenance of a stable oven temperature throughout the OCXO's temperature range (without an internal cooling means), the oven temperature is selected to be above the maximum operating temperature of the OCXO. OCXOs can provide more than a 1000-fold improvement over the crystal's f vs. T variation. A good OCXO may have an f vs. T stability of better than ±5 x 10^-9 for a temperature range of -55°C to +85°C. OCXOs require more power, are larger, and cost more than TCXOs.

A special case of a compensated oscillator is the microcomputer-compensated crystal oscillator (MCXO) [14]. The MCXO overcomes the two major factors that limit the stabilities achievable with TCXOs: thermometry and the stability of the crystal unit. Instead of a thermometer that is external to the crystal unit, such as a thermistor, the MCXO uses a much more accurate, "self-temperature sensing" method. Two modes of the crystal are excited simultaneously in a dual-mode oscillator. The two modes are combined such that the resulting beat frequency is a monotonic (and nearly linear) function of temperature. The crystal thereby senses its own temperature. To reduce the f vs. T variations, the MCXO uses digital compensation techniques: pulse deletion in one implementation, and direct digital synthesis of a compensating frequency in another. The frequency of the crystal is not "pulled," which allows the use of high-stability (small C₁) SC-cut crystal units. A typical MCXO may have an f vs. T stability of ±2 X 10^-8 for a temperature range of -55°C to +85°C.

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