Temperature Compensated Crystal Oscillators: Terms and Definitions
The temperature characteristics of crystal oscillators are largely depending on those of crystal units used which are generally expressed by cubic curves of AT cut quartz blank. The temperature compensating circuit, which must be custom-built for each unit, is used to tune the oscillator just enough to offset the uncompensated frequency change with temperature. TCXO features excellent temperature characteristics, fast warm-up time (typically 50 to 1000 ms), low power consumption (10 to 150 mW), lightweight, compactness, and with a fraction of the cost of an OCXO’s. It is ideally suited for various communications equipment such as cellular phones, two-way radios, cordless telephones, microwave communications equipment and satellite communications system, measurement instrument and many more other applications.
Temperature Stability: With standard compensation techniques, fractional stabilities of around ±1 ppm for a temperature range of –40 C to +85 C can be achieved.&nbs Better stabilities can be achieved over narrower temperature ranges. The actual technique employed in all except the simplest TCXOs is based upon use of a varactor diode in series with the crystal. A change in voltage causes a change in the capacitance of the varactor diode resulting in a change in frequency of oscillation. The thermistor network is tailored to the crystal to cause voltage to vary with temperature in such a manner that will compensate for the crystal's frequency versus temperature characteristic. As each individual TCXO requires that its compensation network be matched to its individual crystal, the cost of a TCXO is closely related to the difficulty of the frequency versus temperature specification.
Thermal Hysteresis: It is a measure of a TCXO to repeat the frequency versus temperature data over multiple temperature cycles. Here the frequency of a TCXO is measured at one temperature. The temperature is changed and then returned to the original temperature and the frequency is measured again. The two frequencies are not the same. The difference between the two frequencies is called “thermally induced hysteresis”. This phenomenon is present even if the unit is allowed to stabilize at the same temperature for a long time. The value of this thermal induced hysteresis is normally of the order of ±0.1 ppm for a good TCXO.
VC-TCXO: Frequency-temperature hysteresis limits the ultimate attainable stability of a TCXO. The crystal resonator is a primary source of this hysteresis, which can be minimized but not eliminated. To allow for aging, most TCXO are made tunable over a small frequency range, using a voltage control function (VC-TCXO). A typical functional tuning range is ±5 ppm. Further, it should be noted that the frequency versus temperature characteristic of a TCXO is not linear; thus a 2xl0-7 total error over O°C to +50°C will not produce a gradient of 2x10-7 ÷ 50 = 4x 10-9 per ºC. Perturbations in the crystal characteristics (activity dips) make it virtually impossible to guarantee exceptional stability on a per degree basis in TCXOs.
Thermal Transient: Thermal transient occurs when the rate of temperature change is high enough for the frequency to no longer tack the well-behaved curve that is generated when measured with slow temperature changes. An acceptable rate of the temperature change would be of the order of 0.5 C per minute. This effect is in a large part due to the transient response of the crystal resonator, and the separation between resonator and temperature sensing devices within the oscillator. In an OCXO, it can also depend on the stability and gain of the error amplifier used in the temperature controller. Typical values are less than ±0.2 ppm. The testing and compensation accuracies of TCXOs can be adversely affected by the thermal-transient effect. As the temperature is changed, the thermal-transient effect distorts the static frequency vs. temperature characteristic, which leads to so-called apparent hysteresis. The faster the temperature is changed, the larger is the contribution of the thermal-transient effect to the frequency vs. temperature performance.
Aging: In clock oscillators with moderate temperature stability, aging is usually of little consequence. However, in highly temperature stable TCXOs, crystal aging becomes a significant factor in the oscillator's overall frequency error. Therefore, it is very common for TCXOs to employ specially processed crystals in evacuated glass or cold weld holders.
Shock: Shock is defined as a sudden powerful blow. A typical shock number for TCXOs is 100g.
Mechanical Trim and EFC (Electrical Frequency Control): Mechanical trim allows the frequency to be adjusted via an internal potentiometer (pot). The pot is accessed through a sealed or unsealed hole. EFC (electrical frequency control) requires an external circuit to adjust the frequency. The external circuit usually consists of a pot or DAC. The power for this circuit can be applied via an external voltage source supplied by the customer or an internal reference voltage supplied by the manufacturer.