Example of TCXO Compensation

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.  Better stabilities can be achieved over narrower temperature ranges.  The actual technique employed in all except the most simple TCXOs is based upon use of a varactor diode in series with the crystal as follows:

       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.

       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 (VCTCXO).  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.