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When the first crystal oscillators were built in the 1920s, the only crystals available, such as the X-cut, exhibited poor temperature performance. The development of the AT-cut crystal was a major step toward making temperature compensation feasible^[1]. The AT-cut provided for a relatively flat frequency vs. temperature curve centered on +25 °C. Until about the mid-1940s, the aging and temperature characteristics of the crystals were not good enough to make precision corrections practical^[2]. Leaky packages led to poor aging drift and deficiencies in crystal plate and wafer design produced crystals with severe activity dips and coupled modes^[3]. This produced significant frequency perturbations that limited the effectiveness of any attempt at compensation. But advances in quartz plate design and crystal packages such as the cold weld holders made it possible to produce crystals with relatively smooth frequency vs. temperature curves and aging rates as low as 1 × 10^-9 (or 1 × 10^-3 ppm) per day.

Thermistor/resistor network compensation

Thermistor/resistor TCXOs have been the mainstay of crystal oscillator temperature compensation for 50 years. A correction voltage generated by a network of one or more thermistors cancels the frequency vs. temperature variation of a voltage-controlled crystal oscillator. The introduction of voltage-variable capacitance varactor diodes along with improvements in negative-temperature coefficient thermistors made it possible to compensate crystals to a greater precision^[4]. As early as 1961, compensation ratios of greater than 100-to-1 were being achieved. This would indicate that a crystal with a peak-to-peak deviation of 40 ppm over temperature could be compensated to a level of 0.4 ppm. Today, ratios of two orders of magnitude are about the limit for thermistor/resistor compensation, although achieving that level is facilitated by improved, automated systems and computer analysis power. But even today, achieving stabilities of better than 0.5 ppm requires multiple temperature runs and repeated network adjustments with at least three thermistors. Some attempts at automation of the compensation process using resistor trimming or digital adjustment of thermistor sensitivities have been moderately successful^[5], but these configurations could not be easily integrated for small package size requirements.