Book excerpt: This paper describes a new frequency temperature compensation method for crystal oscillators. The method is applicable to resonators with second order or third order frequency temperature characteristics. Hence, an oscillator with an AT cut, a BT cut crystal unit or a surface acoustic wave resonator (SAWR) can be efficiently compensated. A conventional temperature compensated crystal oscillator (TCXO) uses an expensive variable capacitance diode (varactor) with good linearity as well as many thermistors. Considerable efforts have been devoted to developing a new reactance circuit to replace the varactor. It was found that a series circuit, involving a capacitor and a silicon diode, has the same characteristics as a varactor, and its linearity could be easily adjusted. In the case of an AT cut crystal unit with third order frequency temperature characteristics, compensation over a wide temperature range can be achieved by using two series circuits.