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Product category: Capacitors
News Release from: Gothic Components | Subject: Quartz crystal resonators
Edited by the Electronicstalk Editorial Team on 07 January 2003

Frequency sources for millimetre wave
applications

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A precision frequency source of megahertz can be multiplied up to several gigahertz and provide a relatively inexpensive and reliable oscillator, says Robin Summers of Advanced Crystal Technology.

A precision frequency source of megahertz can be multiplied up to several gigahertz and provide a relatively inexpensive and reliable oscillator.Quartz crystal resonators are available in a variety of surface mount packages.Each part is usually tested to a maximum frequency tolerance of +/-30ppm at 25C, plus stability over temperature of +/-50ppm for -10 to +60C.A ceramic resonator is also a viable source of a stable frequency, but here the 25C tolerance is likely to be +/-0.5% and frequency stability of +/-0.3% over -20 to +80C.Most crystals are specified in a parallel resonant mode for a given value of load capacitance, that is to say a part resonates at a given frequency if the load capacitance in the circuit matches what is expected by the crystal.In a Pierce oscillator, the load capacitance is connected to ground either side of the crystal.(A Pierce oscillator comprises an inverter - U1, a resistor - R1 between inverter input and output, another resistor - R2 between inverter output and junction of X1 and C2, a crystal - X1 between inverter input and junction of R2 and C2, a capacitor C1 between inverter input and ground and a capacitor C2 between junction of R2 and C1, and ground).So, in the example given, if X1 resonates at 15MHz for a load capacitance of 30pF, then giving both C1 and C2 a value 56pF gives a load capacitance of C1 x C2/(C1 + C2) or 28pF plus 2pF of stray capacitance (a reasonable estimate) to give the correct figure.Hence for crystals with relatively low values of load capacitance, it is possible to "pull" them over several hundred parts per million by switching the size of the capacitive load.A device like a VCXO (voltage controlled crystal oscillator) employs the variation in capacitance of a varactor diode to vary its output frequency up to a bandwidth of 10kHz and