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
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 +/-100ppm.Crystals can be damaged by excessive drive level.The probability of this rises for lower frequency and smaller package parts, where the power limit may be as low as 1uW.Moderate overdriving gives rise to premature ageing, so the resonant frequency climbs faster than the few parts per million per year that would be predicted.Extreme overdrive may cause the quartz substrate to crack and fail.Most oscillator circuits are based around an inverter.The resistor R1 forces the IC into its linear mode of operation.R2 limits the voltage across the electrodes of the crystal, acting as a potential divider of the inverter output down to ESR/(ESR + R2).ESR is equivalent series resistance, an indicator of crystal quality.This value is around 30ohm for a part with a 20MHz fundamental, but rises to 800ohm at 2MHz and 50kohm at 32.768kHz.Parts designed to run on third overtone have an ESR roughly double that of the equivalent fundamental.The IC represented by U1 is usually a 4069UB for CMOS, 74HCU04 for HCMOS, or 74S04 for third and fifth overtone circuits.A ceramic resonator is less stable than a crystal, is less accurate, but cheaper, and less prone to overdriving.Some resonator components have load capacitors built in, so have three terminals rather than two.The third is the earth side of the two capacitors.This reduces the component count by two, so saves on board space as well.As the reactive component of the resonator is less at resonant frequency than that of the crystal, an oscillator using it may be more reluctant to start at extremes of temperature, where the loop gain of the circuit is reduced.Hence some resonator manufacturers offer parts that are characterised for use with specific integrated circuits.This may be achieved by varying the sizes of the load capacitors or varying the selection criteria for the resonator element from that of the standard part.All manufacturers' datasheets specify values of C1, C2, the part number of the test IC and the supply voltage to it.A number of suppliers offer VHF to gigahertz up and down convertors, with mainly solid-state low-power components.Modulation of the VHF oscillator to carry analogue or digital data, and recovery of those data after transmission, detection and amplification is a topic beyond the viable length of this article.A good Internet search engine will find plenty of reference works on the subject
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