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SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $77.50 12 issues (1 year): $145 24 issues (2 years): $270 Online subscription (Worldwide) 6 issues (6 months): $55 12 issues (1 year): $105 24 issues (2 years): $200 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: 1 Huntingwood Dr, Huntingwood NSW 2148 54 Park St, Sydney NSW 2000 2 Silicon Chip Editorial Viewpoint Crystals: more than meets the eye Similar to ICs, we tend to drop crystals into a circuit and expect them to ‘just work’, without realising the amount of engineering involved. When they were first introduced, crystal resonators were expensive devices; today’s high-precision, low-cost crystals are the culmination of a huge amount of research and manufacturing investment. You may be aware that quartz is a piezoelectric material, meaning that when a voltage is applied to it, it changes shape slightly. Similarly, if you apply mechanical force to a quartz crystal, it generates a small voltage. Essentially, quartz acts as an electromechanical transducer. This property is used in piezo buzzers and force/pressure/acceleration sensors as well as crystal resonators. In use, a quartz crystal resonator acts as a very high-Q mechanical resonator, excited by the surrounding electrical circuit. Very few other materials can do this and remain stable in the long term. At the crystal’s natural resonant frequencies, the motional impedance drops drastically, so the electromechanical conversion becomes extremely efficient. Quartz has very low mechanical damping, minimal dislocation mobility, low internal friction in shear modes, and a stable crystalline lattice with very few slip systems. This means that once energy is put into a shear vibration, the lattice does not readily convert it to heat. Most other common solids dissipate orders of magnitude more energy per cycle. For example, a quartz resonator can have a Q value in the range of 104 to 106, while most metals, ceramics and glass operating as mechanical resonators typically have Q values in the range of 102 to 104. The natural resonant frequency of a crystal fragment depends on its size, thickness and the way it is cut relative to the crystalline structure. A crystal can operate in multiple vibration modes: shear, flexural, tuning-fork mode and others, each resonating over a different frequency range. At resonance, the motional reactances cancel, reducing the impedance of the crystal to a low value, often just a few tens of ohms. Off-resonance, the impedance changes rapidly. This is why a crystal oscillator locks so tightly onto one frequency: the crystal’s mechanical resonance dominates the feedback loop. Creating a modern crystal resonator starts with synthetic quartz grown slowly using hydrothermal processes in autoclaves, rather than by melting (as is used for growing silicon crystals in semiconductor manufacturing). The crystalline structure is analysed, then cuts are made at specific angles to create different crystal types (AT-cut, BT-cut etc). The plates are polished to extremely precise thickness, cut into precisely sized and shaped pieces, and electrodes are added with minimal stress to avoid altering the crystal’s behaviour. They are then mounted on tiny flexible supports at vibration nodes and hermetically sealed in a can to reduce ageing. A crystal actually has two closely spaced resonant frequencies. At its series-resonant frequency, its impedance falls to a minimum. Slightly above this is its parallel or anti-resonant frequency, where the crystal’s motional components interact with its electrode and package capacitance to produce a very high impedance. Many oscillator circuits operate between these points, at a frequency determined partly by the external load capacitance. This is why crystals are specified with a particular load capacitance and using the wrong capacitors can shift the frequency. All this work goes into producing a precision device that you can buy for tens of cents each in volume. So next time you use a crystal, consider the effort and technology that went into it behaving predictably and operating seamlessly in your circuit. by Nicholas Vinen Australia's electronics magazine siliconchip.com.au