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by Graham Woods Ubiquitous quartz surrounds us - it’s everywhere, it’s in the soil and in the houses we live and beats at the heart of almost every piece of consumer electronics - we're talking crystals here... Piezoelectricity is the property of quartz that we utilise in our receiver and transmitter crystals. So what is it? Put simply: it is an electric voltage produced by certain crystals and by a number of ceramic materials when they are subjected to pressure. What’s more, the piezoelectric effect works both ways: stress a piece of quartz and you get an electrical output from it that is proportional to the stress it undergoes. That is to say, when the quartz has an electric field applied to it, the crystal becomes deformed or strained by an amount proportional to the applied field; the sense of the strain depends on the direction of the field. Incorporate a crystal in an oscillator circuit (in our Tx and Rx) and it will make the circuit run very accurately at the required frequency.
Of course, there’s much more to it than that because fundamental frequencies or overtones are selected for circuits and some oscillators need to be very temperature stable and so on. In 35MHz R/C equipment 3rd overtone receiver crystals are generally used but some Tx's like Graupner MC-20 use an 8 point something MHz fundamental and multiply it up by four. Designers’ circuitry requirements (single and double superhet receivers, capacitance, etc) require different values for receiver crystals too; DS Rx's have two crystals 10.7 + 24...MHz - this is why crystals are not interchangeable. Nowadays, the designers of electronic equipment don’t go traipsing round the Brazilian jungle looking for the large natural quartz specimens we see in museum gemstone collections to cut up; there is a large business concerned with the growing of manmade crystals and ceramics of all sorts. The process is hardly different from that of your school days when you dangled a seed crystal of copper sulphate on a cotton thread in a saturated aqueous solution of copper sulphate to grow a larger specimen. With quartz the process is essentially the same but uses steel wire suspended in molten quartz under extremes of pressure and temperature with the crystal literally pulled slowly from the solution. Control of the speed of pull, pressure and temperature and doping agents enables crystal bars of all sorts of materials from humble quartz to the complex compounds (for transistors, IC's, l.e.d.'s, etc.) to be created with precisely aligned crystallographic orientations and electronic/quantum physical properties. These bars are then sliced into thin wafers for etching and cutting for the manufacture of electronic components.
The Piezoelectric Effect was discovered in the 1880's and is used widely in a number of transducers and electronic gear. Your old record player had a cartridge that used the piezoelectric effect, the ultrasonic transducers in your ancient car alarm used it, that hospital ultrasound scanner uses it, some gas and cigarette lighters use it. Then there are strain gauges and accelerometers, flow meters and pressure transducers of all sorts including altimeters, variometers and airspeed indicators not forgetting modern barometers, model gyros, radios, TV’s, microphones and computers, your Swatch watch and even artificial limbs. You name it, there's a tiny piece of quartz or piezo-ceramic in there somewhere. As an aside: on a global scale, large earthquake movements are also said to produce massive releases of piezoelectricity in the form of sparks and ball lightning as rock formations are put under extremes of pressure.
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In
my photograph of a damaged 35MHz AT-cut 3rd
overtone receiver crystal (shown here with the
dented can removed) notice how thin the slice of
quartz is; notice too the shaved edge where
quartz slice was aligned with the actual crystal
structure of the crystal when the quartz plates
were cut from the original bar. Now look at the
those hair-thick wires with loops on... your
prized or expensive model is hanging on just such
a pair of delicate wires inside the receiver!
Remember this the next time you crash a model.
Also note that pulling crystals in and out of
equipment doesn't do the crystal pins and sockets
any good at all - these delicate, sensitive
wafers of quartz are hermetically sealed in a dry
nitrogen atmosphere in their cans with their pins
in 'glass' seals and should be handled with care
so as not to damage the seal. If the seal fails
then moisture can penetrate the can and degrade
the electrodes. It goes without saying you should
take special care of your crystals.