THE PIEZOELECTRIC EFFECT

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.

Diagram showing crystal cuts Each slice of crystal has a natural resonant frequency it likes to oscillate at depending on the ‘cut’ of the crystal. The frequency of the crystal is controlled by the thickness of the quartz slice plus the added metal electrodes. Most crystals are made from one of three different cuts (right) of quartz according the frequency required (Click it for larger image). AT-cut (1MHz to 300MHz), BT-cut (2MHz to 38MHz fundamental) and the X-cut (24kHz to 50kHZ).

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.

One of my 35 MHz crstals with the can removed 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.

Once a wafer has been cut, the next stage of the production process involves the reduction in thickness of the quartz to get it to the correct size - this skilled process is called 'lapping'. During this stage the sliver of quartz is 'lapped' to the correct size so it will resonate at the correct frequency. Some say this process is somewhat of a 'black art'. Electrodes of silver or gold are added by vacuum deposition - this is where the wires are connected. The final frequency of the crystal is adjusted by adding an extra layer of silver to one side of the quartz sliver. Tolerances are extremely fine (measured in parts per million) and define how close the resonant frequency is to the required frequency - the smaller the tolerance the more expensive it will be. Crystal frequency is usually specified at 25ºC since crystal accuracy is very temperature dependent - some crystals are made to operate in temperature controlled ovens.

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.

THE UK 35 MHz R/C BAND

CHANNEL NUMBER

TRANSMITTER FREQUENCY

RECEIVER FREQUENCY

Frequencies in red are the newer channels released for use in late 2000. They may not be universally available for a while.  These new frequencies should not be confused with the German B-band in 35MHz.

35 MHz is the preferred frequency band for model flying in the UK. 27 MHz 'solids' can be used but this band is not recommended for flying models. In the table there is a list of regular (non DS)  35 MHz channel numbers and their matching transmitter and receiver frequencies. Certain types of equipment (Multiplex, Futaba FC series, etc.) actually use a lower frequency crystal in the transmitter, and use a doubler circuit, so you may find that you have a transmitter crystal that actually reads half that shown here (e.g. 17.575 MHz instead of 35.150 MHz for channel 75).

Receiver crystal frequencies are different since the internal intermediate frequency (I.F.) into account. This is 455 kHz for standard receivers nowadays. Double superhet (DS) receivers use a second 10.6 MHz I.F. as well as the 455 kHz I.F. (some of these crystals may well read 10.7 MHz lower than those shown in the table. Not all crystals can be used in all radio sets and receivers - you must check for yourself.

Frequencies in red are the new channels released for use in late 2000. They may not be universally available for a while. These new frequencies should not be confused with the German B-band in 35MHz.

55
56
57
58
59

60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90

34.950
34.960
34.970
34.980
34.990

35.000
35.010
35.020
35.030
35.040
35.050
35.060
35.070
35.080
35.090
35.100
35.110
35.120
35.130
35.140
35.150
35.160
35.170
35.180
35.190
35.200
35.210
35.220
35.230
35.240
35.250
35.260
35.270
35.280
35.290
35.300

34.495
34.505
34.515
34.525
34.535

34.545
34.555
34.565
34.575
34.585
34.595
34.605
34.615
34.625
34.635
34.645
34.655
34.665
34.675
34.685
34.695
34.705
34.715
34.725
34.735
34.745
34.755
34.765
34.775
34.785
34.795
34.805
34.815
34.825
34.835
34.845