Q Jon, W4BCT, asks, “I recently bought some radio crystals.
Most are removed from 1940s Navy radios. When I
was young my father had some of these, and I wanted to take them apart to see what was inside. Of course, he wouldn’t allow this. Now I have some to play with, but I was wondering if you could explain how crystals work?”
A A number of crystalline substances found in nature have the ability to transform mechanical strain (movement) into an electrical charge, and vice versa (think of a tuning fork or a church bell which can transform mechanical strain into sound). This property is known as the piezoelectric effect. A small plateor bar cut in the proper way from a quartz crystal and placed between two conducting electrodes will be mechanically strained when the electrodes are connected to a source of voltage. Conversely, if the crystal is squeezed between two electrodes a voltage will be developed between the electrodes.
Crystalline plates also are mechanical resonators that have natural frequencies of vibration ranging from a few thousand hertz to tens of megahertz. The vibration frequency depends on the kind of crystal, the way the plate is cut from the natural crystal, and on the dimensions of the plate (like the tuning fork and the bell). The thing that makes the crystal resonator valuable is that it has extremely high Q, ranging from 5 to 10 times the Qs obtainable with good LC resonant circuits.
Since the crystal has a definite resonant frequency controlled by the crystal lattice, it can be used to “regulate” an oscillator to a high degree of accuracy.
The crystals we use most often resonate in the 1- to 30-MHz region and are of the AT cut, thickness shear type, although these last two characteristics are rarely mentioned. A 15-MHz-fundamental crystal of this type is about 0.15 mm thick. Because of the widespread use of reprocessed warsurplus, pressure-mounted FT-243 crystals, you may think of crystals as small rectangles on the order of a half-inch in size. The crystals we commonly use today are discs, etched and/or doped to their final dimensions, with metal electrodes deposited directly on the quartz. A crystal’s diameter does not directly affect its frequency; diameters of 8 to 15 mm are typical.
AT cut is one of a number of possible standard designations
for the orientation at which a crystal disc is sawn from the original quartz crystal. The crystal lattice atomic structure is asymmetric, and the orientation of this with respect to the faces of the disc influences the crystal’s performance. Thickness shear is one of a number of possible orientations of the crystal’s mechanical vibration with respect to the disc. In this case, the crystal vibrates perpendicularly to its thickness. Place a moist bathroom sponge between the palms of your hands, move one hand up and down, and you’ll see thickness shear in action.
From QST January 2001
