NAZIS SANK MY
Imagine a tuned LC circuit with a Q of 10,000! Incredibly narrowband, if you wanted it to resonate at a frequency of 1 MHz, it would need an L of ten or twenty Henries, and a C of twenty or so fF (femto-Farads=10-15 Farad)… It would be altogether not very practical, since any coil of that size would have tonnes of stray capacitance, and aside from its bulk, it would completely swamp out your fF capacitor. And yet, there is a way to do it, because we've all seen circuits with Q's like that…called crystal oscillators!
Quartz crystal manufacturing technology started development in between the two World Wars, and it's a fascinating story, full of adventures and derring-do worthy of Indiana Jones and his gang. While the work was driven by the requirements of the military, most of the discoveries were made by radio amateurs, who were experimenting with stuff they really didn't understand very well.
The piezoelectric effect started it all off, back in the 1880's: Marie and Pierre Curie discovered that there were a few substances, like quartz and Rochelle salt, that when given a squeeze, would produce a voltage. Likewise, supply a voltage, and the crystal changes its own shape. Quartz crystals are like electric motors and generators in the sense that they convert between electric and mechanical energy. If an AC signal of the right frequency is applied to it, the crystal will resonate, and start to vibrating. Like any object in elastic motion, the crystal has an elasticity, and a "reluctance" to change in motion: the elasticity shows up as a capacitance, and the "reluctance" looks to the circuit like a very large inductance. These two quantities make up the motional reactances of the crystal. And the "right frequency" happens to set up a standing wave inside the crystal structure, at the same frequency that the motional reactances are equal in magnitude and opposite in sign!
Quartz is a crystal, meaning that the molecules in a chunk of it are lined up in a particular pattern. Slicing the crystal at a particular angle to its geometry produces a wafer that can be excited in one fashion or another. The dimensions, primarily the thickness, set the resonant frequency. There are special "magic" angles for the slicing, which can be measured by x-raying the crystal. Prior to 1926, all crystals used the "X-cut." In 1927 the "Y-cut" was found, and in 1934 the "AT-" and "BT" cuts were discovered. Today's general-purpose crystals are 99% AT-cut. The different cuts have different characteristics, including temperature stability. One of the very first niche applications for quartz crystals was in the oscillator circuits of broadcast transmitters.
Finally a few
thousand pounds of the precious material were obtained and loaded onto a
freighter, bound for quartz-hungry
Next time, we'll look at some of the various quartz products used in broadcasting: parallel and series crystal units, and SAW filters and crystal filters.