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.


The US wasn't yet at war, but things were looking pretty grim as 1940 rolled around.  If the States entered the war, they'd need lots of communications sets.  And the two-way radios being developed needed lots of crystals.  At the time, no one knew how to make crystal units from synthetic quartz.  Although natural quartz is common enough on our planet, the stuff that was pure enough to be "electronic grade" was to be found only in one place: in mines high in the mountains of central Brazil, above the tropical jungle.  As production increased, a worldwide shortage of electronic grade quartz rapidly ensued.  Prices for the raw material doubled, which had the strange effect of even further reducing the supply… the Brazilian quartz miners were only looking for enough money to subsist, and raising the price just caused them to quit mining sooner!


Finally a few thousand pounds of the precious material were obtained and loaded onto a freighter, bound for quartz-hungry USA crystal labs.  A Nazi U-boat sank it as soon as it left the harbour.  After that, all wartime shipments of quartz to the States were delivered, at great expense, by government DC-3!  (Today almost all crystal units are made from synthetic quartz, so Brazil's importance to the electronic industry has diminished.)


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.