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.
The
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.