Eric Vittoz - IEEE

TECHNICAL LITERATURE
He was the first to generate the now-familiar electrical
model for a crystal, in which a series-RLC circuit is
shunted by a capacitance. In short order, Cady developed
an oscillator based on insights facilitated by this
model (Fig. 5) [18].
Figure 5 – Two-port crystal oscillator (from Cady’s
patent application [18]).
Cady’s work caught the eye of George Washington
Pierce, an acquaintance who was teaching at Harvard.
Cady graciously demonstrated his two-port oscillator
to Pierce, who then devised a simpler oscillator that
required only one vacuum tube and a one-port crystal
resonator. The Pierce oscillator has been a standard
circuit block ever since (Fig. 6) [19].
Figure 6 – First published schematic of a Pierce oscillator
(from Pierce’s patent [19]).
Both Cady and Pierce were motivated by the need
for frequency-stable oscillators in the nascent radio
art. Standard LC oscillators were hard-pressed to
maintain frequency within a 1% tolerance band.
Quartz-controlled oscillators are at least a hundred
times more stable, an attribute equally valuable for
transmitters and clocks.
“It Doesn’t Tick – It Hums!”
Aside from renewing interest in piezoelectric technology,
the First World War produced a generation of
soldiers who relied on wristwatches instead of the
less-practical pocket watches that had previously
been in fashion. Consumer demand for wristwatches
grew steadily in the postwar years, and watch manufacturers
responded to the growing interest. By the
end of the Second World War, wristwatches were a
commonplace item.
The invention of the transistor made it inevitable that
watches and clocks would eventually benefit somehow.
The first company to put an electronic watch into production
was Bulova, an American company with facilities
in Switzerland. Swiss employee Max Hetzel received permission
in 1952 to begin research on his ideas for what
would be called the Accutron – a wristwatch based on an
electromagnetic tuning fork as the resonant element.
Oscillation would be maintained by placing the tuning
fork in the feedback loop of a single-transistor circuit. In
early 1953, Raytheon delivered a few CK722 germanium
alloy transistors, and Hetzel started work in earnest. Within
a year he had a working prototype with a 5cm fork
oscillating at 200Hz. His Swiss colleagues were not terribly
impressed, and Hetzel eventually moved to Bulova’s
New York headquarters to continue work on the project
[20]. Working closely with fellow employee William Bennett,
the fork was shrunk to fit within a typical wristwatch,
with a resulting resonance at 360Hz. Each Accutron coil
was wound with 8000 turns of 15μm-diameter insulated
copper wire, conveying some idea of the manufacturing
challenges that they had to overcome. An exploded view
of a second-generation Accutron is shown in Fig. 7.
Figure 7 – Exploded view of Accutron model 218 [21].
46 IEEE SSCS NEWS Summer 2008