May 1934 Radio News and Short-Wave
 [Table
of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
acknowledged.
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 Piezoelectric
material's first widespread application in electronics was in the form of quartz
crystals that were cut along certain axes in order to provide resonance at specific
frequencies. Prior to crystals, radio sets used LC tank circuits for beat frequency
oscillators (BFO's). This 1934 Radio News and Short-Wave magazine article
reports on the state of the art at the time. They are still used in modern circuits
as frequency-determining devices in oscillators and as frequency-selective devices
in filters. Many high Q and tailored frequency response applications are now using
polycrystalline ceramics for surface acoustic wave (SAW) devices where crystals
used to claim an exclusive domain. This article provides a bit of theory of operation
as well as application in radio circuits using both fundamental and overtone frequencies.
Part 9
appeared in the July issue.
Phenomena Underlying Radio: Piezo-Electric Effects - Part Eight
Response frequencies of 32 x 47 x 2.760 mm parallel cut quartz
plate in region of major high frequency.
By E. B. Kirk
The strains in a crystal are approximately proportional to the applied potential
difference and if the potential is reversed the direction of the strains is reversed
also. It follows, therefore, that if the potential is alternated in direction the
plate will be set in vibration at the same frequency. The dimensions of the plate
cause it to have various natural periods of vibration and if the frequency of the
applied voltage is made to approach one of these natural frequencies (different
along the different axes) the response of the crystal increases until, if the two
coincide, resonance occurs and the amplitude of vibration may increase several hundredfold.
If the applied voltage is too great, the crystal can be broken by the violence of
its own motion. When a crystal is made to vibrate in this way, thereby calling into
play the converse effect, the direct effect is also involved, for the motion of
the plate with the charges on the faces gives rise to electrical reactions on the
circuit which is being used to supply the alternating potential difference. The
frequency at which the reaction takes place is in most part dependent on the dimensions
and the elastic constants of the material. By accurate grinding the natural periods
of vibration along the various axes can be made any desired value and thus gives
a simple means for obtaining frequency standards. A device of this sort is called
a piezo-electric resonator.
One crystal can furnish a number of frequencies anyone of which can be called
into action by the proper adjustment of the driving circuit, for in addition to
the natural periods along the three axes the overtones of harmonics can be used.
These are 2, 3, 4 (and so on) times the fundamental frequencies.
Response-frequency spectra of a parallel cut crystal plate at
different temperatures illustrating the interchange of activity between the two
frequencies as the frequencies f the two modes of vibration pass through a coincident
value.
Length of plate along optic axis = 47 mm.
Width along electric axis = 19.35 mm.
Thickness along third axis = 2.75 mm.
(After lack: - mode of vibration and temperature coefficients of quartz crystal
plates. Bell telephone laboratories, reprint. B403.)
We have considered a crystal plate working in a circuit which was driven by an
alternating electric potential. A most important advance was made when it was discovered
that a crystal could be used, in a vacuum-tube circuit, for maintaining oscillations
of constant frequency. A number of circuits were worked with before the present
ones were evolved, and although it is still not ordinarily possible to obtain much
more than a watt or so of power directly from a crystal circuit, it is always possible
to amplify to any desired power level.
The uses to which the direct and the converse piezo-electric effects can be turned
are rather impressive. We have mentioned before the use by the Curies for the measurement
of movements of 1/10,000,000 of a centimeter. It is interesting to note that interferometry
allows measurements to slightly less than 1/10,000,000 cm., and although it has
at present many more chances for application, the crystal methods are superior for
some purposes. Crystals can also be used for the measurements of high pressures,
as in the explosion chambers of large guns. High-voltage measurements have been
previously mentioned; it has been used for the detection of currents as small as
1/1,000,000,000,000 of an ampere. (The vacuum tube is still ahead, being able to
detect as little as 1/10,000,000,000,000,000 ampere.) Piezo-electric oscillographs
have been constructed by Wood; the supersonic waves (in water) of Langevin are used
for deep-sea sounding. submarine signaling, and for the detection of underwater
obstacles. Pierce has used a quartz oscillator for the determination of the velocities
of waves of high frequencies in gases.
Crystals have also been applied the study of vibrations in heavy machinery and
we may expect a rapid extension of its use to other laboratory and engineering measurements.
Hund has suggested the use of an unsymmetrical quartz plate for the production of
audio-frequency power. He has also suggested that a vibrating plate could be used
as an optical shutter, making it possible to measure the velocity of light in a
manner similar to Fizeau's mechanical shutter arrangement; a very important suggestion,
since the velocity of light enters in such a fundamental way into the calculations
of radio theory. The optical properties and the behavior of quartz are most interesting.
There are many likely applications ahead in the fields of phototelegraphy, facsimile
transmission and television.
Multi-Resonance in Crystals
A crystal plate cut without reference to the orientation of the three axes will
in general respond to a large number of frequencies. Plotting the response amplitude
against the frequency gives results similar to those shown in the above figure,
which has been called the frequency spectrum of the crystal plate. Usually there
are one or more frequencies in the spectrum of a plate giving responses sufficient
to drive a vacuum tube in the usual crystal oscillator circuit. Relations between
major response frequencies and the dimensions of a plate cut by the Curie or perpendicular
method are as follows: For this type of plate there are two major response frequencies,
one high and one low. The high frequency which is a function of c, the thickness
of the plate, can be determined approximately from the relation f = K/c, where c
is the thickness of the plate in millimeters and K = 2.860 x 106 (giving
a constant of approximately 105 meters per mm. of thickness). The low frequency
is a function of b, the dimension along the electric axis, and is obtained by substituting
the value of b in millimeters for c in the above relation. With the parallel or
30° cut there is a major high and low frequency, but in some cases the high frequency
occurs as two response frequencies a kilocycle or so apart (termed a doublet).
For thin plates of relatively large area the high frequency is given approximately
by the relation f = K/d where d is the thickness in millimeters and K = 1.96 x 106.
The low frequency, a function of the width, e, of the plate, is obtained by substituting
the value of e in millimeters in the place of d.
Posted December 19, 2022
(updated from original
post on 2/14/2014)
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