January 1957 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
According to a 2001 paper published by the
National Institute of Standards and Technology (NIST, formerly National Bureau of Standards, NBS), "The end of the era of quartz
frequency standards began in 1949 with the development at NBS of the world’s first atomic frequency
standard based on an ammonia absorption line at 23.87 GHz." Further, "The Bureau supported work
on both technologies for the next decade, but the rapid advances in the accuracy of atomic frequency
standards could not be matched by quartz devices, and the work on quartz frequency standards was stopped
in 1959." This article from a 1957 edition of Popular Electronics claims that the "master of
all master-clocks" resided at the U.S. Naval Observatory at the time - not quite accurate from what
my research indicates. Still, it is a good introduction to crystal growth and processing for use as
After Class: Special Information on Radio, TV, Radar, and Nucleonics
Some Facts on Quartz Crystals
In the U. S. Naval Observatory at Arlington, Va., is the master of all master-clocks. The primary
standard for all the timepieces of the nation, this clock is never fast or slow by more than a few seconds
in one-hundred million. If you were to measure the distance between New York and Miami, Florida, with
this kind of accuracy, the results would not be in error by more than the length of one standard cigarette!
Such fabulous micro-precision, matched only by the earth itself as it spins on its axis, is keyed
to a tiny slab of crystalline quartz held under rigid temperature control in a special oven. Although
it may soon be supplanted by an even more precise time standard (resonance of cesium atoms), the quartz
crystal still remains the most important frequency-controlling device in existence today.
How They Are Cut.
Oscillating crystals are cut from so-called mother stones by high-speed carborundum wheels. Although
most finished plates come from natural quartz prisms, modern techniques for growing mother stones in
the laboratory have been perfected to the degree where our dependence upon nature soon will be unnecessary.
Synthetic crystals are often superior to natural ones since the conditions during synthesis are held
under rigid laboratory control.
Synthetic quartz mother stone was grown in a laboratory.
Bell Telephone labs.
A crystal exhibits piezoelectric activity. This means that it develops electric charges on its faces
when compressed or otherwise distorted in shape. When connected in the grid circuit of a vacuum tube,
sustained oscillations of one particular frequency are produced. Since the thickness of the crystal
plate is an important factor in determining the frequency of oscillation, the cut blank is ground and
polished until its mechanical resonant frequency arrives at the desired value; the thinner the crystal,
the higher its resonant frequency.
Thin wafers are cut from large mother stones by multiple saw. Wafers must then be ground and polished.
James Knights Co.
At one time, only X-cut and Y-cut crystals were available. These terms indicate that the crystal
slices are taken from the mother stone at right angles to the X and Y axes. The X-axis is a line joining
two opposite corners of the hexagonal prism and the Y-axis is a, line joining the mid-points of the
two opposite faces. (See Fig. 1.) Both of these cuts are subject to temperature effects, changing frequency
over relatively wide ranges as the temperature varies.
Fig. 1. Axes of mother stone and orientation planes of X-cut and Y-cut crystal blanks. A hexagonal
prism has six X and six Y axes.
Crystal blanks are being loaded into a planetary type of lapping machine prior to final polishing.
Between 1934 and 1949, vastly improved cuts were discovered by scientists connected with Bell Telephone
Laboratories and RCA. Labeled AT, BT, V, CT, and DT, these crystals were ground from blanks oriented
at complex angles with the axes of the mother stone. Finally, in 1940, the most stable quartz crystal
ever devised was announced by W. P. Mason. Known as the GT-cut, this crystal shows no appreciable change
in frequency from -25° C to +75° C; the cut remains virtually "on-frequency" over a range of
Disassembled military type FT-243 (7620-kc.) pressure-sandwich type of crystal holder used extensively
in communications equipment.
The kind of cut you might use depends upon the depth of your purse and its contents. X- and Y-cuts
are quite inexpensive while a GT-cut for a specific frequency is still prohibitive in cost to experimenters
of average means.
Characteristics. The most important single crystal parameter is its temperature coefficient. Since
the frequency of a Y-cut crystal rises with temperature, it is said to have a positive coefficient;
conversely, the coefficient of an X-cut crystal is negative because frequency drops with rising temperature.
For example, the temperature coefficient of a certain Y-cut crystal is given as +75 p/m/C°. This
is read as "plus 75 parts per million per °C." It means that for every degree rise in temperature,
the crystal frequency rises 75 cps for each megacycle of its basic frequency. Thus, if you assume that
the temperature of this Y-cut crystal ground for 3 mc. at 0° C goes up 10° C, the change in
frequency will be: 75 cps X 3 mc. X 10° = 2250 cps = .00225 mc. Adding this to the basic frequency
gives 3.00225 mc. On the surface, this does not appear to be a serious deviation, but it is a change
of 750 parts per million for only 10° of temperature variation. The GT-cut, on the other hand, varies
only one part per million over a 100° C range (180° F).
For any given cut at a specified temperature, the frequency of oscillation is determined by the
thickness of the blank. As a crystal is ground thinner and thinner, its natural frequency rises. In
the early days of crystal control, it was virtually impossible to grind the quartz slabs thin enough
to exceed 20 mc. without having them fracture during operation. Since some cuts are worse than others
in this respect, the thickness-frequency specification for each is given in terms of the frequency factor.
This parameter is defined by the simple equation: k: = F X t; where F is the fundamental frequency of
the crystal in mc., t is the thickness in thousandths of an inch, and k is the frequency factor of the
For example, the X-cut has a frequency factor of approximately 112 while the
Y-cut is rated at 77. Suppose that one of each of these were to be ground to oscillate at 4 mc. Their
respective thicknesses would be:
X-cut: t = k/F = 112/4 = .0028"
Y-cut: t = k/F = 77/4 =
From this it is evident that the larger the frequency factor of a crystal, the thicker
it may be for a given frequency. The AT-cut, with a frequency factor of about 66, is just about the
thinnest of all plates.
Overtone Crystals. An overtone or harmonic quartz crystal is one that has been specially ground or
otherwise treated by the manufacturer so that it vibrates in two or more parts rather than as a whole.
Essentially, this process is very similar to overtone production in musical instruments where the sounding
body vibrates in parts showing nodes and loops along its length (Fig. 2). If a crystal were to vibrate
in two equal parts, the output would be exactly double the fundamental or "one-part" frequency. In practice,
this is seldom the case, because overtone crystals do not "break up" into equal sections as they oscillate.
Fig. 2. Analogy showing relation of overtones to fundamental: (A) a string producing its fundamental
tone by vibrating in one part; (B) first overtone or second harmonic of the string; (C) second overtone
or third harmonic.
If, for example, an AT-cut crystal is treated to produce third harmonic output at 21 mc., it might
be marked "7-mc. fundamental" This means that its harmonic frequency is approximately three times its
fundamental; its output may differ from the true third harmonic by several megacycles. When such crystals
are purchased for transmitter control, the buyer should know the harmonic output rather than the fundamental
Overtone crystals are almost always used in special oscillator circuits in which the crystal responds
at its series resonant frequency. Standard oscillators operate at their parallel resonant frequencies.
Mounting. The development of better crystal holders has kept pace with improvements in the fabrication
of finished quartz plates. These holders are designed to avoid interference with the piezoelectric vibrations
of the crystal and to provide protection against mechanical shock. In the pressure-sandwich type of
holder, the crystal is supported between two electrodes which are in intimate contact with a pair of
flat metal plates to insure good electrical connection. Spring loading and the use of fiber and neoprene
make for firm support, excellent protection, and hermetic sealing. This type of holder typifies medium-frequency
mounting techniques; at the higher frequencies, particularly when overtone crystals are employed, other
kinds of holders are favored.
For extra-precise control of frequency, crystal holders are often enclosed in thermostatically controlled
ovens which maintain the frequency constant over extremely wide variations in ambient temperature. Such
holders are very compact and weigh only a few ounces.
Four types of mountings. At left is a 110-kc. X-cut crystal in a military type HC-13/U holder which
produces very little damping and some mechanical resonance in wires supporting the crystal. Next is
a GT-cut in a special holder; the crystal is first plated with gold, then placed in evacuated glass
holder, and is supported by eight wires soldered to plating. In cutaway view, a circular, silver-plated
AT-cut in a glass holder is mounted inside temperature-controlled oven for greater stability. At right
is a close-up view of an NT-cut in a miniature glass holder supported by four wires.
"After Class" Topics
Posted April 3, 2013