Fundamental Crystal Control for Ultra-High Frequencies
April 1932, QST
radio frequencies continued to increase during the early years of
'wireless' development, the use of quartz crystals as a stable reference
source ran into a physical limitation because as crystal slices
reached a certain thinness, overtone and subharmonics appeared that
caused problems in circuits. A new mineral called tourmaline saved
the day. With an elasticity much greater than quartz, tourmaline
is able to vibrate at higher fundamental frequencies for a given
thickness. Since that time, science has provided the means for utilizing
the much more abundant quartz crystals in overtone modes so that
tourmaline is not required. At the time, though, it was a much-welcomed
May 1932 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Fundamental Crystal Control for Ultra-High Frequencies
Tourmaline Oscillators for Wavelengths Down to 1.2 Meters
By Harald Straubel
Fig. 1 - Lycopodium pattern of transverse oscillation of a quartz
plate cut so that vibrations starting at the center are reflected
simultaneously from the circumference. The powder thus collects
at the central nodal point.
Fig. 2 - Lycopodium pattern on the surface of a "raw" tourmaline
crystal plate that was cut perpendicular to the axis.
Fig. 3 - When the crystal of Fig. 2 was made circular the pattern
of the oscillations became more symmetrical, as shown by the
concentric rings of the lycopodium collected at the nodes.
Fig. 4 - Contrast this pattern of a disc-shaped quartz oscillator
with those of the tourmaline shown in Fig. 3.
Fig. 5 - Ii the tourmaline crystal the optic and electric axis
have the same direction.
Fig. 6 - The tourmaline controlled oscillator circuit. It is
of the same type as the familiar circuit generally used for
quartz crystal control.
Fig. 7 - Performance curve for an RE 134 valve with tourmaline
crystals of the same diameter but of different thicknesses to
give various wavelengths. The efficiency decreases rapidly at
the shorter wavelengths.
Fig. 8 - An ultra-high frequency tourmaline crystal that was
cracked near the edges from overloading without impairing its
operation. The diameter of this plate is 8 millimeters (0.312
Fig. 9 - The tourmaline plate with respect to its three axes.
The major surfaces are perpendicular to the optic (Z) axis and
parallel to the plane of the X and Y axes.
Here is confirmation of exciting rumors that have been coming
through from Germany since last fall. Practicable fundamental
crystal control at the frequencies too high for quartz, has
arrived. Tourmaline crystal does the trick. The pleasure of
presenting this article is the keener because of the friendly
cooperation of the A.R.R.L.'s sister societies, the D.A.S.D.
and the R.S.G.B., that have brought it to us. The connection
with the D.A.S.D. of Dr. Straubel, the author, and that with
the R.S.G.B. of Mr. Pilpel, the translator, make this contribution
a fine feather in the cap of amateur radio. - Editor.
The method of fundamental crystal control which has been used
so far presents difficulties on frequencies higher than 6000 to
7500 kc. because very thin quartz oscillators produce side tones
on several neighboring frequencies. This multiplicity of oscillations
is noticeable even on longer waves but can be suppressed by cutting
the quartz plate in a suitable shape. Fig. 1 shows such a plate.
The surface and sides are ground so that all oscillations starting
at the center will be reflected at the same time from the circumference
of the crystal. The radius of the curves at the corners depends
on the square root of the modulus of elasticity of quartz. If this
plate is excited on its fundamental frequency, the whole plate oscillates
in all directions. Lycopodium powder, uniformly distributed over
the crystal plate, is therefore concentrated at the middle point,
which is the nodal point of the whole disc.
higher frequencies than 6000-7500 kc. crystal-controlled short-wave
transmitters nowadays do not often use this method, but employ quartz
crystals with fundamental frequencies such as 3000 to 1500 kc.,
with frequency multiplication; that is to say, one uses harmonics
of the crystal oscillator and amplifies them. An installation of
this sort becomes very cumbersome and complicated when used on the
ultra short waves because the harmonics are so weak that amplification
must take place after every frequency doubling before further frequency
doubling is practicable.
If it were possible to construct
fundamental oscillators for such short waves, the installation would
be extraordinarily simplified.
The tourmaline crystal controlled oscillator operates at 75
megacycles (4 meters) with the stability and other characteristics
of quartz-crystal controlled oscillators operating at much lower
frequencies. Imagine the number of frequency multiplying stages
that would be necessary to duplicate its performance with quart-crystal
On the above-mentioned grounds, quartz is unsuitable for short-wave
fundamental control. The author has found, however, that a tourmaline
crystal produces considerably more uniform oscillations than quartz.
It was noticed even with a "raw" crystal, a plate simply cut perpendicular
to the optical axis, that on exciting the longitudinal oscillations,
extraordinarily uniform transverse oscillations resulted, as shown
in Fig. 2.
The piezo-electric constant of tourmaline is
i.e., about 10% less than quartz.
If, in spite of the
smaller constants and the irregular natural formation of this crystal,
particularly easy oscillations are noticeable, it is highly probable
that this is because there is considerably less tendency to side-tone
oscillation. Actually, in the natural crystal only a few symmetrical
side tones could be detected and the frequencies of these were far
removed from the fundamental.
From these results it can be
assumed that the oscillating properties even of such a thin crystal
will remain constant, as they must do if it is to be used for fundamental
control of ultra short-wave transmitters. A further advantage for
the production of very high frequencies lies in the high speed of
sound, necessitated by the extraordinarily large elasticity modulus
of 1,600,000 kilograms per square centimeter in the direction of
the optical axis. Tourmaline, therefore, supplies a frequency 35%
higher than quartz of the same thickness, the constant being 80
meters wavelength per millimeter thickness.1
Circuit and Tubes
A circuit for the production
of the oscillations is shown in Fig. 6, while the photograph shows
the actual transmitter for use on 7 meters and employing 5 watts.
The simplicity of construction and absence of any type of screening
For waves down to 2 meters (150 mc.) ordinary
detector or power valves were used (such as Telefunken RE 084 or
RE 134), but for shorter waves a special short-wave valve was employed,
the Valvo S 0401. For longer waves of 5 meters and upwards, a larger
valve was found suitable, the RS 241, which is similar to the American
Type '10 and the Philips TB 04/10. It was noticed that, as is usual
with all self-excited transmitters, the higher the frequency the
lower the efficiency, oscillation eventually ceasing altogether.
Fig. 7 shows the performance of an RE 134 valve. The reason for
this is partly the large self-capacity of the crystal which, for
a given valve capacity, upsets the phase of the reaction on short
waves to such an extent that the crystal no longer oscillates. The
crystal then works only as short-circuit capacity while the valve
either oscillates unstabilized or ceases to oscillate altogether.
This can be avoided by reducing the capacity of the crystal, which
can be done only by decreasing the diameter. An increase in the
diameter does not enable the crystal to stand a greater load on
these ultra short waves, however.
The electrodes next presented certain difficulties. Silvering,2
such as used in thick oscillators, must be omitted as this affects
the fundamental frequency of the thin plate quite considerably and
a great load will cause peeling, although the oscillator remains
undamaged. Therefore, silvering was dispensed with and perfectly
level electrodes were used. Even slight unevenness of the electrodes
caused certain parts of the crystal to melt and become disintegrated.
Splintering, such as occurs with quartz, was never experienced.
By using perfectly plane electrodes the load on the crystal
could be considerably increased. Working temperatures of more than
100° Centigrade did not affect the operation in any way. Great overloading
(crystal 8 mm. diameter, RS 241 Valve with 350 volts plate potential)
once caused the edge of the crystal to crack without affecting the
middle portion or causing the efficiency to decrease materially.
When the crystal was in a horizontal position, no
extra weight was placed upon the electrodes. Although an additional
weight reduced the crystal's controlling properties, it was possible,
in the cases of crystals of larger diameter (12 mm.), to subject
them to pressures up to 500 grams per square centimeter before they
stopped oscillating. In order to render them safe from shocks and
vibrations, a light spring was always used to supply pressure.
With the 75-mc. transmitter
depicted experiments were carried out to determine the actual degree
of frequency stabilization obtainable. The modulated transmissions
were received on a simple detector with regeneration over a distance
of 1 kilometer although no aerial was used for either transmitting
or receiving. As far as the transmitter was concerned, no hand-capacity
effects were noticed; but when the hand was placed near the inductance,
reception was louder owing to the body acting as a capacity-coupled
aerial. Only when the coil was actually touched did oscillation
cease, to recommence immediately on the same frequency when the
hand was removed. This was particularly noticeable when the transmitter
was keyed in the plate circuit. The heterodyne note as heard in
an oscillating receiver was absolutely pure on the frequency of
75 mc. (4 meters) and showed d no frequency change whatsoever, better
than could be obtained when using a 7500-kc. quartz crystal. The
well stabilized transmitted frequency in every case could be received
on the detector with regeneration and without the need for the broad
resonance curve of a super-regenerative receiver.
the condenser C (Fig. 6) no jumping in frequency could be noticed.
Naturally, the frequency of the tourmaline, just as that of quartz,
was affected by varying the tuning of the plate circuit, but frequency
jumping never took place. To obtain this condition, however, perfectly
plane parallel crystal surfaces are necessary.
coefficient of tourmaline oscillators is about 10% greater than
the average of quartz and is negative. Repeated measurements in
the region of 20° to 60° Centigrade showed this coefficient to be
46.6 parts in a million per degree Centigrade. For an accurately
constant frequency one must use a thermostat and heater, the cost
of which is inconsiderable in comparison with the amount saved by
the elimination of frequency-doubling and amplifying stages, and
the simplification of the installation. For simple transmitters
one can easily dispense with temperature control. The frequency
change of tourmaline is always in proportion to the temperature
9 shows how the crystal plate is cut from the actual crystal. The
position of the different axis is the same as in Fig. 5. Most suitable
tourmaline crystals are found in Brazil and South Africa.3
In conclusion, I wish to offer my thanks to the firm of Carl
Zeiss, Jena, Germany, for help in my experiments. In particular,
without their great interest it would have been impossible to overcome
the difficulties in construction of the shortest wave oscillators
for 1.2 meters.
to the English system and in terms of frequency, t=146.25f, approximately,
where t is thickness in inches,
f is frequency in kilocycles. The thickness dimension is parallel
to the Z axis for tourmaline whereas it is generally
parallel to either the X or Y axis for quartz. - EDITOR.
Cf. Parsons, "Silvering Electrodes on Quartz Crystals," QST, March,
1932. - EDITOR.
3. We understand that the common black
variety, known as
schorl, is generally unsuitable. - EDITOR.
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