Phenomena Underlying Radio (part
July 1934 Radio News and
the Short-Wave Article
is part 8 in a series published by Radio News and the Short-Wave magazine
in the early 1930s. As with most topics pertaining to electronics, the
theory is still relevant and applicable to many modern circuits and
systems. Piezoelectric principles are introduced for determining the
frequency of oscillators. I have to admit to not having heard of the
'pyroelectric' effect. A pyroelectric crystal when heated or cooled
develops charges on the extremities of its hemihedral (another new word
for me, meaning "exhibiting only half the faces required for complete
symmetry") axes. read on to learn more.
July 1934 Radio News & Short-Wave
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Phenomena Underlying Radio
Part 8 (Piezo-Electric Applications)
E. B. Kirk
Piezo-Electric crystals have also been adapted to use in phonograph
pick-ups and in microphones and loudspeakers. The acoustical actions
give promise of becoming very valuable. Piezo-crystal oscillators and
resonators have furnished a most convenient form of wavemeter and are
an excellent means for maintaining frequency standards. Little more
than mention of the work in this field can be made here, but frequency
determinations and control are of the utmost importance to aural broadcasting
and to television.
A Crystal-Controlled Oscillator
Figure 1. This shows a typical crystal-controlled oscillator
circuit. For ultra-short waves (in the neighborhood of 1 to
5 meters or a little above) tourmaline has proven more suitable
than quartz for fundamental control, since it produces more
uniform oscillation with less tendency toward side-tone oscillation.
It also allows a frequency gain of about 35%, for quartz, for
the same size plate. Tourmaline has a constant of approximately
80 m. per mm. when cut as a disc. The mounting of such a crystal
is very important. It should rest on an electrode having a carefully
lapped plane surface. Even slight unevenness will cause irregular
operation and crystal damage by overheating, fusion or cracking.
Silvering or sputtering can be used with large crystals, but
it affects the period of small plates. In any case, the upper
electrode, although making uniform contact, must not exert excessive
pressure and is therefore best held in place by a spring. One
commercial type of mounting carries the crystal within an evacuated
bulb. The crystal and its mounting should, at least, be hermetically
sealed and some thermostatic means provided for keeping it at
constant temperature. T he values of L and C in the circuit
are chosen for resonance at the desired wavelength and the crystal
dimensions are determined so that either one of its fundamentals
drives the circuit at the desired frequency. The constants of
the intermediate and final power stage are chosen to resonate
to the frequency of the crystal oscillator
Marrison (of the Bell Laboratories) has by
means of a series of circuits reduced the frequency of a circuit controlled
by a crystal from 100,000 kilocycles per second to 10 cycles per second
and at this low frequency has driven a clock. By such an arrangement
it is possible to maintain a frequency constant over a period of days
to within an accuracy of 1 in 10,000,000. So far this is, to my knowledge,
the most accurate timekeeper devised.
One method of calibrating
wavemeters makes use of a peculiar luminous property of a vibrating
crystal first observed by Giebe and Scheibe. They invented (in 1925)
what is called a luminous resonator. This consists of a crystal plate
resting on one electrode, but with the upper electrode separated from
the crystal by a small air-gap and the whole affair mounted in a partial
vacuum (10 to 15 mm. of mercury). When one of the resonance frequencies
of the plate is approached, by tuning the driving circuit, the interaction
of the direct and the converse effects causes luminous bands to appear
on the upper surface of the crystal. The number and the arrangement
of the bands depends on the manner in which the plate is vibrating;
that is, which fundamental or overtone is acting. This gives a convenient
visual indicator and has been used for the comparison of the international
standards of frequency.
Quartz has been used almost exclusively for piezo-electric crystals,
although mica, Rochelle salts, tourmaline. boracite, sugar, d-tartaric
acid and many other substances of the same crystallographic form can
be used. For frequencies below 25 kilocycles, quartz crystals of sufficient
size are difficult and expensive to obtain (magneto-strictive methods
which we shall discuss later are useful below 25 kc.). Recently Rochelle
salt crystals have been "grown" very successfully and are being used
to advantage particularly for phonograph pick-ups and microphones.
We have not discussed the results of twisting a crystal; this
action, although it can be analyzed into a combination of compression
and tension applied in a complex way, is too complicated to be approached
in a non-mathematical manner.
The Pyro-Electric Effect
It was mentioned previously that Dutch travelers returning
from Ceylon about 1703, with tourmaline crystals, discovered the piezo-electric
effect. Although there may be doubt that they recognized the pressure
action as such, it seems clear that they did observe definitely the
pyro-electric effect by noticing that the tourmaline crystals which
had become heated in an open fire strongly attracted the hot ashes.
This unusual action is exhibited by a limited class of crystals (all
crystals having one or more axes with dissimilar ends and which constitute
the class of hemihedric crystals with inclined faces). The Curies, after
trying a number of substances, concluded that all crystals which showed
pyro-electric action showed also piezo-electric response. Some of the
substances tested were sodium chlorate, tourmaline, quartz, topaz, Rochelle
salts and sugar.
A pyro-electric crystal when heated or cooled
develops charges on the extremities of its hemihedral axes. If a crystal,
tourmaline, for example, be heated and then broken, the parts will show
the same polarity as the unbroken piece, and if it be powdered and spread
on a glass plate and its temperature changed, the particles of the crystal
will arrange themselves in lines similar to iron filings in a magnetic
field. This shows that there is a polarity developed even in the smallest
pieces. This action is explained in a manner somewhat similar to the
explanation of the piezo-electric action. The heat causes changes within
the crystal which are unequal along the various axes and, since the
electrons are bound, in the rearrangement of the molecules. There is
a shift or polarization.
Pyro-electricity has not been put to
any startling use, but it is evident that since mechanical change always
involves heat, an application of mechanical force, compression, tension,
bending, twisting, etc. would, by causing inequalities of temperature,
give rise to electric charges on crystals submitted to these forces.
A series of compressions and rarefactions (such as sound waves) would
cause a corresponding variation in the electrical condition which in
turn could be detected or amplified. Further the converse, as in the
case of the piezo-electric effect, is possible: changes in potential
difference applied to the appropriate faces of a crystal cause changes
in temperature within the crystal. However, we see at once that if this
were done a condition exactly similar to that considered under the piezo
action is brought about. Obviously the four effects, the two direct
and the two inverse, are tied together.
Posted July 24, 2013
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