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
This 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. Types other than the familiar quartz include tourmaline, boracite, topaz, Rochelle salts, and even sugar. Read on to learn more.
Phenomena Underlying Radio
Part 8 (Piezo-Electric Applications)
E. B. Kirk
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
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.
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 August 22, 2019 (original 7/24/2013)