Phenomena Underlying Radio: Piezo-Electric Effects
May 1934 Radio News Article

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.

Phenomena Underlying Radio: Piezo-Electric Effects

By E. B. Kirk

Response frequencies of 32 x 47 x 2.760 mm parallel cut quartz plate in region of major high frequency.

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.)

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  February 14, 2014