Radio broadcast .. (1922-30)

264 RADIO BROADCAST JANUARY, 1927 FIG. 2 Several quartz crystals and pieces sawed from larger crystals. The pyramid ends of the large crystal in the picture forming the heading to this article are shown among them each other by scientists. The useful pair now cooperate in many ways to promote improve- ments in radio communication and explorations into various dark corners of the world of science. For instance, the fact that an oscillating crystal gives rise to a system of sound waves in the sur- rounding air has made it possible to measure with great precision the velocity of sound at frequencies far beyond the range audible to the human ear. With suitable apparatus, moreover, such an inaudible sound can be transmitted through water, between, for example, two submerged submarines, and can be made intel- ligible by electrical devices at chosen receiving posts. APPLICATIONS TO BROADCASTING TODAY, crystals find their widest applicability in the radio field due to the fact that, in combination with a vacuum tube oscillator, they act as stabilizers of frequency. There are two principal ways in which radio requires the steadying influence on frequency of the piezo-electric crystal. In the first place, for satisfactory operation of a broadcasting station, the transmitted carrier frequency must be held very nearly constant. And, in order to avoid confusion, every frequency assigned and used must be accurately known in terms of a standard. In either case the re- quirement is met by the quartz crystal when combined with a vacuum tube circuit, such as shown by the diagram in Fig. 8, which will produce sus- tained oscillations at a frequency determined by the crystal. Now in many respects, a crystal is like a piece of any other solid. Take a bar of steel, strike it in one way and, due to its mechanical vibrations, it will give out a sound of a certain pitch; strike the same piece of steel in some other way and another pitch may become audibly more pro- nounced. The crystal, as we have already seen, can be excited to mechanical vibration by alter- nating voltage; furthermore, it can in this way be vibrated at any frequency whatever, Electric AxisE throughout a very wide range, depending upon the frequency of alter- nation of the voltage, but at any one of a number of different fre- quencies its readiness to vibrate is especially marked. A somewhat similar characteristic is found in the ordinary tuning fork. One can force the prongs to vibrate at any reason- able frequency, but at some characteristic frequency they will vibrate with large ampli- tude by very little outside help. A second fork in vibration at this same frequency, across the table from the first, may be all that the latter requires to keep it going. For the present it is best to get acquainted Electric Axis E Optic Axis 0 Axis Optic Axis 0 FIG. 3 Certain properties of a quartz crystal differ in the various directions indicated by arrows FIG. 4 Unless this small convex crystal is completely enclosed when an alter- nating voltage is applied, it shoots out from between its electrodes, as here illustrated. A natural frequency for this crystal is about one and a half million oscillations per second with some of the more general facts about crys- tals that oscillate so that, later on, in dealing with a particular application of the crystal, we can feel more at home in following specific di- rections. An impedance (that is, a coil combined with a condenser) is shown in the plate circuit of Fig. 8. From the point of view of an electric circuit, a piezo-electric crystal is also a coil com- bined with a condenser, since, to the circuit, it acts as an impedance. Now such an arrange- ment of impedances may cause oscillations of current and voltage to start in the tube circuits. Thereby an oscillating voltage is set up at the electrodes of the crystal which is here shown connected between grid and fi 1 a m e n t. Such a voltage, as before des- cribed, excites the crystal to mechanical vibration. If the frequency of this excitation is even only approximately one of those values at which the crystal strongly prefers to go, the counter electric potentials set up by the FIG. 5 A diagram of the crystal shown in Fig! 6. Arrows indicate directions similarly shown in the original crystal of Fig. 3. The natural frequency of this crystal is about 28,000 oscillations per second elongations and contractions of the crystal so influence the action of the tube by way of the grid as to force the oscillations to the frequency chosen by the crystal. The tube drives the crystal but the crystal dictates the frequency. Be- cause of the strong preferences, just mentioned, which the crystal shows for certain frequencies, such an oscillator operates with remarkable con- stancy of frequency. In order to start the oscillator at some frequency preferred by the crys- tal, the coil and condenser must have values within rather liberal limits. In most cases, however, the condenser can be dispensed with altogether by choosing a suitable coil. Surprisingly large changes can be made in the inductance of the coil and in the plate voltage or filament current to the tube without affecting the output fre- quency seriously for ordinary pur- poses. A tuned circuit, such as a wave- meter, coupled closely to the crystal oscillator, will cause small changes in frequency; but it would be very difficult to produce changes as large as one cycle in a thousand per second without "killing" the oscillator by doing so. It is usual also to find that the frequency of a crystal oscillator is affected somewhat by changes in temperature; but for one degree change in temperature a change of one cycle in thirty thousand per second would be unusually large. If special care is taken to keep a crystal at constant temperature, and to avoid undue coupling to other circuits, the frequency of a crystal oscillator may be made to hold as constant as any conceivable need can ever demand. In ordinary usage, without exercising any unusual precau- tions, crystals are found to hold frequencies at FIG. 6 A shudder runs through a quartz crystal when an alternat- ing voltage is applied between its upper and lower sides