Condenser C prevents this d.c. current flowing through coil L, when S is closed, thus preventing discharge of the B battery through coil L, and also making the reactances of the circuit LC to radio frequency current low.
When S is closed a surge in current takes place via condenser C and coil L, which is absorbed from the internal plate circuit. Since the choke maintains the battery current steady, the variable current through coil L induces a voltage in coil L'.
Assuming the induced voltage is such as to cause a surge of current down through coil L', the condenser C' would be charged with the plate D, positive. The voltage across this condenser would then be in opposition to the original grid bias, and would consequently make the grid less negative than before.
This decrease in negative grid voltage would effect an increase in the internal plate current.
Because of the choke coil in the battery circuit, the increased current must flow down through coil L, and up through the condenser C, which in turn would induce a voltage in the coil L', causing a surge of current up through this coil and charging the condenser C' with the plate H, positive.
Now, the voltage across the condenser C' aids the original grid bias and results in an increased negative grid voltage and therefore the current from plate to the filament is decreased again, with the resulting surge of current down through C, and up through L, which induces a voltage in L' and so the cycle is repeated indefinitely.
Oscillating Plate Current Characteristics.—On account of the amplifying action of the vacuum tube, the alternating voltage across the grid condenser C' (fig. 6), during a given cycle would result in an increased alternating voltage across the coil L, in the plate circuit, and consequently with sufficient close coupling between L and L', the voltage induced in coil L', would be greater than that which previously existed across the condenser C'.
For this reason the amplitude of each cycle would be greater than that of the preceding cycle, and the oscillations of the plate current would continue to increase until the instantaneous values of the plate current varied between some maximum or some minimum determined by the coupling and the circuit constants. The form of this oscillating plate current is shown in fig. 7.
Power Supply.—Generally high power radio broadcasting stations receive alternating current of low voltage. This current is stepped up or transformed to high voltage, and then rectified by special designed vacuum tubes, known as mercury-vapor rectifiers.
These tubes when used in connection with large capacity stations, are of considerable size and may be mounted and interconnected, as shown in figs. 10 and 9.
The mercury vapor rectifier tubes are designed for use in high voltage devices and will supply high voltage d.c. power at uniform voltage, which in some instances may reach up to 20,000 volts.
Other large water-cooled vacuum tubes are connected as oscillators producing the high frequency carrier current.
17,000 VOLT RECTIFIER UNIT
17,000 VOLT RECTIFIER UNIT
How Crystal Oscillators Work.—The operation of crystal controlled oscillators depends upon the so called Piezo-electric effect in quartz crystal.
Among the many uses of crystals in radio oscillator circuits is that of a master oscillator or primary frequency standard, controlling the frequency of radio transmitting stations output, as standard for instrument manufacturers, etc.
The natural crystal of quartz is in the shape as shown in fig. 12. The axis parallel to the lengthwise natural edges is called the optic axis, and the axis perpendicular to the natural edges is called the electrical axis. There is also a third axis perpendicular to the other two which in the figure is designated as the B axis.
Considering a crystal shown in fig. 12 with the black volume removed and mounted as shown in fig. 13 it has been found that this slab if subjected to an electric pressure of proper sign along its electric axis, the crystal expands along this axis and contracts along the B axis. The reversal of the electrical pressure causes a reversal of these expansions and contractions.
It has also been found that when subjected to mechanical pressure tending to contract it along the B axis and expand it along the electrical axis, an electromotive force is developed in the crystal along its electrical axis. The electromotive force so developed is opposite in sign to the external electric force which would have produced a corresponding expansion and contraction of the crystal. Hence if an alternating electromotive force is impressed across the crystal as shown in fig. 14 the crystal will expand and contract along its vertical axis and at the same time contract and expand along its horizontal axis.
It is a well known fact that a body capable of mechanical vibrations, has a certain natural frequency of vibration, and so the crystal in this instance has a certain natural frequency of expansion and contraction along the vertical axis and some other natural frequency along its horizontal axis, and each one of these frequencies depends upon the actual dimensions of the crystal itself.
The thicker the crystal be cut the lower the frequency to which it will vibrate. From the above it will be readily understood that the manner of cutting crystal is of paramount importance, not only in regard to its thickness, but also to its optical axis.
LOCAL FREQUENCY GENERATOR
LOCAL FREQUENCY GENERATOR
JIG. 14—Illustrating the action of crystal when an electro-motive force is
FIG. 15—Crystal oscillator circuit. When a crystal is connected as shown, ahd the switch be closed for a short period, the condenser formed between the plates of the crystal mounting will be charged, thus subjecting the crystal to an electrical strain, which causes the crystal to expand along the vertical axis and contract along the horizontal axis. When the switch is opened, the electrical strain on the crystal will be relieved and so will tend to return to its previous form, on account of its mechanical properties. However, the crystal will vibrate at one of its own natural frequencies during a short period. The particular one of the two frequencies is dependent upon the constant of the circuit and may be adjusted from one to the other by adjusting the inductance.
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