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Waveforms - Output Generating Circuits

Electronics

Waveforms

The sine wave (230) is one of the most important known waves and can be obtained from alternating current. It is characterized by the fact that the voltage in it gradually increases until it reaches the peak, then decreases to zero at the same rate and its direction is reversed until it reaches the peak of the negative side and its value is equal to the value of the peak in the positive direction. If we used an oscilloscope, we would see that this shape is repeated 50 times per second if the power in the house is 50 cycles per second. The sine wave may be on another oscillation, and it may represent a musical note issued by a signal generator. As for the word "oscillation" or "frequency", it means the number of complete cycles per second.

The gradual increase or decrease in voltage or current helps to find mathematical relationships between the maximum value and the effective value of each of the voltage or current (where the second is less than the first). It is known that the AC voltmeter cannot respond to rapid changes accompanying voltage and therefore indicates only the effective value. Square waves are useful in testing and other operations, as they increase and decrease almost instantaneously (231), and can be obtained by generating sine waves usually and then cutting off their positive and negative peaks. As for the waves that resemble saw teeth (232), they differ from the previous two waves, and are produced by a circuit consisting of a resistor and a capacitor. If the capacitor is charged with a high voltage, via a resistor in series, its voltage rises, and the shape of this rise from S to Y is in the form of a straight and almost inclined line. When the capacitor reaches point Y, it is discharged at a tremendous speed, after which its voltage returns to the value S. This process is repeated many times, resulting in a saw-shaped wave. It is worth noting that the ascent from S to Y must be free of curvature. As for the descent from Y to S, it must be instantaneous.

230 - Two cycles of the sine wave.
231 - Two cycles of square wave.
232 - Saw waves.
233 - Saw wave generation circuit.

Figure (233) shows a circuit for generating a saw wave in which the current passes through the resistor to the capacitor which is gradually charged (from S to Y) with the possibility of controlling the speed and rate of charging by choosing the appropriate values ​​of the capacitor, the resistor and the supply voltage. When the charge value reaches the level Y, the switch closes and the capacitor begins the discharge process. After that, the switch opens again and the charge begins to rise again, and so on.
As for the practical circuits, they use a thyratron valve instead of a switch to open and close the circuit. It is worth noting that this valve is capable of conducting every time the voltage reaches a certain value that was previously determined (see p. 53).

Saw waves are used in the oscilloscope where the gradual rise in voltage helps move the cathode rays along the screen and the sudden discharge of the capacitor helps return them instantly to the original point to start a new scanning process.