MM0704, Lesson 1
Square Wave Applied to RC Circuits. Square waves of voltage are often applied to series RC circuits in radar. The
reason is either to use the RC circuit for coupling the undistorted square wave to a following stage, or to distort it
purposely. In the latter case, the RC circuit is called a wave shaping circuit.
In AC circuit analysis, you learned that RC circuits do not affect the shape of the sine wave. If a sine wave of voltage
is applied to the RC circuit, no matter what the values of resistance and capacitance are, its shape across either the
resistor or the capacitor will be a sine wave. The only changes that would be evident would be in amplitude and phase.
However, with a square wave applied to a series RC circuit, the values of resistance and capacitance have a direct effect
upon the resultant wave shape across the resistor and the capacitor.
If the R and C of the circuit are such that the capacitor charges very rapidly and reaches full charge during the first part
of the input wave, the output across the resistor will be a sharply peaked wave. The output across the capacitor will be
a square wave with the leading edge rising with a slight curvature rather than rising vertically. On the other hand, if the
capacitor and resistor are of such a value that the capacitor doesn't have time to charge, the output across the resistor
may approximate the input, although the top of the wave will slope down. For values of resistors and capacitors
between these two extremes, the wave shapes will differ.
You can think of a square wave of voltage as a DC voltage that alternates between a maximum and a minimum, each
for the same length of time. One way of developing a square wave of voltage is by using the circuit shown in figure
1-19A. With the switch in position A, the output across the resistor is 50 V. With the switch in position B, the output
is zero. If the switch is left in each position the same length of time and if the switching time is zero, the output is a
square wave of voltage as shown in figure 1-19B.
The frequency of this square wave can be determined from the formula for frequency in terms of time,
Figure 1-19. Square Wave.