Figure 2-22.Phase shifting a sine wave.
The capacitor in series with the resistor forms a phase-shift circuit. With a constant frequency rf
carrier applied at the input, the output across the resistor would be 45 degrees out of phase with the input
Now, lets vary the resistance and observe how the output is affected in figure 2-23. As the resistance
reaches a value greater than 10 times XC, the phase difference between input and output is nearly 0
degrees. For all practical purposes, the circuit is resistive. As the resistance is decreased to 1/10 the value
C, the phase difference approaches 90 degrees. The circuit is now almost completely capacitive. By
replacing the resistor with a vacuum tube, as shown in view (A) of figure 2-24, we can vary the resistance
(vacuum-tube impedance) by varying the voltage applied to the grid of the tube. The frequency applied to
the circuit (from a crystal-controlled master oscillator) will be shifted in phase by 45 degrees with no
audio input [view (B)]. With the application of an audio signal, the phase will shift as the impedance of
the tube is varied.
Figure 2-23.Control over the amount of phase shift.