TUNING CAPACITORS
A tuning capacitor is a variable air-dielectric capacitor with plates that move within other plates to change the overall capacitance value. A single gang-tuning capacitor, as shown in Fig. 1-19, has a set of aluminum plates called the rotor mounted on a shaft so that the plates interleave with a matching set called the stator mounted on a rigid spacer. When the rotor shaft is turned by a knob, the rotor plates move in or out between the stator plates without touching them. A change in knob position alters the capacitance value, which is directly proportional to the area of the interleaved plates. Capacitance values can be from 1 to 500 pF. They are used to tune radio receivers, transmitters, and oscillators.
Figure 1-19 Tuning capacitor.
TRIMMER CAPACITORS
A trimmer capacitor is a small variable capacitor with air, ceramic, plastic, glass, or other dielectric that is used for fine-tuning RF circuits. They have capacitance values from 2 to about 100 pF. Made in many different styles, plate spacing is changed to alter the capacitance value by turning an adjustment screw.
