Resonant frequency meter
The frequency of the tunable resonator is compared with the frequency of the signal under study. A resonator is an oscillating circuit, a cavity resonator, or a quarter-wave line segment. The signal under study arrives at the resonator; from the resonator output, the signal goes to a galvanometer.
The maximum readings of the galvanometer indicate the best coincidence of the natural frequency of the resonator with the frequency of the signal under study. The operator adjusts the resonator using the dial.In some models of resonant frequency meters, amplifiers are used to increase sensitivity.