The measuring mechanism and circuit of a ferromagnetic electro-frequency table, a series resonant circuit composed of a fixed coil with an iron core, an inductor L, and a capacitor C are usually adjusted to resonate at the intermediate frequency of the scale (for example, 50 Hz).
The movable part is composed of two coils, in which the moving coil 1 is connected in series with the capacitor C1 in parallel with the resonance circuit. When the power is turned on, the rotating force on the movable part, I and I1 are the current in the fixed coil and moving coil 1, θ is the angle between the two current phasors, and K is the coefficient. The moving coil 2 forms a closed loop with the resistor R2 and the inductor L2. When the pointer of the movable part deviates from the angle α of the middle position of the scale, the moving coil 2 will receive a reaction torque proportional to the deflection angle α and return the pointer to the middle position. When the measured frequency is equal to the middle frequency of the scale, the resonant circuit resonates. At this time, the angle between the current in the fixed coil and the current in the moving coil 1 is θ = 90 °, so the rotating torque M = 0. So the movable part makes the pointer point on the scale of the intermediate frequency of the scale (for example, 50 Hz) under the action of the torque of the moving coil 2. When the measured frequency deviates from the intermediate frequency, the resonance condition is destroyed, the rotational torque is no longer zero, and the movable part deflects until the rotational torque and the resistance torque are balanced, the movable part will stop at the new frequency corresponding to the measured frequency Location. By changing the parameters of the series resonant circuit, different frequency ranges can be obtained.
Frequency table is used to measure the frequency of power frequency grid. For a frequency of 50 Hz, the measurement error of the frequency meter is less than 0.1 Hz.
Many countries in the world use short-wave frequencies for worldwide broadcast transmission. The short-wave frequency range is usually between 1.6MHz and 30MHz. In general, we also divide the short-wave frequency into many "meter band waves", and each meter band contains a frequency range .