Circuit quality and reception quality of the radio signal

At the heart of any radio receiver is the principle of selective reproduction of a signal modulated by a certain carrier frequency, which, in turn, is determined by the resonance of the oscillatory circuit, which is the main element of the receiver circuitry. The quality of the received signal depends on how correctly this frequency is chosen.

Selectivity, or selectivity of the receiver is determined by how much the signals interfering with a steady reception will be weakened, and the useful ones will be strengthened. The quality of the contour is a quantity that objectively demonstrates the success in solving the problem in numerical terms.

The resonant frequency of the circuit is determined by Thompson's formula:

F = 1 / (2π√LC), in which

L is the inductance value;

C is the value of the electrical capacitance.

In order to understand how the oscillations in the circuit occur, it is necessary to understand how it works.

Both capacitive and inductive loads prevent the occurrence of an electric current, but do so in antiphase. Thus, they create the conditions for the appearance of the oscillatory process, about the same as it occurs on the swing, when two skaters push them in different directions alternately. Theoretically, by changing the capacitance of a capacitor or a coil, it can be achieved that the resonant frequency of the circuit coincides with the carrier frequency of the transmitting radio station. The more they differ, the less qualitative the signal will be. In practice, the receiver is tuned by changing the capacitance of the capacitor.

The whole question is how sharp the peak on the frequency response curve of the receiving device will be. This is how you can visually see how the useful signal will be amplified, how much interference is suppressed. The quality of the circuit is the parameter that determines the selectivity of the reception.

It is determined by the formula:

Q = 2πFW / P, where

F is the resonant frequency of the circuit;

W is the energy in the oscillatory circuit;

P is the power dissipation.

The Q-factor of the loop with parallel connection of the capacitor and inductance is determined by the following formula:

Q = R√ (C / L)

With the values of the inductance and capacitance of the capacitor everything is clear, and as for R, it recalls that in addition to the reactance, the coil also has an active component. Therefore, the outline of the circuit is often represented, including three elements: capacitance C, inductance L and active resistance R.

The quality of the circuit is a quantity inversely proportional to the damping rate in it of the oscillations. The more it is, the slower the relaxation of the system.

In practice, the most significant factor affecting the quality of the circuit is the quality of the coil, which depends on the magnetic permeability of its core, on the number of turns, the degree of isolation of the wire, and on its resistance, as well as on losses during passage of high-frequency currents. Therefore, to adjust the reception frequency, variable capacitors are usually used, which are two sets of plates entering and leaving each other during rotation. Such a system is typical for almost all non-digital radios.

However, in receivers with digital tuning also have their own oscillating circuits, simply their resonant frequency varies differently.