At the heart of any radio is the principle of selective reproduction of a signal modulated by a specific carrier frequency, which, in turn, is determined by the resonance of the oscillating circuit, which is the main element of the receiver circuit. The quality of the received signal depends on how correctly this frequency is selected.
The selectivity, or selectivity of the receiver, is determined by how much the signals that interfere with the stable reception will be weakened, and the useful ones will be amplified. The quality factor of a circuit is a quantity that objectively demonstrates in numerical terms the success of solving this problem.
The resonant frequency of the circuit is determined by the Thompson formula:
f = 1 / (2ฯโLC), in which
L is the inductance value;
C is the value of the electric capacity.
In order to understand how oscillations occur in the circuit, you need to understand how it works.
Both capacitive and inductive loads prevent the occurrence of electric current, but do so in antiphase. Thus, they create the conditions for the occurrence of the oscillatory process, approximately the same as it happens on a swing, when two skaters push them in different directions alternately. Theoretically, by changing the capacitance of a capacitor or coil, it is possible to achieve that the resonant frequency of the circuit coincides with the carrier frequency of the transmitting radio station. The more they differ, the less quality 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 graph of the frequency response of the receiver will be. This is how one can visually understand how a useful signal will be amplified, how much interference is suppressed. The quality factor of the circuit is the parameter that determines the selectivity of reception.
It is determined by the formula:
Q = 2ฯFW / P, where
F is the resonant frequency of the circuit;
W is the energy in the oscillatory circuit;
P is the power dissipation.
The quality factor of the circuit with parallel connection of the capacitor and inductance is determined by the following formula:
Q = Rโ (C / L)
With the values โโof the inductance and capacitance of the capacitor, everything is clear, and as for R, it recalls that, in addition to reactance, the coil also has an active component. Therefore, the circuit circuit is often depicted, including three elements: capacitance C, inductance L and resistance R.
The quality factor of the circuit is a value inversely proportional to the attenuation rate of the oscillations in it. The larger it is, the slower the relaxation of the system.
In practice, the most significant factor affecting the quality factor of a circuit is the quality of the coil, which depends on the magnetic permeability of its core, the number of turns, the degree of isolation of the wire, and its resistance, as well as losses due to the 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, receivers with digital tuning also have their own oscillatory circuits, just their resonant frequency changes differently.