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Resonance - How does a radio receiver work .. Electronics

Resonance

A radio transmitter uses radio waves of different frequencies. To select a signal with a specific frequency, the circuit must be tuned to resonate with the frequency of this signal. The resonance circuit consists in parallel (141) of a coil L A, and a variable capacitance capacitor cm). The resonance frequency is the frequency at which a specific voltage is generated on both ends of the coil. If the coil is of fixed value, the tuning is done only by the capacitor, which moving it from its minimum value to its maximum value covers the different audio frequencies of the transmitting stations. An example of this is the range between 1500 kHz and 600 kHz, i.e. between 200 meters and 500 meters (the medium frequency range). As for another range of frequencies, we choose another coil L2 whose inductance is proportional to the value of the new frequencies, and the number of turns and its size are different from the previous coil. It is possible to select one of the two coils L1 or L2 by means of the switch H in circuit (142). Sometimes we may need to tune two or more circuits at the same time. Figure (143) shows the antenna coil which consists of the primary coil L1 and the secondary coil L2 which is tuned by the variable capacitor SM A. In this circuit, the amplified signals pass to point S and from there to the primary coil of the second transformer L3 which is tuned by another variable capacitor SM2. We notice that the two variable capacitors SM A, SM2 of the two resonant circuits are distinguished by a common drive shaft on which the moving plates of both of them are fixed together. In addition, we find in some receivers that the capacitors SM A, SM2 tune the same frequency of the signal to be heard. It is known that using two resonant circuits instead of one circuit makes the device more “selective” and highly sensitive to selecting the station to be heard from among other nearby stations.

141 - Parallel Tuned Current.
142 - Selection of different frequency ranges.
143 - Composite tuning.
144 - Tunable cores and trimmers.

In receivers already above the heterodyne, the capacitor Sm, tunes to a different frequency than the first circuit. By a process called "frequency shifting" it appears as if all stations are transmitting signals at the same frequency. The subsequent circuits are then tuned to this new frequency.

Usually each coil has a tunable core (144) so ​​that its inductance can be matched with the inductances of the other coils. The coil usually includes a capacitor called a trimmer or trimmer T A. The trimmer compensates for the variable capacitance resulting from the connecting wires in the circuit itself or from valves or transistors, etc. Maintenance engineers adjust the cores of the coils and the trimmers so that the circuits become sensitive and responsive at the same time to the different frequencies so that the collective tuning process is completed in the most complete manner.