Most of these blocks are discussed individually, and in more detail, on other pages. See filters, mixers, frequency changers, am modulation and amplifiers.
There are signals from thousands of radio transmitters on many different frequencies inducing signal voltages in the aerial. The rf filter selects the desired station from the many. It is adjustable so that the selection frequency can be altered. This is called TUNING.
The selected frequency is applied to the mixer. The output of an oscillator is also applied to the mixer. The mixer and oscillator form a FREQUENCY CHANGER circuit. The output from the mixer is the intermediate frequency (i.f.) The i.f. is a fixed frequency of about 455 kHz. No matter what the frequency of the selected radio station is, the i.f. is always 455 kHz.
The i.f. signal is fed into the i.f. amplifier. The advantage of the i.f. amplifier is that its frequency and bandwidth are fixed, no matter what the frequency of the incoming signal is. This makes the design and operation of the amplifier much simpler.
The amplified i.f. signal is fed to the demodulator. This circuit recovers the audio signal and discards the r.f. carrier. It usually incorporates a diode in the circuit.
Some of the audio is fed back to the i.f. amplifier as an AUTOMATIC GAIN CONTROL voltage. This ensures that when tuning from a weak station to a strong one, the loudness from the loudspeaker stays the same.
The audio signal voltage is increased in amplitude by a voltage amplifier.
The power level is increased sufficiently to drive the loudspeaker by the power amplifier. |
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