Most of these blocks are discussed individually, and in more detail, on other pages. See filters, mixers, frequency changers, am modulation and amplifiers.
The f.m. band covers 88-108 MHz. There are signals from many radio transmitters in this band inducing signal voltages in the aerial. The rf amplifier selects and amplifies the desired station from the many. It is adjustable so that the selection frequency can be altered. This is called TUNING. In cheaper receivers the tuning is fixed and the tuning filter is wide enough to pass all signals in the f.m. band.
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 10.7 MHz. No matter what the frequency of the selected radio station is, the i.f. is always 10.7 MHz.
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.
Some of the audio is fed back to the oscillator as an AUTOMATIC FREQUENCY CONTROL voltage. This ensures that the oscillator frequency is stable in spite of temperature changes.
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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