The linear multiplier can be used to demodulated or detect and AM signal. Demodulation can be thought of as reverse modulation. The purpose is to get back the original modulating signal (sound in the case of standard AM receivers). The detector in the AM receiver can be implemented using a multiplier, although another method using peak envelope detection is common
The Basic AM Demodulator
An AM demodulator can be implemented with a linear multiplier followed by a low pass filter, as shown in figure 5.7 The critical frequency of the filter is the highest audio frequency that is required for a given application (15 KHz, for example)
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Figure 5.7 Basic AM demodulator.
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Operation in Terms of the Frequency Spectra
Let's assume a carrier modulated by a signal tone with a frequency of 10 KHz is received and converted to a modulated intermediate frequency of 455 KHz, as indicated by the frequency spectra in figure 5.8. Notice that the upper side and lower side frequencies are separated from both the carrier and the IF by 10 KHz.
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Figure 5.8 An AM signal converted to IF
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When the modulated output of the IF amplifier is applied to the demodulator along with the IF. Sum and difference frequencies for each input frequency are produced as shown in figure 5.9. Only the 10 kHz audio frequency is passed by the filter. A draw back to this type of AM detection is that a pure IF must be produced to mix with the modulated IF.
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Figure 5.9 Example of demodulation.
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IF and Audio Amplifiers
The basic function of the IF Amplifier
The IF amplifier in a receiver is a tuned amplifier with a specified bandwidth operating at a center frequency of 455 KHz for AM and 10.7MHz for FM. The IF amplifier is one of the key features of a super heterodyne receiver because it is set to operate at a signal resonant frequency that remains the same over the entire band of carrier frequency that can be receiver. Figure 5.10 illustrates the basic function of an IF amplifier in terms of the frequency spectra.
Assume, for example. That the received carrier frequency of ƒc=1MHz is modulated by an audio signal with a maximum frequency of ƒm=5 KHz indicated in the following figure.
By the frequency spectrum on the input to the mixer for this frequency, the local oscillator is at frequency of
ƒo = 1MHz + 455KHz = 1.455MHz
The mixer produces the following sum and difference frequencies as indicated in the previous figure.
ƒo + ƒc = 1.455MHz + 1MHz = 2.455MHz
ƒo – ƒc = 1.455MHz – 1MHz = 455KHz
ƒo + (ƒc + ƒc) = 1.455MHz + 1.005MHz = 2.46MHz
ƒo + (ƒc – ƒc) = 1.455MHz + 0.995MHz = 2.45MHz
ƒo – (ƒc + ƒc) = 1.455MHz – 1.005MHz = 450KHz
ƒo – (ƒc – ƒc) = 1.455MHz – 0.995MHz = 460KHz
Since the IF amplifier is a frequency-selective circuit, it responds only to 455 KHz and any side frequencies lying in the 10 KHz band centered at 455 KHz. So, all of frequencies out of the mixer are rejected except the 455KHz IF, all lower-side frequencies down to 450 KHz, and all upper-side frequencies up to 460 KHz. This frequency spectrum is the audio modulated IF.
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Figure 5.11 The audio amplifier in a receiver system.
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