T 7.2.1.3 Amplitude Modulation

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TPS <strong>7.2.1.3</strong> Review s AM (t) C 1 R 1 s D (t) S AM (t) S D (t) S H (t) Fig. 3-9: The envelope demodulator Fig. 3-10: Synchronous demodulation Envelope demodulation always requires the carrier. After rectification this produces a DC voltage, which establishes the working point of the diode. In practice envelope demodulation is frequently used in AM radio communications due to its simple circuitry. 2. Synchronous demodulation In principle synchronous demodulation is simply another modulation process. To carry it out, you need an auxiliary oscillation in the receiver, which in terms of frequency and phase corresponds exactly to the carrier oscillation in the modulator. The auxiliary carrier s Aux (t) and the modulated signal s AM (t) are supplied to a circuit with multiplying capabilities, see. Fig. 3-10: Three cases can be distinguished DSB, DSB sc and SSB: 1. Demodulation of DSB ⎧⎪ m sAM ( t) = AC⎨cos( 2π fC t)+ cos 2π( fC − fM ) t ⎩⎪ 2 m + cos 2π ( fC + fM ) t 2 s t cos 2π f t φ ( 3 10) Aux ( ) = + [ ] ⎫ ⎬ ⎪ ⎭ ⎪ Aux [ ] ( ) − The amplitude of the auxiliary oscillation is negligible, furthermore it is true that f C = f Aux (i.e. frequency equality prevails between carrier and auxiliary carrier). After lowpass filtering we obtain the demodulated signal: AC m sD( t) = ACcosφ+ cos( 2 π fMt) cosφ (3-11) 2 2. Demodulation of DSB sc . The constant DC voltage component A C cos φ is omitted: A m s D ( t ) = C cos ( 2π f t ) M cosφ (3-12) 2 3. Demodulation of SSB SC . s D ( t ) = AC m cos( 2π f ± ) M φ (3-13) 4 In the synchronous demodulation of DSB a phase error φ reduces the amplitude of the demodulated signal by the factor cos φ. In the case of SSB, the phase error leads to a shift in the demodulated signal. In both cases the phase between the carrier and the auxiliary carrier has a noticeable effect on the demodulation process. Due to this phase sensitivity synchronous demodulation is also called coherent demodulation. Questions 3.1 What is meant by modulation? Mixing? 3.2 Name the reasons for performing modulation! 3.3 In DSB the carrier's peak values are affected by the instantaneous value of the message signal, but the spectrum shows that the carrier amplitude remains constant! How do you explain the apparent contradiction? 3.4 Which are the characteristic features of a beat? 3.5 Define amplitude deviation and the modulation index. 3.6 Which methods of AM demodulation are you familiar with and how do they differ? 26
TPS <strong>7.2.1.3</strong> Review 3.7 In radio links the carrier is normally attenuated to 5%...10%. What advantages does this have compared to transmission with 100% carrier amplitude? Why isn't the carrier completely suppressed? 3.8 How high is the maximum efficiency in DSB? How can the efficiency be increased? 3.9 Which methods of carrier suppression are there? 3.10 Which demodulation method is used for AM with supressed carrier? 27
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T 7.2.1.3 Amplitude Modulation

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