In this ppt we learn about amplitude modulation and demodulation techniques.

1. Amplitude Modulation
UNIT-1
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2. Introduction
 A frequency-translated signal from which the baseband
signal is easily recoverable is generated by adding, to the
product of baseband and carrier, the carrier signal itself.
 Such a signal is shown in fig.(1)
 Figure l(a) shows the carrier signal with amplitude Ac, in 1
(b) we see the baseband signal.
 The translated signal l(c) is given by
v(t)=Ac[1+m(t)]cos ωct (1)
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3. Contd… 3
Fig.1.(a)Sinusoidal
carrier
(b)modulating
waveform
(c)modulated
waveform

4. Contd…
 We observe, from Eq. (1) as well as from Fig. l(c), that the
resultant waveform is one in which the carrier Ac cos ωct
is modulated in amplitude.
 The process qf generating such a waveform is called
amplitude modulation.
 A communication system which employs such a method of
frequency translation is called an amplitude-modulation
system, or AM for short.
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5. Contd…
 The designation “carrier” for the auxiliary signal Ac cos
ωct seems especially appropriate in the present connection
since this signal now “ carries ” the baseband signal as its
envelope.
 The term “ carrier ” probably originated, however, in the
early days of radio when this rela-tively high-frequency
signal was viewed as the messenger which actually “
carried ” the baseband signal from one antenna to another
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6. Contd…
 The very great merit of the amplitude-modulated carrier signal is the
ease with which the baseband signal can be recovered.
 The recovery of the baseband signal, a process which is referred to as
demodulation or detection, is accomplished with the simple circuit of
Fig. 2(a), which consists of a diode D and the resistor- capacitor RC
combination
 We now discuss the operation of this circuit briefly and qualitatively.
 For simplicity, we assume that the amplitude-modulated carrier which
is applied at the input terminals is supplied by a voltage source of zero
internal impedance.
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7. Contd… 7
Fig.2(a)A demodulator for an AM signal Fig.2(b)Input waveform and output voltage across capacitor

8. Contd…
 We assume further that the diode is ideal, i.e., of zero or infinite
resistance, depending on whether the diode current is positive or
the diode voltage negative.
 Let us initially assume that the input is of fixed amplitude and
that the resistor R is not present
 In this case, the capacitor charges to the peak positive voltage of
the carrier.
 The capacitor holds this peak voltage, and the diode would not
again conduct.
 Suppose now that the input-carrier amplitude is increased.
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9. Contd…
 The diode again conducts, and the capacitor charges to the new
higher carrier peak.
 In order to allow the capacitor voltage to follow the carrier peaks
when the carrier amplitude is decreasing, it is necessary to
include the resistor R, so that the capacitor may discharge.
 In this case the capacitor voltage vc has the form shown in Fig.
2(b)
 The capacitor charges to the peak of each carrier cycle and
decays slightly between cycles
 The time constant RC is selected so that the change in vc
between cycles is at least equal to the decrease in carrier
amplitude between cycles.
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10. Contd…
 It is seen that the voltage vc follows the carrier envelope except
that vc also has superimposed on it a sawtooth waveform of the
carrier frequency.
 In Fig.2(b) the discrepancy between vc and the envelope is
greatly exaggerated.
 In practice, the normal situation is one in which the time interval
between carrier cycles is extremely small in comparison with the
time required for the envelope to make a sizeable change
 Hence vc follows the envelope much more closely than is
suggested in the figure.
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11. Contd…
 Further, again because the carrier frequency is ordinarily much
higher than the highest frequency of the modulating signal, the
sawtooth distortion of the envelope waveform is very easily
removed by a filter.
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12. Thank You
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