16. AMPLITUDE MODULATION (AM)
Amplitude Modulation (or) Linear Carrier Wave Modulation is
defined as the process of varying the amplitude of the high
frequency carrier signal with respect to the amplitude of the
modulating signal, keeping the frequency and phase of the
carrier signal constant.
Mathematical Expression for Amplitude Modulated Signal:
Consider a sinusoidal carrier signal c(t) defined as:
c(t) = Ac cos(2πf c t)
Ac is the carrier amplitude and f c is the carrier frequency.
47. It is clear from the above equation that:
• There is no output from the ring modulator at the carrier
frequency. Thus the carrier is entirely eliminated.
• The modulator output consists entirely of modulation products.
48. Coherent Detection of DSB-SC Signal:
The message signal m(t) is recovered from a DSB-SC signal s(t) by first multiplying
s(t) with a locally generated sinusoidal signal and then low pass filtering the product
as shown in Figure 1.21. The assumption is that the local oscillator output is exactly
coherent or synchronized in both frequency and phase with the carrier signal c(t)
used in the product modulator to generate s(t). This method of demodulation is
known as coherent detection or synchronous detection.
52. QUADRATURE CARRIER MULTIPLEXING
• A Quadrature Carrier Multiplexing (QCM) or Quadrature Amplitude
Modulation (QAM) method enables two DSBSC modulated waves,
resulting from two different message signals to occupy the same
transmission band width and two message signals can be separated at the
• The transmitter involves the use of two separate product modulators that
are supplied with two carrier waves of the same frequency but differing in
phase by -90o . The multiplexed signal s(t) consists of the sum of the two
product modulator outputs given by the equation
s(t) =Acm1(t)cos(2ᴨfc t)+Acm2(t)sin(2ᴨfc t)
58. Generation of SSB Signal
• The three practical methods for generating the SSB signal are filter method,
phase-shift method and Weaver’s method. The first two techniques are based on
frequency domain and time domain description of the SSB signal respectively
(i) Frequency Discrimination (or) Filter Method:
• This method is used to generate an SSB signal when the baseband is restricted
and appropriately related to the carrier frequency. Under these conditions the
desired sideband will appear in a non overlapping interval in the spectrum in
such a way that it may be selected by an appropriate filter
• In designing the band pass filter in the SSB modulation scheme, it must satisfy
two basic requirements:
• a. The passband of the filter occupies the same frequency range as the spectrum
of the desired SSB signal.
• b. The width of the transition band of the filters separating the passband from
the stopband, where the unwanted sideband of the filter input lies is twice the
lower frequency component of the modulating signal.