Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: May 2015

1. The equation of AM wave in its simplest form i.e. single tone modulation, is expressed as:

$$s(t)=A \cos\omega_ct+A\dfrac{m_a}{2}\cos(\omega_c+\omega_m)t+A\dfrac{m_a}{2}\cos(\omega_c-\omega_m)$$

1. From this equation, it is obvious that the carrier component in AM wave remains constant in amplitude and frequency. This means that the carrier of amplitude modulated wave does not convey any information.
2. In power calculation of AM signal, it has been observed that for single-tone sinusoidal modulation, the ratio of the total power to the carrier power is:where $m_a$ is the modulating index

$$\dfrac{P_t}{P_c}=1+\dfrac{m_a^2}{2}$$

1. Thus, for 100% modulation about 67% of the total power is required for transmitting the carrier which does not contain any information.
2. Hence, if the carrier is suppressed, only the sidebands remain and in this way a saving of two-third power may be achieved at 100% modulation.
3. The resulting signal obtained by suppressing the carrier from the modulated wave is called Double sideband suppressed carrier (DSB-SC) system.

DSBSC generation using balanced modulator

1.The balanced modulators are used to suppress the unwanted carrier in AM wave.

2.The carrier and modulating signals are applied to the inputs of the balanced modulator and we get the DSBSC signal with suppressed carrier at the output of the balanced modulator. Hence, the output consists of the upper and lower sidebands only.