Mumbai University > Information Technology > Sem 3 > Principles of analog and digital communication

Marks:- 5M

Year:- Dec 2015

Single sideband suppressed carrier modulation (SSB-SC)

• The modulation method where only a single band of double sideband suppressed carrier modulation is transmitted is known simply as SSB. It offers even better electrical efficiency and frequency band efficiency than DSB.

• In addition, with digital modulation, modulation is performed at relatively low frequency, and this method is also used to up-convert the signal to a radio frequency. SSB-SC modulation can be achieved with various systems, but in terms of hardware, USB or LSB can be used as a filter. As with DSB-SC, the modulating signal spectrum is shifted directly to the carrier frequency band without loss.

Modulation:

• The symbol for amplitude modulation is a multiplier, but actual circuits use the nonlinearity of transistors or diodes, or switching operations.

• For example, if a modulating signal is input to a circuit using non-linear amplification, several higher harmonics are generated in the output due to the amplification characteristics of the circuit.

• This higher harmonic contains a sum (or difference) component of the information signal frequency and carrier frequency, and if a suitable filter is used to eliminate the unwanted components, the correct amplitude modulation waveform can be obtained.

Demodulation

• Demodulation (detection) methods for amplitude modulation on the receiving side include synchronous detection and asynchronous detection.

• Synchronous detection demodulates the received signal by multiplying it with a carrier frequency which has the same frequency and phase as the transmission carrier wave. Asynchronous detection includes envelope detection and rectification detection.

• With asynchronous detection, the information signal m(t) must be incorporated in the envelope of the receiving waveform.

• With a modulation factor of 100% or more, demodulation is not possible with asynchronous detection, but with synchronous detection, correct demodulation is possible. However, synchronous detection requires complex, costly circuits.