Communication Systems Prof. Chungming Kuo

Double Sideband and Single Sideband (cont.) Chapter 6 Double Sideband and Single Sideband (cont.)

Double Sideband and Single Sideband This module will provide an introduction to amplitude modulation by considering double sideband (DSB) and single sideband (SSB). Historically, these were not the earliest forms of amplitude modulation employed on a wide scale, but they relate very closely to concepts developed in the module frequency conversion.

Double Sideband and Single Sideband (cont.) Hence, they will be covered before conventional amplitude modulation with a large carrier is introduced. The commercial AM broadcast system employs the latter system.

Amplitude Modulation Forms Conventional Amplitude Modulation (Sometimes called AM-LC for AM with “large carrier.” When we use AM without any modifier, it will be assumed to mean conventional AM.)

Amplitude Modulation Forms (cont.) Double Sideband (DSB) (Sometimes called DSB-SC, with SC representing “suppressed carrier.” We will refer to it simply as DSB.) Single Sideband (SSB) Vestigial Sideband (VSB)

Essential Trigonometric Identities

Essential Trigonometric Identities (cont.)

Notation

Continuous Spectrum Signal

Discrete Spectrum Signal

Balanced Modulator

Balanced Modulator Equations Continuous Spectrum

Balanced Modulator Equations Continuous Spectrum (cont.)

Balanced Modulator Equations Discrete Spectrum

Balanced Modulator Equations Discrete Spectrum (cont.)

Balanced Modulator Equations Discrete Spectrum (cont.)

Example 1 A continuous-spectrum signal has components from near dc to 15 kHz and carrier is 1 MHz. Find range of DSB frequencies and bandwidth: Lowest frequency is 1 MHz-15 kHz = 985 kHz

Example 1 (cont.) A continuous-spectrum signal has components from near dc to 15 kHz and carrier is 1 MHz. Find range of DSB frequencies and bandwidth: Highest frequency is 1 MHz+15 kHz = 1015 kHz

Example 2 A discrete-spectrum signal has components at 1, 3, and 5 kHz, and carrier has a frequency of 250 kHz. List DSB frequencies and find bandwidth: LSB: 250 - 1 = 249 kHz 250 - 3 = 247 kHz 250 - 5 = 245 kHz

Example 2 (cont.) A discrete-spectrum signal has components at 1, 3, and 5 kHz, and carrier has a frequency of 250 kHz. List DSB frequencies and find bandwidth: USB: 250 + 1 = 251 kHz 250 + 3 = 253 kHz 250 + 5 = 255 kHz

Single Sideband (SSB) With SSB, only one of the two sidebands is transmitted. It may be the lower sideband (LSB) or the upper sideband (USB). The transmission bandwidth is: The most common method for generating SSB is the filter method illustrated on next two slides.

A DSB signal is first generated

Spectral Plots for USB

Spectral Plot for LSB

SSB Filter Method Generator

SSB Equations for Discrete-Spectrum

Example 3 For system of Example 1, determine range of SSB frequencies and bandwidth for LSB and USB: LSB: 985 kHz to 1 MHz USB: 1 MHz to 1.015 MHz

Example 4 For system of Example 2, list SSB frequencies and determine bandwidth for LSB and USB. LSB: 249 kHz 247 kHz 245 kHz

Example 4 (cont.) For system of Example 2, list SSB frequencies and determine bandwidth for LSB and USB. USB: 251 kHz 253 kHz 255 kHz

Product Detection of DSB and SSB

DSB Product Detection Analysis

DSB Product Detection Analysis (cont.)

DSB Detection Spectral Plots

SSB Product Detection Analysis

SSB Product Detection Analysis

SSB Product Detection Analysis (cont.)

SSB Detection Spectral Plots

Effects of Non-Synchronization The preceding analysis has assumed that the carrier at the receiver is locked in frequency and phase with that at the transmitter. This condition is referred to as synchronous product detection.Next, assume the following form for the receiver carrier:

Results of Mathematical Analysis A rather detailed analysis at the receiver now yields the results below. Recall that the DSB signal was assumed as a continuous spectrum signal while the SSB signal was assumed as a discrete spectrum signal.

Comments Both signals are distorted but effects on DSB are more serious. DSB is useful in the following situations: Systems in which a small pilot carrier is transmitted.

Comments (cont.) DSB is useful in the following situations: Certain complex signal processing schemes that can extract a coherent reference. Automatic control systems of the “ac-carrier” types where reference carrier is available.

Summary The instantaneous product of a baseband signal and a carrier yields a DSB signal. If one of the sidebands is eliminated, an SSB signal is generated. For a baseband bandwidth W, the bandwidth of a DSB signal is 2W, and the bandwidth of an SSB signal is W.

Summary (cont.) Theoretically, both DSB and SSB signals can be demodulated by product detection. In practice, DSB requires an exact synchronized reference while tolerable detection can be achieved with SSB without exact synchronization.