1. Chapter 3Analog Signal Transmission and Reception
2. CONTENTS Introduction to Modulations
Radio and Television Broadcasting
Mobile Radio Systems
3. 3.1 INTRODUCTION TO MODULATION Denote m(t) as the analog signal to be transmitted.
The signal m(t) is assumed to be a lowpass signal of bandwidth W and is a power-type signal with
The message signal m(t) is transmitted through the communication channel by putting it on a carrier signal of the form
5. 3.2 AMPLITUDE MODULATION (AM) The message signal m(t) is impressed on the amplitude of the carrier signal c(t).
Types of amplitude modulation
Double-sideband, suppressed carrier AM (DSB-SC AM)
Conventional double-sideband AM
Single-sideband AM (SSB AM)
Vestigial-sideband AM (VSB AM)
6. 3.2.1 Double-Sideband Suppressed Carrier AM A double-sideband, suppressed carrier (DSB-SC) AM signal is obtained by multiplying the message signal m(t) with the carrier signal c(t).
Amplitude modulated signal
The spectrum of the modulated signal can be obtained by taking the Fourier transform of u(t).
9. The magnitude of the spectrum of the message signal m(t) has been translated or shifted in frequency by an amount
The phase of the message signal has been translated in frequency and offset by the carrier phase
The bandwidth of the AM signal is 2W, where W is the bandwidth of m(t).
The upper (lower) sideband of U(f) contains all the frequency content of the message signal M(f).
u(t) does not contain carrier components - u(t) is called a suppressed-carrier signal (DSB-SC AM signal)
55. Frequency Division Multiplexing The message signals are separated in frequency; each signal modulates a separated carrier.
The lowpass filters at the transmitter ensures that the bandwidth of the message signals is limited to W Hz.
At the receiver, the signals are usually separated by passing through a parallel bank of bandpass filters.
The output of each bandpass filter is demodulated.
59. AM-radio Broadcasting Commercial AM-radio broadcasting utilizes the frequency band 535-1605 kHz for the transmission of voice and music. Carrier spacing is 10 kHz.
The baseband message signal m(t) is limited to 5 kHz.
Conventional AM is used from an economic standpoint. The major objective is to reduce the cost of implementing the receiver.
83. A nonlinear device followed by a bandpass filter tuned to the desired center frequency can be used as frequency multiplier.
For example, assume a nonlinear device has the function
The output signal will be
The frequency is multiplied by a factor of 2.
92. 3.4 RADIO AND TELEVISION BROADCASTING AM Radio
97. FM-Stereo Broadcasting Many FM-stereo stations transmit music programs in stereo by using the outputs of two microphones.
A pilot tone at the frequency of 19 kHz is added to the signal for the purpose of demodulating the DSB-SC AM signal.
A monophonic FM receiver can recover the summed signal L+R by using a conventional FM demodulator. Hence, FM-stereo broadcasting is compatible with conventional FM.
105. The audio portion of the TV signal is transmitted is limited to W= 10 kHz.
The peak frequency deviation in the FM-modulated signal is selected as 25 kHz, and the FM-signal bandwidth is 70 kHz.
109. Terminologies The simultaneous transmission between two parties is called full-duplex operation.
In analog cellular system, the 3 kHz audio signal is transmitted via FM using a channel bandwidth of 30 kHz. This represents a bandwidth expansion of approximately a factor of 10.