5.1

Analog Transmission

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5-1 DIGITAL-TO-ANALOG CONVERSION5-1 DIGITAL-TO-ANALOG CONVERSION

Digital-to-analogDigital-to-analog modulation is the process of modulation is the process of changing one of the characteristics of an analog changing one of the characteristics of an analog signal based on the information in digital data. signal based on the information in digital data.

A sine wave is defined by three characteristics: amplitude, frequency, and phase.

When we vary anyone of these characteristics, we create a different version of that wave.

Any of the three characteristics can be altered in this way, giving us at least four mechanisms for modulating digital data into an analog signal:

Amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK) , quadrature amplitude modulation (QAM).

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Figure 5.2 Types of digital-to-analog conversion

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Figure 5.1 Digital-to-analog conversion

Aspects of Digital-to-Analog Conversion Before we discuss specific methods

of digital-to-analog modulation, two basic issues must be reviewed: bit and baud rates and the carrier signal.

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Bit rate and Baud Rate

Bit rate: the number of bits transmitted in a second

Baud rate: the number of signal units per second that is required to represent those bits

A signal unit is composed of one or more bits

The fewer the signals, the more efficient the system and less bandwidth required

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Baud rate determines the bandwidth required to send a signal

Bit rate = baud rate x number of bits represented by each signal unit

Baud is always less than or equal to bit rate

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A baud is analogous to a car and the bit to a passenger

A car can carry one or more passengers

If 1000 cars travel, carrying one passenger each, 1000 passengers are transported

BUT: A car can carry 4 pple; so 4000 pple will be transported

NOTE: the number of cars NOT passengers determine the traffic

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Bit rate is the number of bits per second. Baud rate is the number of

signalelements per second.

In the analog transmission of digital data, the baud rate is less than

or equal to the bit rate.

Note

Example

A signal carries 8 bits in each signal unit. If 2000 signal units are sent per seconds, find the baud and bit rates

Given that signal’s bit rate is 3000 and that each signal unit carries 6 bits, what is the baud rate

An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need?

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Carrier signal

Sending device produces high frequency signals.

The base signal is called the carrier signal or carrier frequency

The receiving device must be tuned to the frequency that it expects from the sender

Digital info then modulates the carrier signal.

This process is called modulation

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This kind of modification is called modulating the signal OR shift keying

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Amplitude Shift Keying

In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements.

Both frequency and phase remain constant while the amplitude changes.

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Figure 5.3 Binary amplitude shift keying

Bandwidth for ASK

Bandwidth of a signal is the total range of frequencies occupied by a signal

BW = (1 + d)xNbaud

d = factor related to the modulation process (minimum value is 0)

Nbaud = the baud rate In ASK, baud rate and bit rate are always

the same

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Example

Find the minimum bandwidth for and ASK signal transmitting 2000bps. The transmission is half duplex

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Frequency shift keying

The frequency of the carrier signal is varied to represent data.

The frequency of the signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes.

Both peak amplitude and phase remain constant for all signal elements.

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Figure 5.6 Binary frequency shift keying

Phase shift keying

In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements.

Both peak amplitude and frequency remain constant as the phase changes

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Figure 5.9 Binary phase shift keying

Bandwidth for PSK

Is the same as ASK BW = (1 + d) x Nbaud

D = factor related to the modulation process

Nbaud = the baud rate In ASK, baud rate and bit rate are

always the same

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Analog-to-analog modulationAnalog-to-analog modulation

Analog-to-analog modulation is the representation of Analog-to-analog modulation is the representation of analog information by an analog signal. analog information by an analog signal.

This can be done in three ways: This can be done in three ways: •Amplitude modulationAmplitude modulation•Frequency modulationFrequency modulation•Phase modulation Phase modulation

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Figure 5.15 Types of analog-to-analog modulation

Amplitude modulation (AM)

the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal.

The frequency and phase of the carrier remain the same; only the amplitude changes to follow variations in the information.

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The bandwidth of an audio signal (speech and music) is usually 5 kHz.

Therefore, an AM radio station needs a bandwidth of 10kHz.

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Figure 5.16 Amplitude modulation

AM Bandwidth

The modulation creates a bandwidththat is twice the bandwidth of the modulating signal

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Example

We have and audio signal with bandwidth of 4KHz. What is the bandwidth needed if we use AM modulation?

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The total bandwidth required for AM can be determined

from the bandwidth of the audio signal: BAM = 2B.

Frequency Modulation

the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal.

The peak amplitude and phase of the carrier signal remain constant, but as the amplitude of the information signal changes, the frequency of the carrier changes correspondingly.

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Figure 5.18 Frequency modulation

FM Bandwidth

The actual bandwidth is difficult to determine exactly, but it can be shown empirically that it is several times that of the analog signal or 2(1 + β)B where β is a factor depends on modulation technique with a common value of 4.

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The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.

Beta is usually 4Therefore the BW = 10B

Note

Standard bandwidth allocation for FM radios

The bandwidth of an audio signal (speech and music) broadcast in stereo is almost 15 kHz.

The FCC allows 200 kHz (0.2 MHz) for each station. This mean β = 4 with some extra guard band.

FM stations are allowed carrier frequencies anywhere between 88 and 108 MHz. Stations must be separated by at least 200 kHz to keep their bandwidths from overlapping.

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If we have an audio signal of 4Mhz. What is the Bw needed if we are to modulate the signal?

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Phase Modulation

the phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal.

The peak amplitude and frequency of the carrier signal remain constant, but as the amplitude of the information signal changes, the phase of the carrier changes correspondingly.

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Figure 5.20 Phase modulation

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The total bandwidth required for PM can be determined from the bandwidth

and maximum amplitude of the modulating signal:

BPM = 2(1 + β)B.

The value of beta is usually 1 or 3

Note

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Ntuthu Ernest Mbili

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