Learn more about Communications Blockset

Analog Modulation

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Section Overview Analog Modulation Features of the Blockset Representing Signals for Analog Modulation Sampling Issues in Analog Modulation Filter Design Issues

Section Overview

In most media for communication, only a fixed range of frequencies is available for transmission. One way to communicate a message signal whose frequency spectrum does not fall within that fixed frequency range, or one that is otherwise unsuitable for the channel, is to alter a transmittable signal according to the information in your message signal. This alteration is called modulation, and it is the modulated signal that you transmit. The receiver then recovers the original signal through a process called demodulation. This section describes how to modulate and demodulate analog signals with Communications Blockset.

Open the Modulation library by double-clicking its icon in the main Communications Blockset library. Then open the Analog Passband sublibrary by double-clicking its icon in the Modulation library.

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Analog Modulation Features of the Blockset

The following figure shows the modulation techniques that Communications Blockset supports for analog signals. As the figure suggests, some categories of techniques include named special cases.

For a given modulation technique, two ways to simulate modulation techniques are called baseband and passband. This blockset supports passband simulation for analog modulation.

The modulation and demodulation blocks also let you control such features as the initial phase of the modulated signal and post-demodulation filtering.

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Representing Signals for Analog Modulation

Analog modulation blocks in this blockset process only sample-based scalar signals. The input and output of the analog modulator and demodulator are all real signals.

All analog demodulators in this blockset produce discrete-time, not continuous-time, output.

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Sampling Issues in Analog Modulation

The proper simulation of analog modulation requires that the Nyquist criterion be satisfied, taking into account the signal bandwidth.

Specifically, the sample rate of the system must be greater than twice the sum of the carrier frequency and the signal bandwidth.

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Filter Design Issues

After demodulating, you might want to filter out the carrier signal. The particular filter used, such as butter, cheby1, cheby2, and ellip, can be selected on the mask of the demodulator block. Different filtering methods have different properties, and you might need to test your application with several filters before deciding which is most suitable.

Example: Varying the Filter's Cutoff Frequency

In many situations, a suitable cutoff frequency is half the carrier frequency. Since the carrier frequency must be higher than the bandwidth of the message signal, a cutoff frequency chosen in this way properly filters out unwanted frequency components. If the cutoff frequency is too high, those components may not be filtered out. If the cutoff frequency is too low, it might narrow the bandwidth of the message signal.

The following example modulates a sawtooth message signal, demodulates the resulting signal using a Butterworth filter, and plots the original and recovered signals. The Butterworth filter is implemented within the SSB AM Demodulator Passband block.

To build the model, gather and configure these blocks:

• Signal Generator, in the Simulink Sources library

  • Set Wave form to Sawtooth.

  • Set Amplitude to 4.

  • Set Frequency to .3.

• Zero-Order Hold, in the Simulink Discrete library

  • Set Sample time to .01.

• SSB AM Modulator Passband, in the Analog Passband sublibrary of the Modulation library

  • Set Carrier frequency to 25.

  • Set Initial phase to 0.

  • Set Sideband to modulate to Upper.

  • Set Hilbert transform filter order to 200.

• SSB AM Demodulator Passband, in the Analog Passband sublibrary of the Modulation library

  • Set Carrier frequency to 25.

  • Set Initial phase to 0.

  • Set Lowpass filter design method to Butterworth.

  • Set Filter order to 2.

  • Set Cutoff frequency to 30.

• Scope, in the Simulink Sinks library

  • After double-clicking the block to open it, click the Parameters icon and set Number of axes to 2.

Connect the blocks as in the figure. From the model window's Simulation menu, select Configuration parameters. In the Configuration Parameters dialog box, set Stop time to 10. Running the model produces the following scope image. The image reflects the original and recovered signals, with a moderate filter cutoff.

There is invariably a delay between a demodulated signal and the original received signal. Both the filter order and the filter parameters directly affect the length of this delay.

Other Filter Cutoffs.   To see the effect of a lowpass filter with a higher cutoff frequency, set the Cutoff frequency of the SSB AM Demodulator Passband block to 49, and run the simulation again. The new result is shown below. The higher cutoff frequency allows the carrier signal to interfere with the demodulated signal.

To see the effect of a lowpass filter with a lower cutoff frequency, set the Cutoff frequency of the SSB AM Demodulator Passband block to 4, and run the simulation again. The new result is shown in the following figure. The lower cutoff frequency narrows the bandwidth of the demodulated signal.

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Interleaving

Digital Modulation

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