Lab3Amplitude Modulation

7/30/2019 Lab3Amplitude Modulation

1/4

Lab Exercise 3 and 4

Amplitude Modulation

1. Set up the following circuit. Save the following files for the spectrum analyser and store ina directory. Change the Matlab working directory to this directory.

spectrumanalyser.mdlspecanal.m

2. DSB-SC modulation: Set up the following circuit. Set the sine wave (modulating signal)frequency to 4*pi rad/sec. Set the signal generator to produce a sine wave (carrier signal) of

frequency 40*pi rad/sec.

Simulate the circuit and observe the signals on the scope and the spectrum analyser. Tune the

simulation parameters for accurate calculation and display.

3. Amplitude modulation: Set up the circuit shown below.


2/4

Set the signal generator to produce sine wave of frequency 2 Hz and the carrier frequency to 10 Hz. Keep

the modulating signal amplitude such that the modulation index is 50%. Display the modulated signal on

the Scope as well as on the Spectrum analyser plots. You may have to adjust the axes settings of the plot

as well as the scope to get the proper displays. Also, you may have to set the simulation parameters

properly.

You will design AM transmitter and receiver to study their characteristics. You will also study the

design and use of digital filters. The frequency domain spectrum of signals is obtained through a

buffered-FFT scope.

The exercise requires the model files (.mdl files). Save them to the working directory.

am_transmitter.mdl

filtertuning.mdl

coherent_det.mdl

detwithnoise.mdl

(i) AM transmitter:

Consider the equation of the AM

The amplitude modulation will be studied for modulation index = 1 and 0.5 and A=1.

Load the model file am_transmitter.mdland the experimental set up for generating an AM signal looks

like this:

{1 cos(2000 )} cos(20000 )y A t t


3/4

The buffered-FFT scope is used to study the frequency spectrum of the signal. The B-FFT scope

comprises of a Fast Fourier Transform of 128 samples which also has a buffering of 64 of them in one

frame. From the property box of the B-FFT scope the axis properties can be changed and the Line

properties can be changed. The Frequency range can be changed by using the frequency range pop down

menu and so can be the y-axis the amplitude scaling be changed to either real magnitude or the dB (log of

magnitude) scale. The upper limit can be specified as shown by the Min and Max Y-limits edit box. Thesampling time in this case has been set to 1/20000. (Note: The sampling frequency of the B-FFT scope

should match with the sampling time of the input time signal).

Start the simulation. Observe and sketch the modulating signal in both time-domain (using the scope) and

the modulated signal in frequency domain when A=1 for =0.5. Repeat the observations when =1.

(ii) Fil ter adjustments:

You will now study the use of the Band pass and the low pass filters which will be used extensively in

this experiment (and also in later experiments) since most detection schemes require pre-detection (band-

pass) filter and a post detection (low-pass) filter. These filters are readily available in the simulink block

sets.

In the simulink library browser, expand the DSP blockset filtering filter designs. Drag the digital

filter design block into a new model window. Double click the block and study the different parameters

that can be set. See how the type of filter can be chosen and the properties of the order of the filter and

the cut-off frequency is also specified. As a method to double-check the proper tuning of the parameters,

a signal can be filtered and the parameters tuned to adjust the parameters of the filter. The simulation set

up for this is shown below and you can load this set up from loading the filtertuning.mdlmodel file.

It can be observed that by changing the upper cut-off frequency of the filter the filter characteristics can

be changed. Similar procedure can be adopted to study the band-pass filter. Familiarise yourself with the

operation of low pass and band pass filters.

(iii) AM transmitter and Coherent Detection.

(a) Detection without Noise: Load the model file coherent_det.mdlmodel file to set up the circuitshown below. Study its behavior by simulation. Observe and sketch the signals in time andfrequency domains.


4/4

(b) Detecion with noise: The summer represents the communications channel where thetransmitters signal is perturbed by additive noise. The BPF serves as a pre-filter to band-limit thenoise. The bandwidth of the BPF is wide enough to pass the transmitters signal.

The figure below shows the simulation layout for detection with noise. (Load the model file

detwithnoise.mdl) The transmitter is the same as the one used in the previous parts. The modulated input

is added with a band limited white noise. A band limited white noise is used as a random noise generator.

It generates a band limited white noise. The output of this is coherent detected and the final output is

obtained. Observe and sketch the signals at different stages.

Documents

Lab3Amplitude Modulation