Simulation of Digital Low Pass Filter and Amplitude Modulation-Demodulation in Matlab Simulink Environment

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Electrical and electronics
Date

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Abstract.
This work consists of two research section namely; simulation of digital low pass filter and
amplitude modulation-demodulation in Matlab Simulink environment. The purpose of this
experiment is to investigate the characteristics of the digital low pass filter under different
frequencies. In addition, the section on amplitude modulation-demodulation was conducted to
investigate similarities and differences between product detectors and envelop detector as
modes of demodulation. Furthermore, the spectrum analysis of modulated and demodulated
signals was investigated.

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Introduction
A low-pass filter is a circuit that enables frequencies of the range 0 - f_c Hz (zero to cutoff
frequency) to pass and blocks or attenuates frequencies greater than the cutoff frequency. Digital
processors are embedded in digital filters to compute algorithm on signal’s sampled values.
Superiority of digital filters over analog-type filters is demonstrated by the fact that they have higher
accuracy and ability to restore distorted signals (Smith).
Modulation is the act of translating a message signal into higher frequency signal so as to
enable simultaneous transmission of more baseband signals when translated at different frequencies,
and also to effectively minimize the size of higher frequency reception antenna at the receiver
(Spectrum Analysis). Demodulation takes place at the receiver end where the original message is
recovered from the carrier signal.
Results and analysis
a) Low-pass digital filter
When the sampling time is 0.001 seconds and the samples in one period is 256, then analog
frequency of the sine wave was calculated as:
Period of the analog signal ; T = ( samples
period ×ts )
T =256 × 0.001 s=0.256 s
The frequency of the analog signal, f t
f t= 1
T = 1
0.256 =3.906 Hz
Graphical view of the input signal and the output signal when at 256 samples within the
signal’s period is as shown in fig 1.1:

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Fig1.1 The Input waveform and output signals at 256 (samples/ period)
Fig 1.2 The Input waveform and output waveform at 4 samples per period
Setting samples at 256 per period, from (Fig 1.1) the output signal can be described to be
equivalent to the input signal in all property aspects. The signals have a frequency of 3.906 Hz as
from signals’ periodic time (0.256sec). Therefore, it can be concluded that this frequency falls within
the bandwidth frequency of the designed low pass filter. A low-pass filter is permeable to low-
frequency signals and impermeable to high-frequency signals (Low pass filters).
At the sampling rate set at 4, the output signal as shown in (fig 1.2) has been suppressed to a
near-zero datum line. The frequency of the output signal is 250Hz. With regard to properties of the