04 FDM

EXPERIMENT.4
FDM Multiplexer and Demultiplexer
Objectives:
 To understand the operation theory of frequency Division Multiplexing FDM and
Demultiplexing.
 To design and implement the FDM multiplexer and Demultiplexer.
Equipment Required:
 ACS11-1 and ACS12-1 of ETEK ACS-3000-06 module.
 DC Power Supply.
 Connection wires.
Theory:
If the transmission channel consists only of one modulated signal, then the usage of the channel
is very low and the efficiency is also not good. Therefore, in order to comfort with the economic
benefit, the channel must be able to transmit multiple signals, such as in the telephone system.
As you know the frequency range of the sound is 300Hz to 3 KHz so in order to transmit this
kind of signal via a single channel, we must divide the signal into several slots to prevent the
interference then we can obtain the signal at the receiver. There are two types of signal division
Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM).
FDM Multiplexing:
Figure (4.1) is the system block diagram of FDM. Like TDM, FDM is used to transmit multiple
signals over the same communication channel simultaneously. However, unlike TDM, FDM
does not use pulse modulation. Figure (4.1) assumes that all the input audio signals are low pass
pattern and after each input signal, there will be a low pass filter to remove all the unwanted
signals except the audio signals. Then the audio signals will be sent into the modulator so that the
frequency range of the signals will shift to different region. The conversion of the frequency is
controlled by the carrier signal. Therefore, we utilize the simplest technique which is the AM
modulation to implement the modulator. Then the modulated signal will pass through a band
pass filter which can limit the signal bandwidth to prevent the interference between each signal.
Finally, the signals will be added by a linear adder. As compare to TDM, we utilize AM
modulation to implement FDM system and sampling to implement TDM system.
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Fig.(4.1):Block diagram of FDM Multiplexer.
In this experiment, we build each balanced modulator by utilizing MC1496 and use different
carriers for each modulator. As you know, the output from each balanced modulator is a DSBSC signal. Then, the DSB-SC signals will be added by a linear adder in order to produce the
FDM signal.
Fig.(4.2):Circuit diagram of DSB-SC modulation by utilizing MC1496.
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Fig.(4.3):Circuit diagram of the linear adder.
FDM Demultiplexing:
There are two ways to implement FDM demultiplexer. The first way is shown in figure (4.4). Let
the FDM signals pass through a band pass filter, this filter will remove the signal which its
frequency is larger and lower than f0 and only left a single DSB-SC modulated signal. After that,
this signal will pass through the LPF which recover the modulated signal and obtain the original
audio signal. While Figure (4.5) shows the second way to implement the FDM demultiplexer
which is called synchronous product detection. After the signal passes through the synchronous
product detector, we will add a LPF to remove all the unwanted signals and recover the original
audio signal.
Fig.(4.4):Block diagram of FDM demultiplexer (first method).
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Fig.(4.5):Block diagram of synchronous product detector.
Fig.(4.6):Circuit diagram of synchronous product detector.
Fig.(4.7):Circuit diagram of the LPF.
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Procedure:
FDM Multiplexing:
1- Refer to the audio signal generator in ACS11-1 of ETEK ACS-3000-06 module.
2- Using the oscilloscope to observe the audio signal from the signal generator 1 output
(TP1) .Adjust the variable resistors “Audio Frequency Adjust1” and “Audio Gain
Adjust1” to obtain an output audio signal with 500 Hz frequency and 620mV amplitude.
3- Using the oscilloscope to observe the audio signal from the signal generator 2 output
(TP3) .Adjust the variable resistors “Audio Frequency Adjust2” and “Audio Gain
Adjust2” to obtain an output audio signal with 800 Hz frequency and 620mV amplitude.
4- Using the oscilloscope to observe the audio signal from the signal generator 3 output
(TP7) .Adjust the variable resistors “Audio Frequency Adjust3” and “Audio Gain
Adjust3” to obtain an output audio signal with 1.2 kHz frequency and 620mV amplitude.
5- Refer to the carrier signal generator in ACS11-1 of ETEK ACS-3000-06 module.
6- Using the oscilloscope to observe the carrier signal from the carrier signal generator 1
output (TP2) .Adjust the variable resistor “Carrier Gain Adjust1” so that the output
amplitude of the carrier is 620mV.
7- Using the oscilloscope to observe the carrier signal from the carrier signal generator 2
output (TP4) .Adjust the variable resistor “Carrier Gain Adjust2” so that the output
amplitude of the carrier is 620mV.
8- Using the oscilloscope to observe the carrier signal from the carrier signal generator 3
output (TP8) .Adjust the variable resistor “Carrier Gain Adjust3” so that the output
amplitude of the carrier is 620mV.
9- Using the oscilloscope to observe output signal of the balanced modulator 1(TP5) .Adjust
the variable resistor “Modulator Adjust 1” so that the output is DSB-SC modulated
signal.
10- Using the oscilloscope to observe output signal of the balanced modulator 2(TP6). Adjust
the variable resistor “Modulator Adjust 2” so that the output is DSB-SC modulated
signal.
11- Using the oscilloscope to observe output signal of the balanced modulator 3(TP9). Adjust
the variable resistor “Modulator Adjust 3” so that the output is DSB-SC modulated
signal.
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12- Using the oscilloscope to observe output signal waveform of FDM output port (FDM
O/P).
FDM Demultiplexing:
1- To implement a product detector (shown in fig.(4.5)) and the low pass filter (shown in
fig.(4.6)), refer to figure ACS12-1 on ETEK ACS-3000-06 module.
2- Connect (FDM O/P) in ACS11-1 to (FDM I/P) in ACS12-1.
3- Connect the carrier signal (TP2) in ACS11-1 to (Carrier I/P1) in ACS12-1, (TP4) to
(Carrier I/P2) and (TP8) to (Carrier I/P3).
4- Using oscilloscope to observe the output signal waveforms of (Audio O/P1), (Audio
O/P2) and (Audio O/P3), then adjust the variable resistors “Carrier Adjust 1”, “Gain
Adjust 1”, “Carrier Adjust 2”, “Gain Adjust 2”, “Carrier Adjust 3” and “Gain Adjust 3”
so that the output waveforms are maximum without distortion. Record your results.
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