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Experiment No. 1
Double Side Band
Suppressed Carrier
(DSB-SC)
Experiment No. (1)
Double Side Band–Suppressed Carrier DSB-SC
Lab Objective
- Understanding DSB-SC modulation.
- Testing and studying the performance of DSBSC signal.
- Understanding the function of each block of the transmitter and the
receiver.
- The ability to draw the waveform and spectrum of the output of each
step in the modulator/demodulator.
- Investigate the effect of the carrier error n the receiver on the system
performance..
- Design the different components of the DSBSC system to achieve
certain requirements.
- Compensate the effect of channel noise using filters.
- Use virtual lab simulations to investigate the effect of all of the
system parameters on the system performance.
Theoretical Background
Modulation is a process by which a parameter of a high frequency sinusoid is
modified in accordance with the message signal to be transmitted. The high frequency
sinusoid is known as the carrier and the message signal is the modulating signal. The
modified carrier signal is called the modulated signal. A consequence of modulation is a
translation or shifting of the message spectrum to a higher frequency band. Message
signals, by nature, are low frequency or baseband signals. A baseband signal is a signal
whose spectrum is positioned close to dc (ω=0). Examples of baseband signals include
speech signals whose spectrum occupies the frequency band from 0 to 3.5 Khz and video
signals whose spectrum occupies the frequency band. 0 to 6 MHz
There are two broad classes of communication – baseband communication and
carrier (Passband) communication. Modulation is required to match the signal to the
channel (or link). Baseband communication requires no modulation whereas carrier
communication requires modulation. Links such as local telephones using a pair of wires,
coaxial cables and optical fibers do not need modulation. Radio links (radio and TV
broadcast, microwave links, cellular phones and satellite links), on the other hand, must
utilize modulation. The reverse of modulation is called demodulation (or detection).
Demodulation is a process which extracts the message signal from the modulated signal.
In linear modulation the amplitude of the carrier signal is a linear function of the
message signal. Depending on the nature of the spectral (frequency domain) relationship
between the modulated signal and the message signal, we have the following types of
linear modulation schemes:
1- Double-Sideband Suppressed Carrier (DSB-SC) Modulation.
2- Amplitude Modulation (AM).
3- Single-sideband modulation (SSB),
4- Vestigial-Sideband Modulation (VSB).
Each of these schemes has its own distinct advantages, disadvantages, and practical
applications. We will examine these different types of linear modulation schemes. The
emphasis is characteristics such as signal spectrum, power and bandwidth, demodulation
methods, and the complexity of transmitters and receivers.
Double Sideband Suppressed Carrier Modulation
In amplitude modulation the amplitude of a high-frequency carrier is varied in
direct proportion to the low-frequency (baseband) message signal. The carrier is usually a
sinusoidal waveform, that is,
c(t)=Ac cos(ωct+θc)
Or
c(t)=Ac sin(ωct+θc)
Where:
Ac is the unmodulated carrier amplitude
ωc is the unmodulated carrier angular frequency in radians/s; ωc =2πfc
θc is the unmodulated carrier phase, which we shall assume is zero.
The amplitude modulated carrier has the mathematical form
ΦDSB-SC(t)= A(t) cos(ωct)
Where:
A(t) is the instantaneous amplitude of the modulated carrier, and is a linear
function of the message signal m(t). A(t) is also known as the envelope of the
modulated signal
For double-sideband suppressed carrier (DSB-SC) modulation the amplitude is
related to the message as follows:
A(t)=Ac(t) m(t)
Consider a message signal with spectrum (Fourier transform) M(ω) which is band
limited to 2πB as shown in Figure 1(b). The bandwidth of this signal is B Hz and ωc is
chosen such that ωc >> 2πB. Applying the modulation theorem, the modulated Fourier
transform is
A(t) cos(ωct)= m(t) cos(ωct) ⇔ ½( M(ω - ωc)+ M(ω + ωc))
Figure 1-a shows a DSBSC modulator. Figure 1-B shows an example of the
baseband signal waveform and spectrum .The time domain waveform and the frequency
spectrum for the modulated signal are shown in Figure 1(c). The dashed lines represent
the positive (+A(t)=+m(t)) and negative (-A(t)=-m(t)) amplitudes, respectively
Carrier cos(ωct)
Modulating Signal Modulated Signal
m(t) m(t) cos(ωct)
m(t) cos(ωct) m(t)
M(ω)
-m(t)
Figure(1): Amplitude modulated signal in time and frequency domains
Properties of DSB-SC Modulation:
(a) There is a 180 phase reversal at the point where +A(t)=+m(t) goes
negative. This is typical of DSB-SC modulation.
(b) The bandwidth of the DSB-SC signal is double that of the message
signal, that is, BWDSB-SC =2B (Hz).
(c) The modulated signal is centered at the carrier frequency ωc with two
identical sidebands (double-sideband) – the lower sideband (LSB) and
the upper sideband (USB). Being identical, they both convey the same
message component.
(d) The spectrum contains no isolated carrier. Thus the name suppressed carrier.
(e)The 180 phase reversal causes the positive (or negative) side of the
envelope to have a shape different from that of the message signal, see Figure
2(a) and (b). This is known as envelope distortion, which is typical of DSB-
SC modulation.
(f) The power in the modulated signal is contained in all four sidebands.
(a)
Carrier is phase reversed when m(t) goes negative
(b)
Figure(2): DSB-SC Modulation
Generation of DSB-SC Signals
The circuits for generating modulated signals are known as modulators. The basic
modulators are Nonlinear, Switching and Ring modulators. Conceptually, the simplest
modulator is the product or multiplier modulator which is shown in figure 1-a. However,
it is very difficult (and expensive) in practice to design a product modulator that
maintains amplitude linearity at high carrier frequencies.
One way of replacing the modulator stage is by using a non-linear device. We use
the non-linearity to generate a harmonic that contains the product term then use a BPF to
separate the term of interest. Figure 3 shows a block diagram of a nonlinear DSBSC
modulator. Figure 4 shows a double balanced modulator that use the diode as a non-linear
device, then use the BPF to separate the product term.
Figure(3):
Figure(4):A circuit diagram of a double-balanced modulator.
Another method for generation DSBSC is to use a switching circuit. This is similar
to modulating a signal with a square wave rather than a sinusoid. Then, we use a BPF to
separate the harmonic of interest. Figure 5 shows the block diagram and the associated
waveforms and spectrums of a switching modulator. Figure 6 represents a circuit diagram
for a ring modulator that uses diodes as switching device rather than a non-linear device.
The schematic diagrams and waveforms for a ring modulator are shown in Figures
6 and 7. Semi- conductor diodes are ideally suited for use in balanced modulator circuits
because they are stable, require no external power source, have a long life, and require
virtually no maintenance. A balanced modulator has two inputs: a single-frequency
carrier and the modulating signal. For the modulator to operate properly, the amplitude of
the carrier must be sufficiently greater than the amplitude of the modulating signal
(approximately six to seven times greater). This ensures that the carrier and not the
modulating signal controls the on or off condition of the four diode switches (D1 to D4).
Figure 7 shows the input and output waveforms associated with a ring modulator
for a single-frequency modulating signal. It can be seen that D1 and D2 conduct only
during the positive half-cycles of the carrier input signal, and D3 and D4 conduct only
during the negative half-cycles.
Block diagram of basic switching modulator.
Figure(5): (a)Switching modulator block diagram for DSB-SC. (b-d) associated
waveforms and spectrum
Figure(6): Circuit Diagram for Ring modulator
Figure(7): Ring modulator waveforms
Demodulation of DSB-SC Signals
Demodulation or detection is the process of recovering the message signal from the
modulated waveform. The method used to recover message signals from DSB-SC
waveforms is known as coherent or synchronous detection (or demodulation).
Coherent detection
The block diagram for coherent detection is shown in Figure 8. This is similar to
the modulator except that the band-pass filter is replaced by a low-pass filter. The
received DSB-SC signal is
Sm(t) = ΦDSB-SC(t)= Ac (t) m(t) cos(ωct)
The receiver first generates an exact (coherent) replica (same phase and frequency) of the
unmodulated carrier
Sc(t) = Cos(ωct)
The coherent carrier is then multiplied with the received signal to give
Sm(t)* Sc(t) = Ac (t) m(t) cos(ωct)* Cos(ωct)
= ½ Ac (t) m(t)+1/2 Ac (t) m(t) cos(2ωct)
The first term is the desired baseband signal while the second is a band-pass signal
centered at 2ωc. A low-pass filter with bandwidth equal to that of the m(t) will pass the
first term and reject the band-pass component.
Figure(8): Coherent demodulator for DSB-SC signals
Figure 9 shows a block diagram for the DSBSC system that will be used in the virtual lab
experiment.
Figure 9: A block diagram of a DSBSC system
Pre-Lab
Answer the following questions (Assume any missing data)
1- Assume a modulating signal f(t)= cos(2π.1000.t) and the carrier is
cos(2π.100000.t). Draw the output modulated signal and waveform and spectrum when
using DSB-SC
2- Repeat the previous problem assuming that the carrier is a square wave and
triangle instead of a sinusoid.
3- Repeat Q1 if the modulating signal when there is a phase error in the receiver of
30 degrees
Q4: find a relation for the output SNR of a DSBSC system (Asume a suitable
values for the filters BWs).
Q5: Suggest a circuit to achieve synchronization in the DSCBSC receiver (for
example PLL or Costas loop) use any available resources to illustrate the synchronization
circuit)
Virtual lab experiments
The objective of the virtual lab is to see the output of each stage of the DSBSC
system when applying different inputs and different system parameters to see the effect
of each individual design parameter.
Experiment 1:
Use the provided virtual experiment and figure 9 to test the output waveform and
spectrum of each stage in the DSBSC system. Write your comments
• Sinusoid having a frequency of 1Kz
• Carrier frequency of 100 Khz
• Sampling frequency of 600 Khz.
• Input LPF cut off frequency of 1 Khz.
• BPF bandwidth of 1 Khz.
• Output LPF BW of 1 KHz.
• No noise added
Experiment 2:
Repeat when the input is square wave and sawtooth.
Experiment 3
Discuss the effect of additive noise by varying the noise from 0 variance to 5. Find the
relation between the final SNR and the noise variance.
Experiment 4
Test the ability of the receiver BPF to remove the noise by changing the BPF BW and
measuring the output signal SNR.
Experiment 5
Check the relation between the receiver phase error and the SNR for different noise
values.
Experiment 6
Investigate the effect of the final LPF on the output signal SNR.