Laboratory six
Double sideband suppressed carrier (DSBSC)modulation
Theory
During standard amplitude modulation employed, for instance, by long-wave,
medium-wave and short-wave transmitters, the carrier signal's amplitude is higher
than that of the wanted signal, as described earlier. Furthermore, only about 50%
of the wanted signal is present with the two sidebands (refer to the mathematical
description). As a result, the carrier supplies the main component of the
transmission power. The transmission signal's effective power component can be
increased on the basis of the fact that the carrier is not needed to convey the actual
information. Suitable circuits (e.g. filters) can be used to suppress the carrier
signal, leaving only the upper and lower sidebands called double sideband
modulation, this form of amplitude modulation is used, for instance, to transmit
stereo information in the VHF range. By suppressing the carrier signal, it is
possible to transmit wanted signals at higher efficiency without any distortion
caused by inordinate modulation depth. In purely mathematical terms, the
modulation depth is then infinite.
objective
To understand the advantage of double side band suppressed
carrier modulation and to determine the modulation index.
Required material
1: Interface board
2 :BNC connector
3 :AM modulator
4:USB cabels
5 :Computer
5.1 osciloscope
5.2 spectrum analyiser
5.3 function Generator
5.4 Digital Multimeter
Experiment Setup (Procdure)
This experiment is meant to elucidate the principle of double sideband modulation.
Oscilloscope measurements will demonstrate how the carrier signal's amplitude is
suppressed almost fully during DSB, so that just the two sidebands are transmitted.
1.The experiment set up as shown below.
•
the bridging plug was fit on the AM modulator's "DSB/AM"
contacts.
• On the Colpitts oscillator, the carrier signal's amplitude increased to
200 mVpp.
2.On oscilloscope channel A, the signal supplied by oscillator output 1:1 was
measured . After that the signal at the "AMout" output (without the modulation
signal),was measured.
.Observation
A
B
C
The modulator's output supplies the carrier signal with a
very small amplitude, termed residual carrier.
The modulator's output supplies no measurable signal.
Answer: A
The amplitude of the carrier signal at the modulator's
output is larger than in the case of AM.
3. the value (residual carrier) at the "AMout" measurement point was minimized
with the help of the "Carrier Null" potentiometer (near centre setting) , the function
generator was opened via the menu path Instruments | Voltage sources | Function
generator or by clicking on the icon below, and the settings shown in the adjacent
table was performed. .
Function generator settings
Mode:
SINE
Amplitude:
1:10, 50% (approx. 1 Vpp)
Frequency:
10 kHz
4. The oscilloscope was opened by clicking on the
right on the toolbar.
Instrument:
symbol at the top
Oscilloscope
Time base:
10 µs / div
Channel A:
200 mV / div, DC
Channel B:
500 mV / div, AC
Trigger:
Channel A
5. The low-frequency signal on oscilloscope channel A , and the modulated output
signal on channel B were measured. Channel B's input was set to AC coupling and
the low-frequency signal was used as a basis for the trigger.
X/T
X = 10 µs/DIV
(A)
Channel
200mV/DIVDC
A=
Channel
500mV/DIVAC
B=
The signal measured at the modulator's output given a minimal
residual carrier and low-frequency input signal was described as follows.
The measured signal is shaped like an AM signal
A
with a modulation depth of 0.5.
The measured signal is shaped like an AM signal
Answer: C
B
with a modulation depth of 1.
The measured signal is shaped like an AM signal
C
with over-modulation m>1).
The obtained measurement result was viewed using the oscilloscope's X /
Y display mode and copied the oscillogram on the placeholder provided
below.
X = 10 µs/DIVX/Y
Channel
200mV/DIVDC
A=
Channel
500mV/DIVAC
B=
The appearance of the measured signal in the oscilloscope's X / Y display
mode was described.
The signal appears in the form of two adjacent
A
trapezoids.
The signal appears in the form of two adjacent,
B
.Answer: C
vertical lines.
The signal appears in the form of two adjacent
C
triangles.
The oscilloscope was switched back to the X / t display mode and set so
that the modulated signal's characteristic at a zero crossing can be
specifically observed . the trigger point was shifted to the middle of the
oscilloscope screen and a time base of 2 µs was selected.
X=
X/T
(A,
2 µs/DIV
PT
50%)
Channel
200mV/DIVDC
A=
Channel
500mV/DIVAC
B=
Describe the modulator's output signal at the envelope's zero crossing.
A phase jump occurs at the zero
A
crossing.
No phase jump occurs at the zero
ANSWER:A
B
crossing.
The amplitude is irregular at the zero
C
crossing.
When a phase jump occurs, what is its magnitude?
The phase jump amounts to °.
Describe the measurement result.
The observed
phenomenon is
caused by
When both
signals cross
180
the X-axis
simultaneously
but at a phase
displacement
of 180°,
Answer: The addition of upper and lower side band’s values
the direction of addition is reversed
phase jump is produced.
Conclusion
-The double Side Band Suppressed Carrier (DSBSC) Modulation is used to
transmit the wanted signals at higher efficiency without any distortion caused by
inordinate modulation depth.
- When upper and lower side band values are added, the direction of addition is
reversed and phase jump is produced in DSBSCM
- The double Side Band Suppressed Carrier (DSBSC) Modulation is used for
power efficient modulation.
REVIEW QUESTIONS
1.Describe the advantage of DSBSC modulation over AM modulation.
Ans. - In DSBSC since carrier is suppressed ,hence power consumption is reduced
and nominal.
- The power in the modulator signal is contained in all four sidebands
- The Bandwidth of the DSBSC signal is double that of the message signal that
is BWDSB-SC=2B(Hz).
2. Describe the disadvantage of DSBSC modulation over AM modulation.
Ans. - carrier signal is not available in receiver.
- envelop detection is not possible.
3. Describe the application of DSBSC modulation.
Ans. - Analogue Tv system: to transmit color information
-CB Radio
-Air traffic control Radio
-Two way Radio communications
-Tv broadcasting
-For transmitting stereo information in FM sound broadcasting at VHF
4. Design and simulate DSBSC modulator circuit.
Ans. It is shown in the following page.
1
1.11113ddble sideband suppressed carrier (DSBSC)modulation