ENGN 3226 Digital Communication
Australian National University
Assignment on Modulation and Demodulation
In this Project down conversion receiver architecture is analyzed from the given architecture by
scrutinizing the output signals at different nodes and comparing the visual characteristics using an
appropriate wireless propagation standard. It was realized that step down conversion architecture
is usually used in Single Sideband (SSB) modulation /demodulation which is a form of sidebamd
filtering used widely in radio communications.
Single-sideband modulation or Single-sideband suppressed-carrier (SSB-SC) is a refinement of
amplitude modulation that more efficiently uses electrical power and bandwidth. Amplitude
modulation produces a modulated output signal that has twice the bandwidth of the original
baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power
wasted on a carrier, at the cost of somewhat increased device complexity and more difficult tuning
at the receiver.
The Components used in this project to carry to out SSB demodulation include
1) Sine and Cosine wave generator
2) Phase Shifter
3) Multipliers
4) Adder
5) Tunable RF Low pass filters
Figure one shows a simple phase type demodulator:
Fig.1 SSB-SC Demodulator.
Single - Sideband Demodulation
The block diagram shown can be configured to receive and demodulate SSB signal. It consists of
the following in details:
RF Filter
,ENGN 3226 Digital Communication
Australian National University
This is a tuneable band-pass filter (BPF). It should be tuned to pass the desired transmission band
and reject other noise and out-of-band interference.
Mixer
This is used to translate the signal from its RF band to an intermediate frequency (IF).
IF Filter
This is a good quality (high Q) band-pass filter (BPF) that is factory-tuned at a fixed IF
frequency. This filter can be tuned to provide the desired selectivity to select the AM band for a
given transmitting station.
Envelope Detector
This is used for non-coherent envelope detection of AM-modulated signals.
Product Demodulator
This is used for coherent demodulation of DSB and SSB signals.
Audio Filter
This is a LPF that passes the audio band and rejects higher frequencies.
Principle of Operation
It is convenient, for the purpose of investigating the operation of this demodulator, to use for the
input signal two components, one ωH rad/s, above ω0, and the other at ωLrad/s, below ω0. This
enables us to follow each sideband through the system and so to appreciate the principle of
operation. The multipliers produce both sum and difference products. The sum frequencies are at
or about 2ω rad/s, and the difference (wanted) products near DC. The Fig.2 shown below & we
assume there are two identical filters, one each in the I (in phase) and Q (Quadrature) paths,
which remove the sum products.
, ENGN 3226 Digital Communication
Australian National University
Fig.2 The Ideal Phasing - Type SSB Demodulation
Consider the upper path I: into the ‘I’ input of the summer go two contributions; the first is that
from the component at ωH, the second from the component ωL. Two more contributions to the
summer come from the lower path ‘Q’. You can show that these four contributions are so phased
that those from one side of ω0 will add, whilst those from the other side will cancel. Thus the
demodulator appears to look at only one side of the carrier. The purpose of the adjustable phase
α is to vary the phase of the local carrier source ω0 with respect to the incoming signal, also on
ω0 .
Fig.3 SSB Demodulation
Principle of Operation:
General Architecture of the demodulator is very similar the architecture provided. Operation of
demodulator consists of two sinusoidal inputs one of which is a local oscillator the frequency of
which is very close to the frequency of the other input. Two multipliers produce sum and difference
products. The sum and difference frequencies are at different values with a wide separation
between them. Hence if the low pass filter is tuned at the frequency of the local oscillator one
component of the output is cut off and the output of the filter will only contain the sum frequency
component. The same action takes place with the other filter. After it is mixed with the second
oscillator frequencies again sum and different frequencies are produced. Finally when the two
channels are feed into an adder the output is sinusoidal wave that comprises of only one frequency
component. In this case, as a Low pass filter is used, so the output signal will contain the difference
frequency component.
Australian National University
Assignment on Modulation and Demodulation
In this Project down conversion receiver architecture is analyzed from the given architecture by
scrutinizing the output signals at different nodes and comparing the visual characteristics using an
appropriate wireless propagation standard. It was realized that step down conversion architecture
is usually used in Single Sideband (SSB) modulation /demodulation which is a form of sidebamd
filtering used widely in radio communications.
Single-sideband modulation or Single-sideband suppressed-carrier (SSB-SC) is a refinement of
amplitude modulation that more efficiently uses electrical power and bandwidth. Amplitude
modulation produces a modulated output signal that has twice the bandwidth of the original
baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the power
wasted on a carrier, at the cost of somewhat increased device complexity and more difficult tuning
at the receiver.
The Components used in this project to carry to out SSB demodulation include
1) Sine and Cosine wave generator
2) Phase Shifter
3) Multipliers
4) Adder
5) Tunable RF Low pass filters
Figure one shows a simple phase type demodulator:
Fig.1 SSB-SC Demodulator.
Single - Sideband Demodulation
The block diagram shown can be configured to receive and demodulate SSB signal. It consists of
the following in details:
RF Filter
,ENGN 3226 Digital Communication
Australian National University
This is a tuneable band-pass filter (BPF). It should be tuned to pass the desired transmission band
and reject other noise and out-of-band interference.
Mixer
This is used to translate the signal from its RF band to an intermediate frequency (IF).
IF Filter
This is a good quality (high Q) band-pass filter (BPF) that is factory-tuned at a fixed IF
frequency. This filter can be tuned to provide the desired selectivity to select the AM band for a
given transmitting station.
Envelope Detector
This is used for non-coherent envelope detection of AM-modulated signals.
Product Demodulator
This is used for coherent demodulation of DSB and SSB signals.
Audio Filter
This is a LPF that passes the audio band and rejects higher frequencies.
Principle of Operation
It is convenient, for the purpose of investigating the operation of this demodulator, to use for the
input signal two components, one ωH rad/s, above ω0, and the other at ωLrad/s, below ω0. This
enables us to follow each sideband through the system and so to appreciate the principle of
operation. The multipliers produce both sum and difference products. The sum frequencies are at
or about 2ω rad/s, and the difference (wanted) products near DC. The Fig.2 shown below & we
assume there are two identical filters, one each in the I (in phase) and Q (Quadrature) paths,
which remove the sum products.
, ENGN 3226 Digital Communication
Australian National University
Fig.2 The Ideal Phasing - Type SSB Demodulation
Consider the upper path I: into the ‘I’ input of the summer go two contributions; the first is that
from the component at ωH, the second from the component ωL. Two more contributions to the
summer come from the lower path ‘Q’. You can show that these four contributions are so phased
that those from one side of ω0 will add, whilst those from the other side will cancel. Thus the
demodulator appears to look at only one side of the carrier. The purpose of the adjustable phase
α is to vary the phase of the local carrier source ω0 with respect to the incoming signal, also on
ω0 .
Fig.3 SSB Demodulation
Principle of Operation:
General Architecture of the demodulator is very similar the architecture provided. Operation of
demodulator consists of two sinusoidal inputs one of which is a local oscillator the frequency of
which is very close to the frequency of the other input. Two multipliers produce sum and difference
products. The sum and difference frequencies are at different values with a wide separation
between them. Hence if the low pass filter is tuned at the frequency of the local oscillator one
component of the output is cut off and the output of the filter will only contain the sum frequency
component. The same action takes place with the other filter. After it is mixed with the second
oscillator frequencies again sum and different frequencies are produced. Finally when the two
channels are feed into an adder the output is sinusoidal wave that comprises of only one frequency
component. In this case, as a Low pass filter is used, so the output signal will contain the difference
frequency component.
