Block diagram of a communication system

ADC
Module 1 Information Source
• A communication system serves to communicate a message or
information.
• This information originates in the information source.
• In general, there can be various messages in the form of words,
group of words, code, symbols, sound signal etc.

Transmitter
Input Transducer
• The function of the transmitter is to process the electrical
• A transducer is a device which converts one form of energy into signal.
another form. • For example in radio broadcasting the electrical signal obtained
• The message from the information source may or may not be from sound signal, is processed to restrict its range of audio
electrical in nature. frequencies (upto 5 kHz in AM) and is amplified.
• In a case when the message produced by the information source is • Modulation is the main function of the transmitter.
not electrical in nature, an input transducer is used to convert it • In modulation, the message signal is superimposed upon the
into a time-varying electrical signal. high-frequency carrier signal.
• For example, in case of radio-broadcasting, a microphone converts • In short, we can say that inside the transmitter, signal
processing such as restriction of range of audio frequencies,
the information or massage which is in the form of sound waves amplification and modulation of signal are achieved.
into corresponding electrical signal.
• All these processings of the message signal are done just to
ease the transmission of the signal through the channel.
The Channel and The Noise • On the other hand, the broadcast channel provides a capability
• The term channel means the medium through which the message travels where several receiving stations can be reached simultaneously
from the transmitter to the receiver. from a single transmitter.
• There are two types of channels, namely point-to-point channels and • An example of a broadcast channel is a satellite in
broadcast channels. geostationary orbit, which covers about one third of the earth’s
surface.
• Example of point-to-point channels are wire lines, microwave links and
optical fibres. • During the process of transmission and reception the signal
gets distorted due to noise introduced in the system.
• Wire-lines operate by guided electromagnetic waves and they are used
for local telephone transmission. • Noise is an unwanted signal which tend to interfere with the
required signal.
• In case of microwave links, the transmitted signal is radiated as an
electromagnetic wave in free space. Microwave links are used in long • Noise signal is always random in character. Noise may interfere
distance telephone transmission. with signal at any point in a communication system.
• An optical fibre is a low-loss, well-controlled, guided optical medium.
Optical fibres are used in optical communications.

Receiver
• The main function of the receiver is to reproduce the message
signal in electrical form from the distorted received signal.
• This reproduction of the original signal is accomplished by a Destination
process known as the demodulation or detection.
• Destination is the final stage which is used to convert an
• Demodulation is the reverse process of modulation carried out electrical message signal into its original form.
in transmitter.
• For example in radio broadcasting, the destination is a
loudspeaker which works as a transducer i.e. converts the
electrical signal in the form of original sound signal.
 Need for Analog Modulation Components of a Continuous-wave Modulation System
• Increase The Signal Strength
The baseband signals transmitted by the sender are not capable of direct transmission. The
strength of the message signal should be increased so that it can travel longer distances. The
most vital need of modulation is to enhance the strength of the signal without affecting the
parameters of the carrier signal.
• Wireless Communication System
Modulation has removed the necessity for using wires in the communication systems. It is
because modulation is widely used in transmitting signals from one location to another with
high speed.
• Prevention Of Message Signal From Mixing
Modulation and its types prevent the interference of the message signal from other signals.
The message signals may interfere with each other. However, by using carrier signals having a
high frequency, the mixing of the signals can be prevented.
• Size Of The Antenna
The signals within 20 Hz to 20 kHz frequency range can travel only a few distances. To send
the message signal, the length of the antenna should be a quarter wavelength of the used
frequency. Thus, modulation is required to increase the frequency of the message signal and to Transmitter Reciever
enhance its strength to reach the receiver.

Amplitude Modulation (AM)

AM
Modulator
 • DSO CH1 shows AM
signal (Blue)

Carrier USB LSB

• CH2 shows
Modulating signal
(yellow)

20
 A closer look at the AM signal

The envelope of the AM signal has the same shape as the message signal and if
we extract this envelope we can recover the message signal

Phase
reversal
of carrier
 Power and Current Relations in the AM wave

Carrier Sideband
power power

We desire for the sideband power to be as high as possible since the

information is contained in the sidebands and after demodulation we require
SNR>>1
Modulation Efficiency

Major portion of the total power is carrier power--- disadvantage of AM

Special case of multitone modulation
 Disadvantages of AM

1. AM is wasteful of transmitted power

BW=2*20k Transmission of carrier wave takes up a major portion of the total

μt2=0.42+0.52+0.62
transmitted power even though it does not carry any information .
2. AM is wasteful of channel bandwidth
The USB and LSB are symmetric about the carrier frequency and if
USB given one we can determine LSB and vice-versa. Hence only one
needs to be transmitted which would mean that the transmission
bandwidth is reduced to half the actual value and is equal to the
bandwidth of the message signal.
 To overcome these disadvantages, we can have variants of AM
Double Sideband-Suppressed Carrier (DSB-SC)
1. Double sideband-suppressed carrier (DSB-SC) modulation, in
Modulation
which the transmitted wave consists of only the upper and lower
sidebands. Transmitted power is saved here through the
suppression of the carrier wave, but the channel bandwidth
requirement is the same as before (i.e., twice the message
bandwidth
2. Single sideband (SSB) modulation, in which the modulated wave
consists only of the upper sideband or the lower sideband. Hence
bandwidth requirement is reduce to half that of the AM wave
 +

Power Relations in the DSC-SC signal

Total power = Total sideband power

 SINGLE SIDE BAND MODULATION (SSB)
• To reduce the bandwidth requirement, only one sideband is transmitted
• An SSB modulator consists of a product modulator (whose output is a
DSC-SC signal) followed by a bandpass filter which chooses either the USB
or the LSB

Ac=20,Am=5, R=1
 AM Transmitters
• In an AM transmitter, amplitude modulation can be generated at any point
after the radio frequency source.
• If the output stage in a transmitter is collector modulated in a low power
transmitter, the system is called high level modulation.
• If modulation is applied at any other point, including some other electrode of
the output amplifier, then so called low level modulation is produced.
• The end product of both systems is the same, but the transmitter circuit
arrangements are different.

• Figure shows a typical block diagram of an AM transmitter, which may be • Higher the level of modulation, the larger the audio power is required to
either low level or high level modulated. produce modulation – A disadvantage.
• There are a lot of common features. Both have a stable RF source and buffer • If any stage except the output stage is modulated, each following stage
amplifiers followed by RF power amplifiers. must handle a sideband power as well as the carrier. All these subsequent
• In both types of transmitters, the audio voltage is processed, or filtered, so as amplifiers must have sufficient bandwidth for the sideband frequencies.
to occupy the correct bandwidth, and compressed to reduce the ratio of
maximum to minimum amplitude. • All these stages must be capable of handling amplitude variations caused
• In both modulation systems, audio and power audio frequency (AF) amplifiers by the modulation. Such stages must be class A and consequently are less
are present and the modulator amplifier is the highest power audio amplifier. efficient than class C amplifiers.
• In fact, the only difference is in the point at which the modulation takes place. • Each of the systems is seen to have one great advantage; low modulating
• To show the difference, an amplifier is shown in low level scheme following power requirements in low level case; more efficient RF amplification
the modulated RF amplifier, i.e., class B. with simpler circuit design in high level case.
• This would also have been called low-level modulation if the modulated
amplifier had been the final one, modulated at any electrode other than the
collector.
• Collector modulated class C amplifiers have better efficiency, lower
distortion and much better power-handling capabilities than a base-modulated
amplifier. Superheterodyne Receiver
• Broadcast AM transmitters use high-level modulation.
• Other methods may be used in low power and miscellaneous applications,
AM generators and test instruments.
• Broadcasting is the major application of AM, with typical output powers
ranging over several kilowatts.

• In superheterodyne receiver, the incoming signal voltage is combined with a

signal generated in the local oscillator and is converted into a signal of a • The local oscillator is a variable oscillator capable of generating a signal from
lower fixed frequency. The signal at this intermediate frequency contains the 0.995 MHz to 2.105 MHz.
same modulation as the original carrier, and it is now amplified and detected • The incoming signal from the transmitter, amplified by the RF stage is then
to reproduce the original information. combined (mixed) with local oscillator signal in the mixer stage. (Mixer is a
• The RF stage is a wide band RF amplifier tunable from approximately 540 class C nonlinear device processes the signals, producing the sum, difference,
kHz to 1650 kHz. and original frequencies)
• It is used to select and amplify the incoming frequency and simultaneously to • The signal from the mixer is supplied to the IF (intermediate-frequency)
reject all others. amplifier. This amplifier is a very-narrow-bandwidth class A device capable
of selecting a frequency of 0.455 kHz ± 3 kHz and rejecting all others.
• It is mechanically tied to the local oscillator to ensure precise tuning
characteristics. • The IF amplifier generally uses two or three transformers, each consisting of a
pair of mutually coupled tuned circuits.
• A constant frequency difference is maintained between the loca1oscillator and
the RF circuits through capacitance tuning, in which all the capacitors are • With this large number of double-tuned circuits operating at a constant
ganged together and operated in unison by one control knob. frequency, the IF amplifier provides most of the gain and bandwidth
requirements of the receiver.
 • Since the characteristics of the IF amplifier are independent of the SSB Transmitter
frequency to which the receiver is tuned, the selectivity and sensitivity of
the superhet are fairly uniform throughout its tuning range.
• The IF signal output is amplified and sent to the detector stage, which
eliminates one of the sidebands still present and separates the RF from the
audio components of the other sideband.
• The RF is filtered to ground, and audio is supplied to the audio stages for
amplification and then to the speakers.

SSB Transmitter SSB Receiver

The difficulty associated with SSB is due to the suppression of carrier

Similar to AM transmitter, except the replacement of an amplitude component. The approach followed for demodulation at the receiver is to re-insert
modulation block with SSB modulation block. the carrier.
• This requires excellent frequency stability on the part of both transmitter and
receiver, because, any frequency shift, anywhere along the chain through
which the information must pass, will cause an equal frequency shift to the
received signal.