Communication device

1. DISCOVER . LEARN . EMPOWER
Communication System
University Institute of Engineering
Electronics & Communication Engineering
Bachelor of Engineering (Electronics & Communication Engineering)
Foundation of Wireless Mobile Communication (ECO-355)
Prepared By: Priya Rana
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2. Transmitter Block Diagram
AM transmitter takes the audio signal as an
input and delivers amplitude modulated
wave to the antenna as an output to be
transmitted.
The block diagram of AM transmitter is
shown in the following figure.
• The audio signal from the output of the
microphone is sent to the pre-amplifier,
which boosts the level of the modulating
signal.
• The RF oscillator generates the carrier
signal.
• Both the modulating and the carrier signal
is sent to AM modulator.
• Power amplifier is used to increase the
power levels of AM wave. This wave is
finally passed to the antenna to be
transmitted. 2

3. FM TRANSMITTER
FM transmitter is a unit, which takes the audio signal as an input and
delivers FM wave to the antenna as an output to be transmitted. The
block diagram of FM transmitter is shown in the following figure.
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4. • The audio signal from the output of the microphone is sent to the
pre-amplifier, which boosts the level of the modulating signal.
• This signal is then passed to high pass filter, which acts as a pre-
emphasis network to filter out the noise and improve the signal to
noise ratio.
• This signal is further passed to the FM modulator circuit.
• The oscillator circuit generates a high frequency carrier, which is sent
to the modulator along with the modulating signal.
• Several stages of frequency multiplier are used to increase the
operating frequency. Even then, the power of the signal is not enough
to transmit. Hence, a RF power amplifier is used at the end to
increase the power of the modulated signal. This FM modulated
output is finally passed to the antenna to be transmitted.
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5. Receiver block diagram
• Superheterodyne is mixing two frequencies together in order to produce
a difference frequency component called as intermediate frequency.
• A superheterodyne receiver usually consists of an antenna, RF amplifier,
mixer, local oscillator, IF amplifier, detector, AF amplifier and a speaker.
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• In the superheterodyne receiver, the incoming signal through the antenna is
filtered to reject the image frequency and then amplified by the RF amplifier.
• RF amplifier can be tuned to select and amplify a particular carrier frequency
within the AM broadcast range. Only the selected frequency and it two sidebands
are allowed to pass through the amplifier.
• The carrier of the received signal is called radio frequency carrier and its
frequency is radio frequency fRF and the local oscillator signal operates at fOSC.
The amplified RF frequency is then mixed with the local oscillator frequency.
• The combining of these two signals is done at the mixer which produces sum and
difference frequency signals of the incoming carrier signal and local oscillator
signal, which are fOSC+fRF and fOSC−fRF

7. • The sum frequency (fOSC+fRF) is rejected by the filter and the remaining difference frequency
(fOSC - fRF) signal which is a down converted frequency signal is called as intermediate frequency
(IF) carrier (fIF=fOSC−fRF)
• The modulation of the IF carrier signal is same as that of the original carrier signal and it has a
fixed frequency of 455kHz which is amplified by one or more stages of amplification.
• The IF signal is amplified with the help of IF amplifier which raises its level for the information
extraction process. Also the IF amplifier fulfills most of the gain and bandwidth requirements of
the receiver.
• This amplified IF signal is applied to the detector to detect the information signal component, to
reproduce the original information data, which is generally in the form of audio signal.
• The RF component is filtered out and audio is supplied to the audio stages for amplification.
• The generated audio signal is then applied to the AF amplifier to increase the audio frequency
level of the signal and to provide enough gain to drive the speaker or headphones.
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8. • A speaker is connected to the AF amplifier to play the audio information signal.
• An important part of superheterodyne receiver is Automatic gain control (AGC) which is
given to the RF, IF and mixer stages in order to generate constant output irrespective of
the varying input signal.
• Superheterodyne radio receiver offers many advantages in terms of
performance,
selectivity.
It is more efficiently able to remove unwanted and distorting signals than other forms of
receivers.
• Due to the enormous advantages provided by the superheterodyne receivers compared
to the other radio receivers, they are widely used in all broadcast radio receivers,
commercial radios as well as televisions operate on the basis of the superheterodyne
principle.
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9. Sensitivity
• Receiver sensitivity is a measure of the minimum signal strength that
a receiver can detect.
• It tells us the weakest signal that a receiver will be able to identify
and process. Receiver sensitivity is expressed in dBm.
• Since it represents how faint an input signal can be to be successfully
received by the receiver, the lower the power level of the signal, the
better.
• So for example a receiver sensitivity of -90 dBm is better than -80
dBm i.e this means that the -90 dBm receiver is more sensitive and
can interpret lower power signals.
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10. SIGNAL TO NOISE RATIO(S/N or SNR)
• In analog and digital communications, a signal-to-noise ratio, is a
measure of the strength of the desired signal relative to
background noise (undesired signal).
• In the linear scale, the SNR is the ratio of the signal power to the
noise power.
• The ratio is typically expressed as a single numeric value in decibels
(dB).
• A positive SNR indicates that the signal power is greater than the
noise power while a negative SNR indicates the opposite.
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11. Friis Transmission Equation
The Friis Transmission Equation is used to calculate the power received from
one antenna (with gain G1), when transmitted from another antenna (with
gain G2), when both the antennas are separated by a distance R, and
operating at frequency f or wavelength lambda.
Derivation of Friis equation:
Consider two antennas in free space (no obstructions nearby) separated by a
distance R:
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12. • Assume that Pt Watts of total power are delivered to the transmit
antenna.
• Assume that the transmit antenna is omni-directional & lossless.
• Then the power density p (in Watts per square meter) of the plane
wave incident on the receive antenna a distance R from the transmit
antenna is given by:
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13. • If the transmit antenna has an antenna gain in the direction of the
receive antenna given by Gt , then the power density equation above
becomes:
The gain term factors in the directionality and losses of a real antenna.
Assume now that the receive antenna has an effective aperture given
by AER . Then the power received by this antenna (PR) is given by:
Since the effective aperture for any antenna can also be expressed as:
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14. • The resulting received power can be written as:
• This is known as the Friis Transmission Formula. It relates the free
space path loss, antenna gains and wavelength to the received and
transmit powers.
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15. THANK YOU
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