08/05/2018
Radio Transmitter
Introduction
A radio transmitter is an electronic device, when connected to an antenna, produces an
electromagnetic signal such as in radio and television broadcasting signal for two way
communications.
 Classification of Radio Transmitter
Radio transmitters may be classified according to the following methods according to the
Type of modulation used : Amplitude modulation, Frequency modulation and Pulse modulation
transmitter.
Service involved: Radio broadcast, Radio telephone, Radio telegraph, Television, Radar,
Navigation transmitter.
Frequency range involved: Long wave, Short wave, microwave and VHF & UHF transmitter
Application of Various Radio Transmitter
Amplitude Modulation Transmitter: the modulating signal amplitude modulates the carrier.
Such transmitters are used for radio broadcast on long, short, medium waves, radio telephony on short
waves, radio telegraphy on short waves etc.
Frequency Modulation Transmitter: the signal voltage frequency modulates the carrier.
Such transmitters are used for radio broadcast in VHF and UHF range, television sound broadcast in
VHF and UHF ranges, radio telephone communication in VHF and UHF ranges over short distances.
Pulse Modulation Transmitter: the signal voltage alters some pulse characteristics of the pulses,
modulation may be pulse width. The pulse modulation are: Pulse width modulation, pulse position
modulation, pulse amplitude and pulse frequency modulation.
Radio Broadcast Transmitter: These transmitters are designed for transmitting speeches, talks, music,
dramas etc for the information. These braodcast transmitters may be either amplitude modulated or
frequency modulated.
1
08/05/2018
Application of Various Radio Transmitter
Radio Telephone Transmitter: are designed for transmitting telephone signals over long distance
by radio means. Radio telephone transmitters may be either of amplitude or frequency modulation
type. The AM telephone transmitters usually work on short wave, FM radio telephone
transmitters usually work on ultra high frequencies.
Radio Telegraph Transmitter: Transmits telegraph signals from one radio station to another radio
station. It may used Amplitude modulation or Frequency Modulation.
Television Transmitter: It requires two transmitters one for transmitting video another for sound
transmission. Both operate in very high frequency. The video transmitter is amplitude and sound
carrier is frequency modulated.
Radar Transmitter: Two types pulse Radar (uses pulse modulation) and Continuous Radar (uses
frequency modulation) of carrier signal. Typically operates at microwave frequency.
Navigation Transmitter: A number of navigational aids using special types of radio transmitter
and receivers are used for sea and air naviagation.
Application of Various Radio Transmitter
Long Wave Transmitter: Operate on long waves on frequencies below kHz. Such transmitters are
used for broadcast in temperature countries where atmospheric disturbances on long waves are
not severe.
Medium Wave Transmitter: Operate on frequencies in the range of 550 to 1650 kHz and are
usually used for broadcast. Therefore, the band of frequency extending from 550 to 1650 kHz
commonly referred to as the broadcast band.
Short Wave Transmitter: Operate on frequencies in the short wave range of 3 to 30 MHz.
Ionospheric propagation of electromagnetic waves takes place at such short waves.
VHF and UHF Transmitter: operate either in VHF range or in the UHF range and are used for
FM broadcast, television broadcast, FM radio telephony etc.
Microwave Transmitter: operate on microwave frequencies beyond about 1000 MHz and are used
in radar, television relay, microwave link between two adjacent island, countries etc.
2
08/05/2018
Amplitude Modulation Radio Transmitter
Block diagram of amplitude modulation radio transmitter using modulation at high power level of the
carrier.
1
Master
Oscillator
TX antenna
2
Buffer
Amplifier
Desired oscillation, f
 f is required to remain constant
variations with V, T and load
3
Harmonic
Generator
1
 Power drawn from the Oscillator 2
 Causes the loading effect of the tank
circuit and f variation
 MO generates signal at multiple of 3
the carrier f of radiated Power
 Tuned the RF voltage and selects the
desired frequency
4
Class C Power
Amplifier
5
Modulated
Amplifier
 Amplifier the low level RF signal
4
 High output circuit efficiency of the order of
70%
 Pushpull amplifier and is modulated by
audio modulating voltage
 High η series plate modulation is used in
high power radio broadcast
5
Class B pushpull amplifier and feeds audio, p
Class B operation used for high circuit
6
efficiency
Feeder Line
6
Modulating
Amplifier
Audio
Amplifier
Modulating Signal
Amplitude Modulation Radio Transmitter
Block diagram of amplitude modulation radio transmitter using modulation at low power level of the
carrier.
Master
Oscillator
Buffer
Amplifier
Harmonic
Generator
Class C
Class B
Modulated
Amplifier
Tuned
Power
Amplifier
TX antenna
Feeder Line
Modulating
Amplifier
Modulating Voltage
3
08/05/2018
Harmonic Generator
6.5 Harmonic Generator
See page number 107 G.K Methal
A simple harmonic generator using half wave rectifier
Operation of AM Transmitter
See the PDF file
4
08/05/2018
Radio Receiver
In radio communications, a radio receiver (radio) is an electronic device that receives radio waves
and converts the information carried by them to a usable form. Generally, the received signal is
first amplified in an RF amplifier stage to increase the signal strength and rejected the unwanted
signal. Finally the RF carrier must be demodulated to get back the original modulating signal.
The main function of Radio receiver:
Select the desired signal and reject the unwanted signals
Amplify the RF signal
Detect the RF carrier to get back the original modulation frequency voltage
Amplify the modulation frequency voltage
Classification of Radio Receiver
(i) AM Broadcast Receivers: to receive the speech or music AM broadcasting signals at long
wave, medium wave (broadcast band) or short wave.
(ii) FM Broadcast Receivers: To receive signals from FM broadcast transmitters operating in VHF
or in UHF.
(iii) Television Receivers: These receivers are for receiving television broadcast in VHF or in
UHF.
(iv) Communication Receivers: These are superheterodyne receivers used for reception of
telegraphy or telephony signals.
(v) Radar Receivers: These are receivers used for receiving Radar signals.
5
08/05/2018
Basic Function of AM Receiver
A radio receiver in its most elementary form performs the following four essential functions:
Reception: This consists in receiving or picking up energy from the various electromagnbetic
radiated by radio transmitter. This function is performed by the receiving antenna.
Selection: This consists in selecting or responding to desired radio wave with the exclusion of all
others. The selector circuit or tuner in the form of parallel tuned circuit responds to the desired
signal only and rejects all others signals.
Detection or Demodulation: The desired signal in the form of a modulated carrier voltage is
detected in a detector circuit to recover the original modulating voltage.
Reproduction: This consists in feeding the detected signal to a loudspeaker or headphones to
reproduce the sound waves giving the original programme.
TRF Receivers
First amplifying the signal is one or more tuned RF amplifier stages tuned to the signal frequency and then
feeding the signal to the detector is simply referred to as Tuned Radio Frequency (TRF) Receivers.
Receiving Antenna
Loud Speaker
Tuned Radio
Frequency
Amplifier
Detector
Audio
Frequency
Amplifier
Advantages:
 Has enough sensitivity and selectivity
Limitation:
Circuit becomes too selective and may lower the fidelity of the receiver due to too many tune stages
The selectivity of the receiver varies considerably with the frequency of the received signal
Selectivity decreases with the carrier frequency increases
6
08/05/2018
Operation of AM Receiver
See the PDF file
Superheterodyne Receiver
Heterodyne means to mix.
Heterodyne reception stands for the radio reception after converting the modulated carrier voltage into similarity
modulated voltage at a different carrier frequency.
Therefore, the heterodyneing process involves a simple change or translation of carrier frequency, this change in
carrier frequency can be achieved by heterodyning or mixing the modulated carrier voltage with a locally generated
high frequency voltage in a non-linear device to obtain at the output similarly modulated carrier voltage at different
carrier frequency called the intermediate frequency.
Superheterodyne reception is a form of heterodyne reception in which frequency conversion takes place one or more
times before the modulated carrier voltage is fed to the detector to recover the original modulation frequency
voltage.
The receiver in which only one frequency conversion takes place before detection is called superheterodyne
receiver.
The receiver in which frequency conversion takes place twice before detection is called a double superheterodyne
receiver or triple detection receiver.
7
08/05/2018
Principle of Superheterodyne receiver
In a simple superheterodyne receiver, the modulated carrier voltage of frequency, fc is fed to a non-linera device called the
frequency mixer to which is also fed the voltage of frequency, fo generated in a local oscillator and at the output we get voltage of
sum and difference frequencies (mfc - nfo) where m and n are integers.
A tuned circuit in the output of the mixer stage tuned to the difference frequency (fc - fo) picks up this difference frequency
component constituting the intermediate frequency (I.F). The IF voltage is modulated exactly similar to the incoming modulated
carrier voltage. There results only a translation or change in the carrier frequency from fc to fi. This intermediate frequency, fi is
fixed for a receiver.
The constant difference frequency, fi is maintained between the local oscillator frequency and the RF signal frequency, usually
through use of capacitance tuning wherein the capacitors in RF tuned circuits and local oscillator are ganged together and operated
in unison through use of a single control knob.
The RF stage is a two or three stage tuned amplifier tuned to the IF and it provides most of the gain and hence the sensitivity of the
receiver.
It is superior to TRF receiver in which both the sensitivity and selectivity change greatly with the carrier frequency. Further the RF
amplifier stage in superhet receiver rejects the image frequency.
Most popularly used in almost all radio receiver applications such as AM broadcast, AM Communications receivers, FM receivers,
SSB receivers, TV receivers etc.
Constituent Stage of Superheterodyne Receiver
Receiving Antenna
The IF frequency is 456 or 465 Khz
Audio Signal
RF
Amplifier
fs
fi
Mixer
fo
Local
Oscillator
IF
Amplifier
Second
Detector
Linear diode
Two/more stages
detector
3-dB at 10 KHz
Amplification & selectivity
AF voltage
Amplifier
AF Power
Amplifier
Loud Speaker
Frequency Converter stage
8