radio transmitters

Radio Transmitters 2011

Radio Transmitters

1. INTRODUCTION

The fundamental equipment used for radio communication are the transmitter and receiver.

The transmitter must generate a radio frequency signal of sufficient power at the desired

frequency. It must have some means of varying (or modulating) the carrier frequency so that it

can carry an intelligible signal.

The receiver must select the desired frequency you want to receive and reject all unwanted

frequencies. In addition, receivers must be able to amplify the weak incoming signal to

overcome the losses the signal suffers during transmission.

Fig 1: Radio transmission block diagram


Table 1: Radio frequency Spectrum

TRANSMITTER CLASSIFICATION

Radio transmitter may be classified into following categories

• According to the type of modulation used

• According to the service involved

• According to the frequency range involved.


CLASSIFICATION ACCORDING TO THE TYPE OF MODULATION

i) Amplitude Modulation Transmitters:

In A.M transmitter, the modulating signal amplitude modulates the carrier. Such transmitters

are used for radio broadcast on long, medium and short eaves, radio telephony on short waves,

radio telegraphy on short waves, television picture broadcast on every short waves or ultra short

waves.

ii) Frequency Modulation transmitters:

In an FM transmitter, the signal voltage frequency modulates the carrier. Such transmitters are

used for broadcast in V.H.F and U.H.F range, television sound broadcast in V.H.F and U.H.F

ranges, radio telephony communication in V.H.F and U.H.F range over short distances

iii) Pulse Modulation Transmitter: In a pulse modulation transmitter, the signal voltage alters

some characteristic of the pulses. Theses characteristics of the pulse altered on modulation may

be pulse width, pulse position, pulse amplitude, pulse frequency or pulse code.

CLASSIFICATION ACCORDING TO THE TYPE OF SERVICE

(i)Radio Broadcast Transmitters:

Theses transmitters are designed for transmitting speech, music etc for the information and

reception of people. These broadcast transmitters may be either amplitude modulated or

frequency modulated. The A.M transmitters operate on long waves, medium waves and short

waves

The F.M broadcast transmitters operate on very short waves or on ultra waves .

ii) Radio Telephony Transmitters:

These transmitters are designed for transmitting telephone signals over long distance by radio

means. A radio telephone transmitter uses certain special as volume compressors, privacy

devices, peak limiters etc. Radio telephone transmitters may be either of amplitude modulation

type or frequency modulation type.

iii) Radio Telegraph Transmitters:

A radio telegraph transmitter transmits telegraph signals from one radio station to another

radio station. It may use either amplitude modulation or frequency modulation. When point to-


point radio communication is involved, the transmitting antennas are highly directive so that the

electromagnetic energy is beamed into a narrow beam directed towards the receiving antenna

at the receiving radio station.

iv)Television transmitters:

Television broadcast requires two transmitters one for transmission of picture and other for

transmission of sound. Both operate in very high frequency or ultra high frequency range. The

picture transmitter is amplitude modulated by the picture signal occupying a band about 75

MHz. The total bandwidth occupied by one television channel is about 7 MHz.

v) Radar Transmitters:

Radar may be Pulse radar or Continuous Wave radar.

Pulse radar transmitter uses pulse modulation of carrier operates at microwave frequencies

typically 3000 MHz or 10,000 MHz.

Continuous Wave radar transmitter uses frequency modulation of the carrier voltage or may

utilize Doppler Effect.

CLASSIFICATION OF RADIO TRANSMITTERS ACCORDING TO THE CARRIER FREQUENCY

In this classification, transmitters may be classified as below

(i) Long Wave Transmitters:

These transmitters operate on long waves . Long wave radio signals travel along the surface of

earth and are rapidly attenuated, for reasonably high signal strength at the distant receiving

aerial, the carrier power radiated from the transmitting aerial must be large.

(ii) Medium Wave Transmitters:

These transmitters operate on frequencies in the Broadcast band ranging from 550 to 1650 kHz

(iii)Short Wave Transmitters:

These transmitters operate on frequencies in the short wave range of 3 to 30MHz. The

attenuation of radio waves travelling from the transmitting aerial to the distant receiving aerial

though the ionosphere is small


2. AMPLITUDE MODULATED TRANSMITTERS

Amplitude modulation process is completed in two stages, one is modulation which is

performed at the transmitters and other is demodulation which is done at receiver.

A transmitter not only performs the modulation process, but also raises the power level of a

modulated signal to the desired extent for effective radiation.

In AM transmitters, the instantaneous amplitude of the rf(radio frequency) output signal is

varied in proportion to the modulating signal. The modulating signal may consist of many

frequencies of various amplitudes and phases.

AM transmitters can be broadly divided into two categories:-

(i) Low level modulation: The block diagram for this type of transmitter is shown in figure 2

(ii) High level modulation: The block diagram for this type of transmitter is shown in figure 3

High level modulation

In this type of AM modulation, modulation takes place at the final power amplifier stage of the

transmitter, just before it is transmitted by the antenna.

This is done to avoid taking the modulated composite signal through non linear devices because

this generates new frequencies, which would have the effect of introducing audible distortion

into the modulated signal.

The disadvantage of AM-modulating the final amplifier stage is that doing so requires audio

power equal to 50% of the carrier power. Thus for a 10 KW transmitter, the audio power

amplifier of 5kW would be required.


Fig 2.1: High level modulation AM transmitter.

Low level modulation

Low level modulation means modulation applied to a low-power stage before the power

amplifier boosts it to the full transmit power level. That's obviously a great benefit, because you

don't need nearly as much audio power, so the audio stages are much less expensive.

But also this means that every stage after modulation must be a highly linear stage, in order to

avoid distortion. Thus the power amplifier must be class A instead of Class-C final power

amplifier.

Class-A means much lower efficiency, so it has to be bigger, and use more DC power supply than

the Class-C, in order to send the same power up to the antenna.


Fig 2.2: Low level modulation AM transmitter

STAGES OF THE AMPLITUDE MODULATED TRANSMITTER

Master oscillator

The master oscillator generates a stable sub harmonic carrier frequency (i.e. the fraction of a

desired carrier frequency). This stable sub-harmonic oscillation is generated by using a crystal

oscillator which is an electronic oscillator circuit that uses the mechanical resonance of a

vibrating crystal of piezoelectric material to create an electrical signal with a very precise

frequency. The frequency is then raised to the desired value by harmonic generator.

Buffer amplifier

This is a tuned amplifier providing high input impedance at the master oscillator frequency. Any

variation in load current does not affect the master oscillator due to this high input impedance

of buffer amplifier at the operating frequency of the master oscillator.


Harmonic generator

This is an electronic circuit that generates harmonics of its input frequency by applying the signal

to a non-linear circuit, to generate harmonics of input frequency.

The desired harmonic is selected by a properly tuned circuit. The circuit uses a class C tuned

amplifier.

Driver Amplifier.

One or more stages of a class C tuned amplifier are used to increase the power level of a carrier

signal to provide a large drive to the modulated class C amplifier

Modulation stage

The collector modulation circuit is used for modulation in high power transmitters. The

modulating amplifier is a class A, or class B amplifier amplifying the base-band signal.

In figure 2.2.1, the rf carrier is applied to the base of modulator Q1. The modulating signal is

applied to the collector in series with the collector supply voltage through T3. The output is

then taken from the secondary of T2. With no modulating signal, Q1 acts as an rf amplifier for

the carrier frequency. When the modulation signal is applied, it adds to or subtracts from the

collector supply voltage. This causes the rf current pulses of the collector to vary in amplitude

with the collector supply voltage. These collector current pulses cause oscillations in the tank

circuit (C4 and the primary of T2). The tank circuit is tuned to the carrier frequency. During

periods when the collector current is high, the tank circuit oscillates strongly. At times when the

collector current is small or entirely absent, little or no energy is supplied to the tank and

oscillations become weak or die out. Thus, the modulation envelope is developed.


Fig 2.2.1: Collector modulation circuit

Feeder and antenna

The transmitter power is fed to a transmitting antenna for effective radiation. The length of the

antenna (a conductor) should be of the order of the wavelength for effective radiation.


SINGLE-SIDEBAND TRANSMITTER (SSB)

A carrier that has been modulated by voice or music is accompanied by two identical sidebands,

each carrying the same intelligence. In the conventional amplitude-modulated (AM)

transmitters, the carrier and both sidebands are transmitted. In a single-sideband transmitter

(SSB), only one of the sidebands, the upper or the lower, is transmitted while the remaining

sideband and the carrier are suppressed. Suppression is the elimination of the undesired

portions of the signal.

Figure 2.3 is the block diagram of a single-sideband transmitter. It is seen that the audio

amplifier increases the amplitude of the incoming signal to a level adequate to operate the SSB

generator. Usually the audio amplifier is just a voltage amplifier.

The SSB generator (modulator) combines its audio input and its carrier input to produce the

two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and

suppresses the other one.

By eliminating the carrier and one of the sidebands, intelligence is transmitted at a savings in

power and frequency bandwidth.

In most cases SSB generators operate at very low frequencies when compared with the

normally transmitted frequencies. For that reason, we must convert (or translate) the filter

output to the desired frequency. This is the purpose of the mixer stage. A second output is

obtained from the frequency generator and fed to a frequency multiplier to obtain a higher

carrier frequency for the mixer stage. The output from the mixer is fed to a linear power

amplifier to build up the level of the signal for transmission.


Figure 2.3: SSB transmitter block diagram.

3. FREQUENCY MODULATED TRANSMITTER

In frequency modulation (fm) the modulating signal combines with the carrier to cause the

frequency of the resultant wave to vary with the instantaneous amplitude of the modulating

signal.

Figure 3.1 shows you the block diagram of a frequency-modulated transmitter. The modulating

signal applied to a varicap causes the reactance to vary. The varicap is connected across the

tank circuit of the oscillator. With no modulation, the oscillator generates a steady center

frequency. With modulation applied, the varicap causes the frequency of the oscillator to vary

around the center frequency in accordance with the modulating signal. The oscillator output is

then fed to a frequency multiplier to increase the frequency and then to a power amplifier to

increase the amplitude to the desired level for transmission.

Harmonics

True harmonics are always exact multiples of the basic or fundamental frequency generated by

an oscillator and is created in amplifiers and their associated circuits. Even harmonics are 2, 4,


6, and so on, times the fundamental; odd harmonics are 3, 5, 7, and so on, times the

fundamental.

The series ascends indefinitely until the intensity is too weak to be detected. In general, the

energy in frequencies above the third harmonic is too weak to be significant.

Fig 3.1: Simplified Fm transmitter block diagram.

Frequency Multiplication

It is easier to design and build low frequency crystal oscillators. In some cases the crystal

oscillator operates at frequencies as low as 1/100 of the desired output frequency.

The oscillator frequency is raised to the required output frequency by passing it through one or

more frequency multipliers. Frequency multipliers are special power amplifiers that multiply

the input frequency. Stages that multiply the frequency by 2 are called doublers; those that

multiply by 3 are triplers; and those multiplying by 4 are quadruplers.

You will find the main difference between low-frequency and high-frequency transmitters is the

number of frequency-multiplying stages used.

Figure 3.2 shows the block diagram of the frequency-multiplying stages of a typical uhf/vhf

transmitter. The oscillator in this transmitter is tunable from 18 megahertz to 32 megahertz.


You have multiplier stages that increase the oscillator frequency by a factor of 12 through

successive multiplications of 2, 2, and 3.

Fig 3.2: Frequency multiplying stages of a typical vhf/uhf transmitter.

Figure 3.3 is a block diagram of an fm transmitter showing waveforms found at various points.

In high-power applications there will one or more intermediate amplifiers added between the

second doubler and the final power amplifier.

Fig 3.3: Detailed Block diagram of an FM transmitter showing waveforms.

radio transmitters

Documents

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collector supply voltage

final power amplifier

high level modulation

collector modulation circuit

low level modulation

pulse modulation transmitter

collector current