Am Transmission

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AM Transmission

Dr. arnas Paulikas

Telekomunikacij ininerijos katedraElektronikos fakultetas, VGTU

ver. 2007 Communication Engineering 2

Introduction

Information signals are transported between atransmitter and a receiver over some form oftransmission medium.

However, the original information signals are seldom in aform that is suitable for transmission.

Therefore, they must be transformed from their originalform into a form that is more suitable for transmission.

The process of impressing low-frequency informationsignals onto a high-frequency carrier signalis calledmodulation.

Demodulationis the reverse process where the receivedsignals are transformed back to their original form.


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AM Principles

Amplitude modulation(AM) is the process of changing the amplitudeof a relatively high frequency carrier signal in accordance with theamplitude of the modulating signal (information).Amplitude modulation is a relatively inexpensive, low-quality form ofmodulation.AM modulators are nonlinear devices with two input signals: asingle-frequency, constant-amplitude carrier signal and theinformation signal.Frequencies that are high enough to be efficiently radiated by anantenna and propagated through free space are commonly calledradio frequenciesor simply RF.

The information acts on or modulates the RF carrier.The information signal may be a single frequency or a complexwaveform made up of many frequencies.


AM Envelope

There are several types of amplitudemodulation.

AM double-sideband full carrier(DSBFC)

is probably the most commonly used.

AM DSBFC is sometimes calledconventionalAM or simply AM.


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AM Envelope


AM Envelope

Vc sin(2fct) - the carrier;Vm sin(2fmt) - the modulating signal;Vam (t)- the modulated wave.The output waveform contains all the

frequencies that make up the AM signal and it isused to transport the information through thesystem.Therefore, the shape of the modulated wave iscalled the AM envelope.Note that with no modulating signal, the outputwaveform is simply the carrier signal.


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AM Frequency Spectrum and Bandwidth

An AM modulator is a nonlinear device.Therefore, nonlinear mixing occurs and the output envelope is acomplex wave made up of a dc voltage, the carrier frequency, andthe sum (fc +fm) and difference (fc -fm) frequencies (that is, the crossproducts).The sum and difference frequencies are displaced from the carrierfrequency by an amount equal to the modulating signal frequency.Therefore, an AM signal spectrum contains frequency componentsspaced fm Hz on either side of the carrier.The modulated wave does not contain a frequency component thatis equal to the modulating signal frequency.

The effect of modulation is to translate the modulating signal in thefrequency domain so that it is reflected symmetrically about thecarrier frequency.


AM Frequency Spectrum and Bandwidth

The bandwidth (B) of an AM DSBFC wave isequal to, B = 2 fm max.


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Phasor Representation of an AM


Coefficient and Percent of Modulation

Coefficient of modulationis a term that is used todescribe the amount ofamplitude change

(modulation) present inan AM waveform.

Percent modulationissimply the coefficient ofmodulation stated as apercentage.

c

m

E

Em =

100c

m=

E

EM


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AM Voltage Distribution

An unmodulatedcarrier can be

describedmathematically as

Therefore, theinstantaneous

amplitude of themodulated wave canbe expressed as

)2sin()( ccc tfEtv =

( )

( )[ ] ( )[ ]( )[ ] ( )[ ]tfEtfm

tftfEE

tv

ccm

cmmc

am

2sin2sin1

2sin2sin

+=

+=

=


AM Voltage Distribution

( ) ( )

( )[ ]

( )[ ]tffmE

tffmE

tfEtv

mm

mm

ccam

2cos2

2cos2

2sin

+

+

=


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AM Power Distribution

The average powerdissipated in a loadby an unmodulatedcarrier is equal to therms carrier voltagesquared, divided bythe load resistance:

Mathematically,power in anunmodulated carrieris:

( )R

E

R

EPc 2

707.0 2c2

c==



The upper and lowersideband powers areexpressedmathematically as:

The total power in anamplitude-modulatedwave is equal to the sumof the powers of thecarrier, the uppersideband, and the lowersideband:

( )42

2/ c22

clsbusb

Pm

R

mEPP ===

lsbusbct PPPP ++=


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Note that with 100% modulation the maximum power inthe upper or lower sideband is equal to only one-fourththe power in the carrier.Thus, the maximum total sideband power is equal toone-half the carrier power.

One of the most significant disadvantages of AM DSBFCtransmission is the fact that the information is containedin the sidebands although most of the power is wasted inthe carrier.Actually, the power in the carrier is not totally wastedbecause it does allow for the use of relatively simple,inexpensive demodulator circuits in the receiver, which isthe predominant advantage of AM DSBFC.


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AM Modulator Circuit

The location in a transmitter where modulationoccurs determines whether the circuit is a low-or high-level transmitter.

With low-level modulation, the modulation takesplace, prior to the output element of the finalstage of the transmitter, in other words, prior tothe collector of the output transistor.

An advantage of low-level modulation is that lessmodulating signal power isrequired to achieve a high percentage ofmodulation.


AM Modulator Circuit

In high-level modulators, the modulation takes place inthe final element of the final stage where the carriersignal is at its maximum amplitude and, thus, requires amuch higher amplitude modulating signal to achieve areasonable percent modulation.

With high-level modulation, the final modulating signalamplifier must supply all the sideband power, whichcould be as much as 33% of the total transmit power.

An obvious disadvantage of low-level modulation is inhigh-power applications when all the amplifiers thatfollow the modulator stage must be linear amplifiers,which is extremely inefficient.


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Low-Level AM Modulator

A small signal class A amplifier can be used to performamplitude modulation, however, the amplifier must havetwo inputs: one for the carrier signal and the second forthe modulating signal.

With no modulating signal present, the circuit operatesas a linear class A amplifier and the output is simply thecarrier amplified by the quiescent voltage gain.However, when a modulating signal is applied, theamplifier operates nonlinearly and signal multiplicationoccurs.

The modulating signal varies the gain of the amplifier ata sinusoidal rate equal to the frequency of themodulating signal.The depth of modulation achieved is proportional to theamplitude of the modulating signal.




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Medium-Power AM Modulator

The modulation takes place in the collector, which is theoutput element of the transistor.

Therefore, if this is the final active stage of thetransmitter (that is, there are no amplifiers between itand the antenna), it is a high-level modulator.

To achieve high power efficiency, medium- and high-power AM modulators generally operate class C.

Therefore, a practical efficiency of as high as 80% ispossible.

Because the transistor is biased class' C, it operatesnonlinear and is capable of nonlinear mixing(modulation).


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Medium-Power AM Modulator


More Practical Medium-Power AM


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More Practical Medium-Power AM


Simultaneous Emitter and CollectorModulation

Collector modulators produce a more symmetricalenvelope than low-power emitter modulators, andcollector modulators are more power efficient.

However, collector modulators require a higheramplitude-modulating signal, and they cannot achieve afull saturation-to-cutoff output voltage swing, thus,preventing 100% modulation from occurring.

Therefore, to achieve symmetrical modulation, operateat maximum efficiency, develop a high output power, andrequire as little modulating signal drive power aspossible, emitter and collector modulations aresometimes used simultaneously.


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High-Power AM


High-Power AM

It is AM modulator that uses a combination of bothemitter and collector modulations.

The modulating signal is simultaneously fed into thecollectors of the push-pull modulators (Q2 and Q3) and tothe collector of the driver amplifier (Q1).

Collector modulation occurs in Q1 thus, the carrier signalon the base of Q2 and Q3 has already been partiallymodulated and the modulating signal power can bereduced.

Also, the modulators are not required to operate overtheir entire operating curve to achieve 100% modulation.


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AM Transmitters

According to modulators classificationthere are:

Low-Level Transmitters and

High-Level Transmitters


Low-Level Transmitters


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Low-Level Transmitters

For voice or music transmission, the source of themodulating signal is generally an acoustical transducer,such as a microphone, a magnetic tape, a CD disk, or aphonograph record.The preamplifieris typically a sensitive, class A linearvoltage amplifier with a high input impedance. The function of the preamplifier is to raise the amplitude of the

source signal to a usable level while producing minimumnonlinear distortion and adding as little thermal noise aspossible.

The driver for the modulating signal is also a linearamplifier that simply amplifies the information signal toan adequate level to sufficiently drive the modulator. More than one drive amplifier may be required.


Low-Level TransmittersThe RF carrier oscillatorcan be any of the oscillator configurationsdiscussed previously.Due to requirements on transmitter accuracy and stability the crystal-controlled oscillators are the most common circuits used.The buffer amplifieris a low-gain, high-input impedance linear amplifier. Its function is to isolate the oscillator from the high-power amplifiers. The buffer provides a relatively constant load to the oscillator, which helps to

reduce the occurrence and magnitude of short-term frequency variations. Emitter followers or integrated-circuit op-amps are often used for the buffer.

The modulator can use either emitter or collector modulation. The intermediate and final power amplifiers are either linear class A or class B

push-pull. This is required with low-level transmitters to maintain symmetry in the AM

envelope.

The antenna coupling network matches the output impedance of the finalpower amplifier to the transmission line and antenna.Low-level transmitters are used predominantly for low-power, low-capacitysystems such as wireless intercoms, remote-control units, pagers...


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The modulating signal is processed in the samemanner as in the low-level transmitter except forthe addition of a power amplifier.

With high-level transmitters, the power of the

modulating signal must be considerably higherthan is necessary with low-level transmitters.

This is because the carrier is at full power at thepoint in the transmitter where modulation occursand, consequently, requires a high-amplitudemodulating signal to produce 100% modulation.


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The RF carrier oscillator, its associated buffer, and the carrier driverare also essentially the same circuits used in low-level transmitters.However, with high-level transmitters, the RF carrier undergoesadditional power amplification prior to the modulator stage, and thefinal power amplifier is also the modulator.Consequently, the modulator is generally collector-modulated classC amplifier.With high-level transmitters, the modulator circuit has three primaryfunctions It provides the circuitry necessary for modulation to occur (that is,

nonlinearity),

it is the final power amplifier (class C for efficiency), and it is a frequency up-converter.

An up-converter simply translates the low-frequency intelligencesignals to radio-frequency signals that can be efficiently radiatedfrom an antenna and propagated through free space.


Carrier Shift

Carrier shift is a term that is often misunderstood or misinterpreted.Carrier shift is sometimes called upwardor downward modulationand has absolutely nothing to do with the frequency of the carrier.

Carrier shiftis a form of amplitude distortion introduced when thepositive and negative alternations in the AM modulated signal arenot equal (that is, nonsymmetrical modulation).

Carrier shift may be either positive or negative. If the positive alternation of the modulated signal has a larger amplitude

than the negative alternation, positive carrier shift results. If the negative alternation is larger than the positive, negative carrier

shift occurs.

Carrier shift is an indication of the average voltage of an AMmodulated signal. If the positive and negative halves of the modulated signal are equal,

the average voltage is 0 V. If the positive half is larger, the average voltage is positive, and if the

negative half is larger, the average voltage is negative.


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Carrier Shift

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Am Transmission