Chapter 3Amplitude Modulation

Circuits

AMPLITUDE MODULATORS• AM is the process of multiplying the

carrier by the modulating signal.

• Two ways:multiply with gainor attenuation factor that varies with modulating signal or linearly mix /algebraically ad the carrier and the modulating signal,then apply the composite signal to a non liner device

Cont’d

• Non-linear mixing is another name for AM.

• An amplitude modulator must multiply analog signals or mix them together in a non-linear device like a diode.

DIODEMODULATOR

Information

CarrierAM signal

Envelope matchesinformation signal

Diode providesmultiplication

HOW THE DIODE MODULATOR WORKS

• The resistive network adds the carrier and modulating signals.

• The diode rectifies the result producing non-linear mixing.

• The parallel tuned circuit selects the carrier and sidebands and rejects the modulating signal.

DIFFERENTIAL AMPLIFIER MODULATOR

• A differential amplifier makes a good amplitude modulator. The modulating signal is used to vary the common mode current in the transistors that amplify the carrier.

• Most integrated circuit (IC) amplitude modulators are based upon the differential amplifier modulator.

DIFFERENTIAL AMPLIFIER(CONT’D)

• ADVANTAGES:

• MAKES EXCELLENT AMPLITUDE MODULATOR.

• HIGH GAIN AND GOOD LINEARITY, CAN BE MODULATED 100%

PIN DIODE MODULATOR• A PIN diode acts as a voltage variable

resistor at very high frequencies.• PIN diodes are special diodes made to be

used for frequencies above 100MHz• An amplitude modulator can be made with

PIN diodes if the carrier frequency is VHF, UHF, or microwaves.

• The PIN diodes are connected as resistive attenuators for the carrier. The attenuation is varied by the modulating signal producing AM.

PIN DIODE MODULATOR• Disadvantages:-produces attenuation, thus needs of

amplifying the signal back before ready for transmission.

-but widely used, because the only modulator for microwave frequencies.

AM LEVEL• Low level AM produces the AM

signal at a very low power level. High power amplifiers increase the power to the desired level. Less efficient linear amplifiers must be used to amplify the AM signal.

• High level AM is produced by amplitude modulating the final amplifier stage in a transmitter. More efficient class C amplifiers can be used.

AM(HIGH LEVEL)• Produce best type of AM, but needs

extremely high power modulator circuit.

• For 100% modulation, the modulator must have at least half the total power of class c amplifier. Say if class C amplifier has input power of 1000W, the modulator must be able to deliver 500W

AM DEMODULATOR• A demodulator is a circuit that recovers

the original information signal from the carrier that is received.

• The simplest AM demodulator is a half wave rectifier circuit called a diode detector.

• The diode detector is also the simplest AM receiver. It is called a crystal radio.

AM DIODE DEMODULATOR (DETECTOR)

AM signal in

Information out

Diode current(no filter)

Filtered waveform

BALANCED MODULATOR• A balanced modulator is an amplitude

modulator that suppresses the carrier but produces both upper and lower sidebands.

• A balanced modulator produces DSB.

• Diode ring or diode lattice modulators are widely used.

• IC balanced modulators based upon differential amplifiers are very popular.

DIODE LATTICE BALANCED MODULATOR

Carrier oscillator

DSBoutput

Modulatingsignal

When the modulating signal is 0, there is nooutput since the currents in the output transformerare equal and opposite (flux cancellation).

When the modulating signal is non-zero,the diodes are unbalanced and so are thecarrier currents which produce an output.

GENERATING SSB

• A balanced modulator eliminates the carrier and provides DSB.

• A filter removes the undesired sideband producing SSB.

• Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

Filterresponse

curve

Sidebandfilter

Balancedmodulator

Carrieroscillator

Microphone Audioamplifier

Linearamplifier

Antenna

fC Uppersidebands

DSBsignal

SSBsignal

TYPICAL SSB TRANSMITTER

Lowersidebands

THE PHASING METHOD OF SSB

• Another way to produce SSB uses a phase shift method to eliminate one sideband.

• Two balanced modulators driven by carriers and modulating signals 90º out of phase produce DSB.

• Adding the two DSB signals together results in one sideband being cancelled out.

DEMODULATING SSB AND DSB• The basic diode detector will not recover the

information from a DSB or SSB signal.• A balanced modulator is used to recover the

SSB or DSB signal.• A SSB/DSB demodulator is often called a

product detector.• The incoming SSB or DSB signal is mixed with

the carrier generated in the receiver to reproduce the original modulating signal.

DSB SIGNAL WITHSINE WAVE MODULATION

Note: the envelope is not a sine wave

The basic diode detector will produce this waveformwhich is not the original information signal.

Balancedmodulator

Carrieroscillator

TYPICAL DSB OR SSB DETECTOR

Information out

DSB (or SSB) signal

Also called aproduct detector

• Carrier=9MHz• Modulating signal=2kHz=0.002MHz• After modulation, and SSB generated, only

upper sideband=9.002MHz transmitted.• In receiver, add with local oscillator at

9MHz, output is sum and difference:18.002MHz, and 0.002MHz=modulating signal.

• Use filter to filter everything else

Example of how product detector works..