AM Modulators

7/30/2019 AM Modulators

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Agenda

AM Modulator circuit.

AM Demodulator circuit.


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AM Modulator Type

Non-Linear Modulator.

Linear Modulator.


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Non Linear modulator

Use non-linear devices suchas diode and transistor.

Fig below (next slide) showthat:


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Non-Liner Modulator

RL

N.L.D

m

f

cf

)(tsAM


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Linear Modulator

Linear Anode such as(class A, class B, class C)

amplifiers.

Fig below (next slide) showthat:


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Linear Modulator

Antenna

Class B

AF amp

Class C

RF amp

RF I/P


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Product Modulators

)(tsm

)(tsAM

twAtS ccc cos)(


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Product Modulators

Consist of Analog productmultiplier and adder. As shownbelow:

)(tsm)(tsAM

)(tsc


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Modulators using theSquared Law

give highest frequency thanProduct Modulators. As shownbelow:

Filter)(tsm

twAtS ccc cos)(

)(tVo)(tSAM

Non-Liner

Element


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Modulators using theSquared Law

Verifying the following relation:

This relation is verified by usingDiodes and Transistors.

)()()(

2

21 tVinatVinatVo


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Balance Modulators

Simple method to generate aDSB-SC signals

Use two conventional AM

modulators arranged in theconfiguration .


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Balance Modulators

Use two square-Law AMModulators.

Select modulators must beapproximately identicalcharacteristics so that the carriercomponent cancels out at thesumming junction.


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Balance Modulator Diagram


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Balance Modulator circuitusing two Diodes

R2

C1 L1R1

C1 L2

D2

)(tsm

twts cc cos)(

)(1

tV

)(tVo

)(2

ti

)(1 ti

)(2

tV

D1


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Analysis

The characteristic curve for each

Diodes verify the following relation:

)()()(2

12111 tVatVati

the amplitude of )(1 tV is given by:

--------------------- (1)


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Analysis

)(cos)(1 tStwtV mc

While the amplitude of )(2 tV is:

)(cos)(2 tStwtV mc

--------------------- (2)

--------------------- (3)

2

211 )]([cos)]([cos)( tStwatStwati mcmc

so

---------- (4)


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Analysis

2

212 )]([cos)]([cos)( tStwatStwati mcmc

And------- (5)

So we can find the amplitude of )(tVo Is:

RtititVo

)]()([)( 21

( ) [2 1 ( ) 4 2 ( ) cos ]o m c

V t R a S t a sm t t ---------- (7)

---------- (6)


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Analysis

Since the resonance circuit operates at the

carrier frequency ( cf ) sothe output

)(tVo is:

twtSRatVcmo

cos)(4)( 2 ---------- (8)

I.e. the circuit works like DSB-SCModulator.

I.e. it works like Product Modulator.

AM Demodulators (Detector )


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AM Demodulators (Detector )

The restoration of the massagesignal from the modulatedsignal is called Demodulation

and detection. There are two Method of

demodulation:- Asynchronous (Envelope

Detector)-Synchronous.


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Demodulators (Detectors)for AM signals

AM De-Modulation

)(tsm)(tsAM

Asynchronous Envelope Detector)


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Asynchronous Envelope Detector)

Consists of a diode and anRC circuit, which isbasically a simple low

pass filter.

Circuit diagram for anenvelope detector isshown in Figure below:


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Asynchronous(Envelope Detector)

R1

R2

R3

R6

R4

R5

C1

C3 C4

D1 C5

C2

12 V

741

741

12 V

-12 V-12 V

Vin

)(tsAM

sync ronous nve ope etector na ys s


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sync ronous nve ope etector na ys s

During the positivehalf-cycle

of the input signal, the diode isconducting and the capacitorcharges up to the peak value of

the input signal. When the input falls below the

voltage on the capacitor, thediode becomes reverse-biasedand the input becomes

disconnected from the out ut. D i thi i d th


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During this period, thecapacitor discharges slowly

through the load resistor R. On the next cycle of the

carrier, the diode conductsagain when the input signal

exceeds the voltage acrossthe capacitor.

Th it h


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The capacitor charges upagain to the peak value of

the input signal and theprocess is repeated again.

The time constant RC mustbe selected so as to follow

the variations in theenvelope of the carrier-modulated signal.


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In such a case, thecapacitor dischargesslowly through the resistorand, thus, the output of theenvelopedetector closelyfollows the message signal.

mcw

RCw

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Synchronous Detector


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We can also demodulatethe signal using BalanceModulator (BM), which is

called SynchronousDetector as shown

below:


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Local Oscillator

Filter)(tsAM

)(1ty )(ty

twALco

cos

Synchronous Detector Analysis


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Synchronous Detector Analysis

Depending on multiply the

modulated signal with sinewave signal that generated

form (LocalOscillator),which is denoted by .

Suppose the following:

twALtVL coo cos)(


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twtEtS cAM cos)()(

)](1[)( tSAtEmc

twALtVL coo cos)(

---------- (1)

---------- (2)

Where oAL

is the carrier amplitude.


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For simplicity suppose the

oAL = 2

twtEty c2

1 cos)(2)(

)]2cos1(

2

1)[(2 twtE

c

---------- (3)

)]2cos1)[(( twtE c


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twtEtE c2cos)()( ---------- (4)

We can eliminate the frequency

cw2 using low pass filter (LPF),

so we get the following o/p:


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)()( tEty

But )](1[)( tSAtE mc

)()( tSAAtymcc

---------- (5)

---------- (6)


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Synchronous DetectorAnalysis

We can also suppress the

DC component by usingcapacitor, the finalo/p ell be:

)(tSAmc

AM Modulators

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