Communications I Angle Modulation and Demodulation

EELE 3370

Communications I

Angle Modulation and Demodulation

Dr. Talal Skaik 2016

Islamic University of Gaza

Electrical Engineering Department

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Generating FM Waves Two Methods: [1] Indirect method using NBFM Generation

[2] Direct method

NBFM Generation

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Dr. Talal Skaik IUG 2016

Generating FM Waves

Bandpass Limiter

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With the NBFM generation, the

amplitude of the NBFM modulator

will have some amplitude variation.


Generating FM Waves Circuit Realization of Hard Limiter The circuit consists of a high gain amplifier, a current limiting resistor R

and two zener diodes arranged in a back-to-back configuration.

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Generating FM Waves Indirect Method of Armstrong

We start with the generation of a NBFM with frequency deviation ∆f

as described previously.

Then we use a frequency multiplier ( x N ) to obtain a WBFM.

After filtering using a bandpass filter centered at Nfc, we get an FM

signal with N ∆f.

Sometimes the frequency increase of the carrier is not needed.

Solution: after the multiplier we insert a mixer to down convert the

carrier to the wanted one.

5 Dr. Talal Skaik IUG 2016

Generating FM Waves Indirect Method

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Generating FM Waves

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A frequency multiplier can be realized by a nonlinear device:

Generally, a nonlinear device may have the characteristic:

If , then y(t) is of the form:

c fx ( t ) A cos t k m( )d

0 1 2 2 2c f c f

n c f

y ( t ) c c cos t k m( )d c cos t k m( )d

+ +c cos n t nk m( )d

Use Bandpass filter to recover

the desired FM signal

Generating FM Waves

Example: The final output is desired to have a carrier frequency of

91.2 MHz and ∆f=75 KHz. We begin with carrier frequency

fc1=200KHz. The baseband signal has frequency range 50Hz -15

KHz. ∆f is initially chosen 25 Hz [gives β= ∆f / fm = 0.5 for worst

case fm=50 Hz].


Generating FM Waves Example solution


Generating FM Waves

Example: Design an Armstrong indirect FM modulator to generate

an FM signal with carrier frequency 97.3 MHz and Δf = 10.24 kHz.

A NBFM generator of fc1 = 20 kHz and Δf = 5 Hz is available. Only

frequency doublers can be used as multipliers. Additionally, a local

oscillator (LO) with adjustable frequency between 400 and 500 kHz

is readily available for frequency mixing.


Generating FM Waves Frequency Doubler Circuit Frequency multipliers are special class C amplifiers that are used to

generate a frequency that is a multiple (harmonic) of a lower frequency. The shown circuit is second harmonic generator.

The LC tank circuit is tuned to the second harmonic 2MHz.


Generating FM Waves Direct Generation

The frequency of a voltage-controlled oscillator (VCO) is controlled

by the voltage m(t).

Use an operational amplifier to build VCO.

Another way is to vary one of the reactive parameters (C and L) of

the resonant circuit of an oscillator.

A reversed biased semiconductor diode acts as a capacitor whose

capacitance varies with the bias voltage.

These diodes are called Varicaps or Varactors.

The frequency of oscillation is given by:

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0 1/ LC


Generating FM Waves If the capacitance C is varied by the modulating signal m(t),

C=C0 - k m(t) Then

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1 1 1n

Here we use the approximation x nx x


Generating FM Waves Because C=C0 – k m(t) , the maximum capacitance deviation is:

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Varactor symbols and capacitance curve.


The varactor diode is a semiconductor diode whose junction capacitance

changes voltage.

This diode is shunted with the tuned circuit (tank circuit) of the carrier

oscillator as shown in the figure.

V0 is the polarizing voltage to maintain a reverse bias across the varactor

diode. 17


Demodulation of FM Signals A frequency-selective network with a transfer function of the form

|H(f)|=2af + b over the FM band would yield an output proportional to

the instantaneous frequency.

A possible circuit is an ideal differentiator with transfer function j2πf.

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Demodulation of FM Signals 19

Differentiator


Demodulation of FM Signals

Practical Frequency Demodulators Differentiation: Operational Amplifier (OPAMP) differentiator can

be used to convert frequency variation to amplitude variation that can

be detected using a simple envelop detector.

Slope detection: Any tuned circuit which has a linear segment of

positive slope in the frequency response under or above the resonance

can be used instead of the OPAMP differentiator.

Examples: high pass RC filter

Tuned RLC filter:

Limitation: narrow bandwidth. 20


Demodulation of FM Signals High Pass Filter

The slope is linear over small band, so distortion occurs if the signal band is

larger than the linear band.

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Demodulation of FM Signals Tuned LC circuit The transfer characteristic of a tuned circuit in a small region off

resonance is approximately linear.


Superheterodyne Analog AM/FM Receivers The radio receiver used in broadcast AM and FM systems is called the

superheterodyne receiver.

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fIF = 455KHz (AM radio); 10.7 MHz (FM); 38 MHz (TV)

AM: 530 KHz -1710 KHz FM: 88MHz – 108 MHz


Superheterodyne Analog AM/FM Receivers

AM stations that are 2fIF apart are called image stations and both

would appear simultaneously at the IF output.

Example: the stations at 1000 KHz and 1910 KHz are called image

stations. They are separated by (2* 455 KHz=910 KHz).

If we pick 1000 KHz station, the LO frequency is K1455 Hz.

Another station 1910 KHz also appears at the IF output

(1910 KHz-1455 KHz=455 KHz)

But the 1910 KHz station is filtered out using the RF filter.

The tunable RF filter has poor selectivity by can filter out the image

station.

The IF section filter has good selectivity and it is fixed at 455 KHz

and it filters out undesired neighboring stations (adjacent stations are

separated by 10 KHz). 24


Communications I Angle Modulation and Demodulation

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