Chapter 6

2008 The McGraw-Hill Companies

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Principles of Electronic

Communication Systems

Third Edition

Louis E. Frenzel, Jr.


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Chapter 6

FM Circuits


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Topics Covered in Chapter 6

6-1: Frequency Modulators

6-2: Phase Modulators

6-3: Frequency Demodulators


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There are many circuits used to produce FM and PM signals. There are two types of frequency modulator

circuits: direct circuits and phase modulation circuits.

A frequency modulator is a circuit that varies carrier frequency in accordance with the modulating signal.

The carrier is generated by LC or crystal oscillator circuits.


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In LC oscillators, the carrier frequency can be changed by varying either the inductance or

capacitance.

The idea is to find a circuit or component that converts a modulating voltage to a corresponding change in

capacitance or inductance.

In crystal oscillators, the frequency is fixed by the crystal.

A varactor is a variable capacitance diode used to change oscillator frequencies.


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Varactor Operation

A junction diode is created when P- and N-type semiconductors are formed during the manufacturing

process.

A depletion region, where there are no free carriers, holes, or electrons, is formed in the process.

This region acts like a thin insulator that prevents current from flowing through the device.

A forward bias will cause the diode to conduct.

A reverse bias will prevent current flow.


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Varactor Operation

A reverse-biased diode acts like a small capacitor.

The P- and N-type materials act as the two plates of the capacitor.

The depletion region acts as the dielectric material.

The width of the depletion layer determines the width of the dielectric and, therefore the amount of capacitance.

All diodes exhibit variable capacitance.

Varactors are designed to optimize this characteristic.


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Figure 6-2: Schematic symbols of a varactor diode.


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Figure 6-4: A direct-frequency-modulated carrier oscillator using a varactor diode.


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Varactor Modulator

In Figure 6-4, the capacitance of varactor diode D1 and L1 form the parallel tuned circuit of the oscillator.

The value of C1 is made very large so its reactance is very low.

C1 connects the tuned circuit to the oscillator and blocks the dc bias on the base of Q1 from being shorted to ground through L1.

The values of L1 and D1 fix the center carrier frequency.

The modulating signal varies the effective voltage applied to D1 and its capacitance varies.


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Varactor Modulator

Most LC oscillators are not stable enough to provide a carrier signal.

The frequency of LC oscillators will vary with temperature changes, variations in circuit voltage, and

other factors.

As a result, crystal oscillators are normally used to set carrier frequency.


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Frequency-Modulating a Crystal Oscillator

Crystal oscillators provide highly accurate carrier frequencies and their stability is superior to LC

oscillators.

The frequency of a crystal oscillator can be varied by changing the value of capacitance in series or parallel

with the crystal.

By making the series capacitance a varactor diode, frequency modulation can be achieved.

The modulating signal is applied to the varactor diode which changes the oscillator frequency.


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Figure 6-5: Frequency modulation of a crystal oscillator with a VVC.


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Frequency-Modulating a Crystal Oscillator

Varactors are made with a wide range of capacitance values, most units having a nominal capacitance in the 1- to 200-pF range.

A frequency multiplier circuit is one whose output frequency is some integer multiple of the input frequency.

A frequency multiplier that multiplies a frequency by two is called a doubler.

A frequency multiplier that multiplies a frequency by three is called a tripler.

Frequency multipliers can also be cascaded.


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Figure 6-6: How frequency multipliers increase carrier frequency and deviation.


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Voltage-Controlled Oscillators

Oscillators whose frequencies are controlled by an external input voltage are generally referred to as

voltage-controlled oscillators (VCOs).

Voltage-controlled crystal oscillators are generally referred to as VXOs.

VCOs are primarily used in FM.

VCOs are also used in voltage-to-frequency conversion applications.


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Reactance Modulator

A reactance modulator is a circuit that uses a transistor amplifier that acts like either a variable

capacitor or an inductor.

When the circuit is connected across the tuned circuit of an oscillator, the oscillator frequency can be varied by

applying the modulating signal to the amplifier.

Reactance modulators can produce frequency deviation over a wide range.

Reactance modulators are highly linear, so distortion is minimal.


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Figure 6-10: A reactance modulator.


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Direct FM Transmitter

Produce an output waveform in which the frequency deviation is directly proportional to the modulating

signal


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Crosby Direct FM Transmitter

Frequency modulator can be reactance modulator or VCO

Three aspect of frequency conversion

a. when the frequency of the frequency-modulated carrier is multiplied, its frequency and phase deviation

are multiplied

b. the rate at which the carrier is deviated is unaffected by the multiplication process

c. when an angle-modulated carrier is heterodyned with another frequency in a nonlinear mixer, the carrier

can either up- or down-converted


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Automatic Frequency Control

(AFC)

Compares the frequency of a noncrystal carrier oscillator to a crystal reference oscillator and then

produces a correction voltage proportional to the

difference between the two frequencies

Correction voltage is fed to the carrier oscillator to automatically compensate for any drift that may

occurred


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Sample Problem

Use the transmitter block diagram values to answer the following questions. For a total frequency

multiplication of 20 and a transmitted carrier frequency

ft=88.8 MHz determine:

a. Master oscillator center frequency

b. Frequency deviation of 75 kHz at the antenna

c. Deviation ratio at the output of the modulator for a

maximum modulating signal of 15 kHz


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Sample Problem

Using the transmitter block diagram values, determine

the reduction in frequency drift at the antenna

transmitter without AFC compared to a transmitter with

AFC. Use VCO stability = +200 ppm, ko=10kHz/V and

kd=2V/kHz


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Most modern FM transmitters use some form of phase modulation (PM) to produce indirect FM.

In PM the carrier oscillator can be optimized for frequency accuracy and stability.

Crystal oscillators or crystal-controlled frequency synthesizers can be used to set the carrier frequency accurately and maintain stability.

The output of the carrier oscillator is fed to a phase modulator where the phase shift is made to vary in accordance with the modulating signal.


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Simple phase shifters do not produce a linear response over a large range of phase shift.

To compensate for this, restrict the total allowable phase shift to maximize linearity.

Multipliers must also be used to achieve the desired deviation.


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Figure 6-11: RC phase-shifter basics.


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Varactor Phase Modulators

A simple phase-shift circuit can be used as a phase modulator if the resistance or capacitance can be made to vary with the modulating signal.

A varactor can be used to vary capacitance and achieve phase shift modulation.

If the modulating signal becomes more positive, it adds to the varactor reveres bias from R1 and R2, causing the capacitance to decrease and the

reactance to increase wherein the circuit produces less phase shift and

deviation

A more negative signal, from A subtract from the reverse bias on the varactor diode, increasing the capacitance and decreasing the reactance, thus the

increase phase shit and deviation are produced.

Inverting amplifier is used to correct the inverse relationship with the modulating signal polarity and the direction of frequency deviation


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Figure 6-12: A varactor phase modulator.


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Transistor Phase Modulator

A transistor can be used as a variable resistor to create a phase modulator.

A standard common emitter class A amplifier biased into the linear region is used in PM.

The transistor from collector to ground acts like a resistor.

The transistors resistance forms part of the phase shifting circuit.


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Figure 6-13: A transistor phase shifter.


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Sample Problem

Calculate the Total phase shit of the transistor phase modulator operating at the carrier frequency of 1.4

MHz with IC =1.22mA Vc=6.28V if there is no

modulation. With a positive modulating signal Ic rises

by 0.5mA and becomes 1.72 mA while collector

voltage drops to 3.9V. Likewise, if the modulating

signal becomes more negative, Ic drops to 0.72 mA

and the collector vo


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Tuned-Circuit Phase Modulators

Most phase modulators are capable of producing a small amount of phase shift. The limited phase shift,

therefore, produces a limited frequency shift.

Phase and frequency shift can be increased by using a parallel tuned circuit.

At resonance, a parallel resonant circuit acts like a large resistor.

Off resonance, the circuit acts inductively or capacitively and produces a phase shift.


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Tuned-Circuit Phase Modulators Phase modulators are easy to implement, but they have two main

disadvantages.

1. The amount of phase shift they produce and the resulting frequency

deviation are relatively low.

2. All the phase-shift circuits produce amplitude variations as well as

phase changes.

3. When modulating signal goes negative, it subtract to the reverse bias of

D1, increasing the capacitance and lower the reactance making the

circuit capacitive producing a leading phase shift and the output lags the

input

4. A positive going modulating voltage decreases the capacitance the

tuned circuit becomes inductive producing lagging phase shift so the

output leads the input


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Sample Problem

A transistor must operate at a frequency of 168.96 MHz with a deviation of + 5kHz. It uses 3 frequency

multipliers, a doubler, a tripler and a quadrupler.

Phase modulation is used. Calculate:

a. Frequency of the carrier oscillator

b. Phase shift required to produce the necessary

deviation at 2.8 kHz modulation frequency.


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Sample Problem

Using the values in the previous example using RC phase shifter with C=capacitor and R=1k. Assume that the total phase shift range is centered on 45deg,

Calculate the 2 capacitive values required to achieve

the total deviation.


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Any circuit that will convert a frequency variation in the carrier back into a proportional voltage variation can

be used to demodulate or detect FM signals.

Circuits used to recover the original modulating signal from an FM transmission are called:

Demodulators

Detectors

Discriminators


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Slope Detector

The slope detector makes use of a tuned circuit and a diode detector to convert frequency variations into

voltage variations.

The main difficulty with slope detectors lies in tuning them.


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Figure 6-16: Slope detector operation.


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Pulse-Averaging Discriminators

A pulse-averaging discriminator uses a zero crossing detector, a one shot multivibrator and a low-pass filter in order to recover the original modulating signal.

The pulse-averaging discriminator is a very high-quality frequency demodulator.

Originally this discriminator was limited to expensive telemetry and industrial control applications.

With availability of low-cost ICs, this discriminator is used in many electronic products.


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Figure 6-17: Pulse-averaging discriminator.


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Quadrature Detector

The quadrature detector is probably the single most widely used FM demodulator.

The quadrature detector is primarily used in TV demodulation.

This detector is used in some FM radio stations.

The quadrature detector uses a phase-shift circuit to produce a phase shift of 90 degrees at the unmodulated

carrier frequency.


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Figure 6-19: A quadrature FM detector.


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Phase-Locked Loops

A phase-locked loop (PLL) is a frequency- or phase-sensitive feedback control circuit used in frequency

demodulation, frequency synthesizers, and various

filtering and signal-detection applications. PLLs have

three basic elements. They are:

Phase detector

Low-pass filter

Voltage-controlled oscillator


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Figure 6-21: Block diagram of a PLL.


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Phase-Locked Loops

The primary job of the phase detector is to compare the two input signals and generate an output signal that,

when filtered, will control the VCO.

If there is a phase or frequency difference between the FM input and VCO signals, the phase detector output

varies in proportion to the difference.

The filtered output adjusts the VCO frequency in an attempt to correct for the original frequency or phase

difference.


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Phase-Locked Loops

This dc control voltage, called the error signal, is also the feedback in this circuit.

When no input signal is applied, the phase detector and low-pass filter outputs are zero.

The VCO then operates at what is called the free-running frequency, its normal operating frequency as

determined by internal frequency-determining

components.

Chapter 6

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