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Electronic Communication

EG 245

Frequency ModulationGeneration and Detection

Dr. Amit MehtaSlides References

Louis E. Frenzel Jr: Principles of Electronic Communication SystemsBeasley and Miller: Modern Electronic Communication

FM Generation

• Need Frequency Sources– LC Oscillators

– Crystal Oscillators

• When controlling them using external voltage: VCOs

2

LC Oscillator

f = ½.π.√LC

If we can amplify and feedback, we got an oscillator

With Amplifier and Feedback• Hartley oscillator

With spark generate first sinusoid

3

Variable Frequency

f = ½.π.√LC

If we very C or L electronically, we change frequency, so get FM

HOW

Varactor Diode

Texture Notes• 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.• 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.– Varactor Operation

• A reverse-biased diode acts like a small capacitor• More reverse bias, less is the capacitance

Direct FM Generation

Direct-frequency-modulated carrier oscillator using a varactor diode.

This concept of changing frequency with voltage makes a VCO

Notes

– 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, hence in turn the output frequency vary

– No mod Vol, output is carrier.

– More Vol, Less C, Higher Frequency, and vice versa

Varactor with Crystal

– 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

Crystal Oscillators

Voltage

Equivalent circuit

Very stable frequency output. Used as local oscillator

FM With Crystal

No feedbackTransistor only as amp.

Series

Direct FM- Frequency modulation of a crystal oscillator with a VVC

Notes

– 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.

– Limitation: Limited frequency shift

Use of Multipliers

4 MHz 96 MHz

+75MHz

Broadcast FM

Deviation of +3.125 KHz

Frequency multipliers increase carrier frequency and deviation

VCO-In Chip Form

– Oscillators whose frequencies are controlled by an external input voltage are generally referred to as voltage-controlled oscillators (VCOs, E.g. before).

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

– VCOs in chip form are primarily used in FM.

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

Phase Modulation

• Most modern FM transmitters use some form of phase modulation (PM) to produce indirect FM (Remember: FM can be extracted from PM).

• 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.– 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.

Phase Shift Circuitry

RC phase-shifter basics

Leading effect from RL Circuit

No electronic variable inductor

Varactor is capacitive, Transistor can be variable R, we use RC circuit

Indirect FM

– 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

The first FM Broadcast by Edwin Armstrong in 1930 was based on Indirect FM

FM Demodulators

AND

fc

Digital:Analog: Differential Amp

A quadrature FM detector

Phase Detector

As frequency increases phase differences reduces

Calibration

Shift = 90 deg; f2 = fcNo output

Shift > 90 deg; f1

Decreasing

Shift < 90 deg; f3

Increasing

Quadrature Detector

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

– The quadrature detector is primarily used in TVdemodulation.

– Working– Due to C1, A and B phase always stays at 90 deg quad– C2 and L tuned always to fc;– When no mod signal, A =fc;

• Shift is of 90 degrees, output calibrated to zero– When input frequency decreases

• Phase shift between B and A increases, hence lower output

– When input frequency decreases• Phase shift reduces, thus higher output

PLL

• Used in Radios

• Available in chip form

– It 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 (instead of LC oscillator, crystallized VCO, all in chip form)

PLL- FM Demodulator

Capacitor store Voltage value and continue to give to vco

Block diagram of a PLL

Working

RC LPF

• No mod signal (Similar to Quadrature detector)– Phase shift of 90, VCO at fc (running frequency), no output

• As input frequency increases, phase difference decreases, error voltage increases and vice versa.

• The error voltage is the recovered signal.

• Capacitor hold the error signal so as to keep VCO at a frequency where the error is zero.