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FM Circuits Tomasi

This document summarizes frequency modulation (FM) circuits and components. It discusses frequency modulators that vary the carrier frequency according to the modulating signal using either an LC or crystal oscillator circuit. It also describes varactor diode operations and how they can be used for frequency modulation. Phase modulators are also covered, including how a varactor can be used for phase shifting. Finally, it reviews common frequency demodulation techniques like slope detectors, pulse-averaging discriminators, and quadrature detectors. Phase-locked loops are also described as a frequency demodulation method.

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Joyce Ann Martes
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188 views25 pages

FM Circuits Tomasi

This document summarizes frequency modulation (FM) circuits and components. It discusses frequency modulators that vary the carrier frequency according to the modulating signal using either an LC or crystal oscillator circuit. It also describes varactor diode operations and how they can be used for frequency modulation. Phase modulators are also covered, including how a varactor can be used for phase shifting. Finally, it reviews common frequency demodulation techniques like slope detectors, pulse-averaging discriminators, and quadrature detectors. Phase-locked loops are also described as a frequency demodulation method.

Uploaded by

Joyce Ann Martes
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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FM CIRCUITS

BY: Maria Victoria C. Padilla, ECE

TUP-COE-ECE 1 st SEM, AY 2017-18


I. FREQUENCY
MODULATORS
- A circuit that varies carrier frequency in
accordance with the modulating signal. The carrier
is generated by either LC or a crystal oscillator
circuit.

• Varactor Operations
• Varactor Modulators
• Frequency-Modulating a Crystal Oscillator
• Voltage-Controlled Oscillator
Varactor Operations

Schematic symbols of a Capacitance versus reverse junction voltage for


varactor diode. a typical varactor.
Varactor Modulators
A direct-frequency-modulated carrier oscillator using a varactor
diode.
Example 1:
• The value of capacitance of a varactor at the center
of its linear range is 40 pF. This varactor will be in
parallel with a fixed 20-pF capacitor. What value
of inductance should be used to resonate this
combination to 5.5 MHz in an oscillator? Total
capacitance CT = 40 + 20 = 60 pF.
Frequency-Modulating a
Crystal Oscillator

Frequency modulation of a crystal oscillator with a VCO.


Frequency-Modulating a
Crystal Oscillator
How frequency multipliers increase carrier
frequency and deviation.
Voltage-Controlled
Oscillator

A 10-GHz SiGe integrated VCO.


CMOS VCO

A CMOS VCO for a 2.4-GHz FSK.


IC VCO

Frequency modulation with an IC VCO. (a) Block diagram with an


IC VCO. (b) Basic frequency modulator using the NE566 VCO
II. PHASE MODULATORS
• Phase shift

• Phase angle

RC phase-shifter basics.
Varactor Phase Modulator
Example 2:
• A transmitter must operate at a frequency of
168.96 MHz with a deviation of 65 kHz. It uses
three frequency multipliers––a doubler, a tripler,
and a quadrupler. Phase modulation is used.
Calculate (a) the frequency of the carrier crystal
oscillator and (b) the phase shift ¢ϕ required to
produce the necessary deviation at a 2.8-kHz
modulation frequency.
• A.
fd = 5 5 kHz/24 5 6208.33 Hz

• B.
Example 3:
• For the transmitter in Example 6-2, a phase
shifter like that in Fig. 6-10 is used, where C is a
varactor and R 5 1 kV. Assume that the total
phase-shift range is centered on 45°. Calculate the
two capacitance values required to achieve the
total deviation. The phase range is centered on
45°, or 45°± 4.263° = 40.737°and 49.263°. The total
phase range is 49.263 - 40.737 = 8.526°. If ϕ = 5
tan^-1 (R/XC), then tan ϕ = R/XC.
III. FREQUENCY
DEMODULATORS
• Slope Detectors
• Pulse-Averaging Discrimators
• Quadrature Detectors
• Phase-Locked Loop
Slope Detectors

Slope detector operation.


Pulse-Averaging
Discrimanators
Pulse-Averaging
Discrimanators

(a) FM input. (b) Output of zero-crossing detector. (c) Output of one


shot. (d) Output of discriminator (original modulating signal).
Quadrature Detectors
Phase-Locked Loop
• 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. All
phase-locked loops have the three basic elements,
shown in next slide
• 1. A phase detector is used to compare the FM
input, sometimes referred to as the reference
signal, to the output of a VCO.
• 2. The VCO frequency is varied by the dc output
voltage from a low-pass filter.
• 3. The low-pass filter smoothes the output of the
phase detector into a control voltage that varies
the frequency of the VCO..
Phase-Locked Loop

Phase-Locked Loop
Phase-Locked Loop
• free-running frequency, its normal operating
frequency as determined by internal frequency-
determining components

Capture and lock ranges of a PLL.


A PLL FM demodulator
using the 565 IC
END

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