Download Chapter Two: Radio - Frequency Circuits

Chapter Two:
Radio-Frequency Circuits
Introduction
• There is a need to modulate a signal using an information
signal
• This signal is referred to as a baseband signal
• The carrier needs to be a higher frequency than the baseband
• RF Amplifiers, Oscillators, Mixers, and frequency
synthesizers are used to meet these conditions
High-Frequency Effects
• At very low frequencies, capacitors and other components
behave in very straightforward ways
• A capacitor is considered an open circuit to DC voltages
and a short circuit for AC at low frequencies
• As frequencies become higher, component interaction
becomes more critical both directly and as “stray”
reactances, inductances, and capacitances
Effect of Frequency on Device
Characteristics
• All electronic devices have
capacitances and inductances
• As frequency increases, so
does inductive reactance
• As frequency increases,
capacitive reactance
decreases
• At some point, the two
reactances will be equal and
the circuit will self-resonate
Lumped & Distributed Constants
• At low frequencies, the capacitances and inductances found
between the traces on a printed circuit board are negligible
• As frequency increases, the stray capacitances and
inductances are considered as distributed along the length of
the pc board
• They are said to be distributed constants
High-Frequency
Construction Techniques
• Circuits are designed to reduce the “stray” capacitances
and inductances resulting from the wiring and circuit board
• Traces and wires are kept short and well separated
• Conductors and inductors in close proximity are kept at
right angles
• Toroidal cores for inductors are used to reduce stray
magnetic fields
• Shielding is used
• A gimmick is used in some circuits
Radio-Frequency Amplifiers
• RF amplifiers differ from audio amplifiers in that wide
bandwidth may or may not be required
• Linearity of the output may or may not be required
• Efficiency can be improved through the use of Class C
amplifiers
Narrowband Amplifiers
• Many RF amplifiers are
required to operate only
within a narrow range of
frequencies
• Filters are used to reduce
the bandwidth
• The tuned amplifier is set
according to the formula:
1
fo 
2π L1C1
Miller Effect
• Inter-electrode capacitance and inductance is a problem
in RF circuits
• This problem is especially severe for the collector-base
capacitance in a common-emitter amplifier
• The multiplication of the effect of capacitance in this
configuration is called the Miller Effect
Common-Base Amplifier
• One solution to the
Miller Effect is to use a
common-base amplifier
configuration as shown
at the right
Wideband Amplifiers
• Baseband parts of RF systems may make use of wideband
amplifiers
• Wideband amplifiers typically use transformer coupling
• Typical wideband amplifiers need negative feedback to
compensate for higher low-frequency gain: as frequency
increases, negative feedback decreases
Amplifier Classes
• Amplifiers are classified according to the portion of the
input cycle the active device conducts current
• This is referred to as the conduction angle and is
expressed in degrees
• Single-ended audio amps are operated in Class A where
the device conducts for 360°
• Push-pull amps can be a Class B if one of the two
devices is conducting at all times
• Most audio power amps operate in Class AB - a
compromise between Class A and Class B
Class B RF Amplifier
• A simple Class B amplifier
is shown at the right
• It uses transformer coupling
• Both transistors are biased
near cutoff
Class C Amplifiers
• Class C amplifiers
conduct for less than 180°
of the input cycle
• Class C amplifiers can be
single-ended or push-pull
• Class C amplifiers are
very efficient in RF
applications but inherently
induce severe distortion
Neutralization
• Transistors or tubes may
introduce sufficient feedback
to cause the circuit to
oscillate and become unstable
• Neutralization can cancel
this type of feedback by
feeding back a portion of the
output signal to the input in
such a way that it has the
same amplitude as the
unwanted signal but the
opposite phase
Frequency Multipliers
• Sometimes it is useful to use harmonic operation to
generate a frequency higher than is conveniently
generated by using a frequency multiplier
Radio-Frequency Oscillators
• RF oscillators do not differ in principle than other
oscillators but practical circuits are quite different
• Any amplifier can be made to oscillate if a portion of the
output signal is fed back to the input
• The Barkhausen criteria establishes the requirements for a
circuit to oscillate
LC Oscillators
• Practical RF circuits whose frequency is
controlled by a resonant LC circuit are:
– Hartley Oscillator
– Colpitts Oscillator
– Clapp Oscillator
Hartley Oscillator
• Common configurations for a Hartley Oscillator
Colpitts Oscillator
• Common configurations
for a Colpitts Oscillator
Clapp Oscillator
• Common
configuration for a
Clapp Oscillator
Varactor-Tuned Oscillator
• The frequency of an oscillator may be tuned by varying
the inductance or capacitance of the circuit
• Varactors are more convenient substitutes than variable
capacitors in many circumstances
Crystal-Controlled Oscillators
• Crystal-controlled oscillators are more stable than LC oscillators
• Crystal oscillators utilize the piezoelectric effect to generate a
frequency-variable signal
Mixers
• Mixers are nonlinear circuits that combine two signals to
produce the sum and difference of of the two input frequencies
Types of Mixers
• Square-law mixers: output is derived by the formula:
vo  Avi  Bvi  Cvi
2
3
• Diode Mixers use a diode operated in the forward bias mode
• Transistor Mixers use bipolar and FET transistors
• Balanced Mixers are mixers where the input frequencies do
not appear at the output
Frequency Synthesizers
• Conventional LC oscillators tend to be unstable because of:
–
–
–
–
Vibration
Temperature changes
Voltage changes
Component aging
• Crystal oscillators are more stable but are are limited to a
narrow range of operating frequencies
• Frequency Synthesizers overcome these limitations and
may end up being more cost effective
Phase-Locked Loops
• The phase-locked loop is the basis of nearly all modern
synthesizer designs
• The loop consists of a:
–
–
–
–
Phase detector
Voltage-controlled oscillator (VCO)
Low-pass filter
The purpose of the PLL is lock the VCO to the reference signal
Simple Frequency Synthesizer
• In addition to the phase detector, VCO, and filter, a
programmable divider is necessary for frequency
synthesis using a PLL as shown below
Prescaling
• Because programmable dividers are unavailable at
frequencies above 100MHz, fixed- and two-modulus
prescalers are used
• Two-modulus prescalers can be programmed to divide by
two consecutive integers