Transcript switching amplifier

Chapter 7
Radio Transmitters
Topics Covered in Chapter 7
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7-1: Transmitter Fundamentals
7-2: Carrier Generators
7-3: Power Amplifiers
7-4: Impedance-Matching Networks
7-5: Typical Transmitter Circuits
7-1: Transmitter Fundamentals
• A radio transmitter takes the information to be
communicated and converts it into an electronic
signal compatible with the communication
medium.
• This process involves carrier generation,
modulation, and power amplification.
• The signal is fed by wire, coaxial cable, or
waveguide to an antenna that launches it into
free space.
• Typical transmitter circuits include oscillators,
amplifiers, frequency multipliers, and impedance
matching networks.
7-1: Transmitter Fundamentals
• The transmitter is the electronic unit that
accepts the information signal to be
transmitted and converts it into an RF signal
capable of being transmitted over long
distances.
7-1: Transmitter Fundamentals
Every transmitter has four basic requirements:
1. It must generate a carrier signal of the correct
frequency at a desired point in the spectrum.
2. It must provide some form of modulation that causes
the information signal to modify the carrier signal.
3. It must provide sufficient power amplification to
ensure that the signal level is high enough to carry
over the desired distance.
4. It must provide circuits that match the impedance of
the power amplifier to that of the antenna for
maximum transfer of power.
8-1: Transmitter Fundamentals
Transmitter Configurations
– The simplest transmitter is a single-transistor
oscillator connected to an antenna.
– This form of transmitter can generate continuous
wave (CW) transmissions.
– The oscillator generates a carrier and can be switched
off and on by a telegraph key to produce the dots and
dashes of the International Morse code.
– CW is rarely used today as the oscillator power is too
low and the Morse code is nearly extinct.
7-1: Transmitter Fundamentals
Figure 8-1: A more powerful CW transmitter.
7-1: Transmitter Fundamentals
Transmitter Types
– High-Level Amplitude Modulated (AM)
Transmitter
1.
2.
3.
4.
Oscillator generates the carrier frequency.
Carrier signal fed to buffer amplifier.
Signal then fed to driver amplifier.
Signal then fed to final amplifier.
7-1: Transmitter Fundamentals
– Low-Level Frequency Modulated (FM) Transmitter
1.
2.
3.
4.
5.
6.
Crystal oscillator generates the carrier signal.
Signal fed to buffer amplifier.
Applied to phase modulator.
Signal fed to frequency multiplier(s).
Signal fed to driver amplifier.
Signal fed to final amplifier.
7-1: Transmitter Fundamentals
– Single-Sideband (SSB) Transmitter
1.
2.
3.
4.
Oscillator generates the carrier.
Carrier is fed to buffer amplifier.
Signal is applied to balanced modulator.
DSB signal fed to sideband filter to select upper or
lower sideband.
5. SSB signal sent to mixer circuit.
6. Final carrier frequency fed to linear driver and power
amplifiers.
7-2: Carrier Generators
• The starting point for all transmitters is carrier
generation.
• Once generated, the carrier can be modulated,
processed in various ways, amplified, and
transmitted.
• The source of most carriers is a crystal oscillator.
• PLL frequency synthesizers are used in
applications requiring multiple channels of
operation.
7-2: Carrier Generators
Crystal Oscillators
– The only oscillator capable of maintaining the
frequency precision and stability demanded by
the FCC is a crystal oscillator.
– A crystal is a piece of quartz that can be made to
vibrate and act like an LC tuned circuit.
– Overtone crystals and frequency multipliers are
two devices that can be used to achieve crystal
precision and stability at frequencies greater than
30 MHz.
7-2: Carrier Generators
Crystal Oscillators
– The Colpitts-type crystal oscillator is the most
commonly used crystal oscillator.
– Feedback is derived from a capacitive voltage divider.
– Transistor configuration is typically an emitterfollower.
– The output is taken from the emitter.
7-2: Carrier Generators
Figure 7-6: An emitter-follower crystal oscillator
7-2: Carrier Generators
Crystal Oscillators
– Pulling, or rubbering capacitors are used to make fine
adjustments to the crystal oscillator frequency.
– Field-effect transistors (FETs) make good crystal
oscillators. The Pierce oscillator is a common
configuration that uses a FET.
– An overtone crystal is cut so that it optimizes its
oscillation at an overtone of the basic crystal
frequency.
– The term harmonic is often used as a synonym for
overtone.
7-2: Carrier Generators
Crystal Switching
– If a transmitter must operate on more than one
frequency, but crystal precision and stability are
required, multiple crystals can be used and the
desired one switched on.
– Mechanical rotary switches and diode switches
are often used in this kind of application.
– Diode switching is fast and reliable.
7-2: Carrier Generators
Figure 7-9: Using diodes to switch crystals.
7-2: Carrier Generators
Frequency Synthesizers
– Frequency synthesizers are variable-frequency
generators that provide the frequency stability of
crystal oscillators but the convenience of incremental
tuning over a broad frequency range.
– Frequency synthesizers provide an output that varies
in fixed frequency increments over a wide range.
– In a transmitter, a frequency synthesizer provides
basic carrier generation.
– Frequency synthesizers are used in receivers as local
oscillators and perform the receiver tuning function.
7-2: Carrier Generators
Phase-Locked Loop Synthesizer
– The phase-locked loop (PLL) consists of a phase
detector, a low-pass filter, and a VCO.
– The input to the phase detector is a reference
oscillator.
– The reference oscillator is normally crystalcontrolled to provide high-frequency stability.
– The frequency of the reference oscillator sets the
increments in which the frequency may be
changed.
7-2: Carrier Generators
Figure 8-10: Basic PLL frequency synthesizer.
7-2: Carrier Generators
Direct Digital Synthesis
– A direct digital synthesis (DDS) synthesizer
generates a sine-wave output digitally.
– The output frequency can be varied in increments
depending upon a binary value supplied to the
unit by a counter, a register, or an embedded
microcontroller.
7-2: Carrier Generators
Direct Digital Synthesis
– A read-only memory (ROM) is programmed with the
binary representation of a sine wave.
– These are the values that would be generated by an
analog-to-digital (A/D) converter if an analog sine
wave were digitized and stored in the memory.
– If these binary values are fed to a digital-to-analog
(D/A) converter, the output of the D/A converter will
be a stepped approximation of the sine wave.
– A low-pass filter (LPF) is used to remove the highfrequency content smoothing the sine wave output.
7-2: Carrier Generators
Figure 7-15: Basic concept of a DDS frequency source
7-2: Carrier Generators
Direct Digital Synthesis
– DDS synthesizers offer some advantages over PLL
synthesizers:
• The frequency can be controlled in very fine
increments.
• The frequency of a DDS synthesizer can be changed
much faster than that of the PLL.
– However, a DDS synthesizer is limited in its output
frequencies.
7-3: Power Amplifiers
• The three basic types of power amplifiers used
in transmitters are:
– Linear
– Class C
– Switching
7-3: Power Amplifiers
Linear Amplifiers
– Linear amplifiers provide an output signal that is
an identical, enlarged replica of the input.
– Their output is directly proportional to their input
and they faithfully reproduce an input, but at a
higher level.
– Most audio amplifiers are linear.
– Linear RF amplifiers are used to increase the
power level of variable-amplitude RF signals such
as low-level AM or SSB signals.
7-3: Power Amplifiers
• Linear amplifiers are class A, AB or B.
• The class of an amplifier indicates how it is
biased.
– Class A amplifiers are biased so that they conduct
continuously. The output is an amplified linear
reproduction of the input.
– Class B amplifiers are biased at cutoff so that no collector
current flows with zero input. Only one-half of the sine
wave is amplified.
– Class AB linear amplifiers are biased near cutoff with some
continuous current flow. They are used primarily in pushpull amplifiers and provide better linearity than Class B
amplifiers, but with less efficiency.
7-3: Power Amplifiers
• Class C amplifiers conduct for less than onehalf of the sine wave input cycle, making them
very efficient.
– The resulting highly distorted current pulse is used
to ring a tuned circuit to create a continuous sinewave output.
– Class C amplifiers cannot be used to amplify
varying-amplitude signals.
– This type amplifier makes a good frequency
multiplier as harmonics are generated in the
process.
8-3: Power Amplifiers
• Switching amplifiers act like on/off or digital
switches.
– They effectively generate a square-wave output.
– Harmonics generated are filtered out by using
high-Q tuned circuits.
– The on/off switching action is highly efficient.
– Switching amplifiers are designated class D, E, F,
and S.
8-3: Power Amplifiers
Linear Amplifiers
– Class A Buffers
• A class A buffer amplifier is used between the carrier
oscillator and the final power amplifier to isolate the
oscillator from the power amplifier load, which can
change the oscillator frequency.
8-3: Power Amplifiers
Figure 8-21: A linear (class A) RF buffer amplifier
7-3: Power Amplifiers
Linear Amplifiers
– Class B Push-Pull Amplifier
• In a class B push-pull amplifier, the RF driving signal is
applied to two transistors through an input transformer.
• The transformer provides impedance-matching and
base drive signals to the two transistors that are 180°
out of phase.
• An output transformer couples the power to the
antenna or load.
7-3: Power Amplifiers
Figure 7-23: A push-pull class B power amplifier
7-3: Power Amplifiers
Class C Amplifiers
– The key circuit in most AM and FM transmitters is
the class C amplifier.
• These amplifiers are used for power amplification in
the form of drivers, frequency multipliers, and final
amplifiers.
• Class C amplifiers are biased so they conduct for less
than 180° of the input.
• Current flows through a class C amplifier in short
pulses, and a resonant tuned circuit is used for
complete signal amplification.
7-3: Power Amplifiers
Tuned Output Circuits
– All class C amplifiers have some form of tuned circuit
connected in the collector.
– The primary purpose of a tuned circuit is to form the
complete AC sine-wave output.
– A parallel tuned circuit rings, or oscillates, at its
resonant frequency whenever it receives a DC pulse.
7-3: Power Amplifiers
Tuned Output Circuits
– The pulse charges a capacitor, which then discharges
into an inductor.
– The exchange of energy between the inductor and the
capacitor is called the flywheel effect and produces a
damped sine wave at the resonant frequency.
8-3: Power Amplifiers
Figure 8-27: Class C amplifier operation
8-3: Power Amplifiers
• Any class C amplifier is capable of performing
frequency multiplication if the tuned circuit in
the collector resonates at some integer
multiple of the input frequency.
8-3: Power Amplifiers
Neutralization
– Self-oscillation exists when some of the output
voltage finds its way back to the input of the
amplifier with the correct amplitude and phase,
and the amplifier oscillates.
– When an amplifier circuit oscillates at a higher
frequency unrelated to the tuned frequency, the
oscillation is referred to as parasitic oscillation.
7-3: Power Amplifiers
Neutralization
– Neutralization is a process in which a signal equal
in amplitude and 180° out of phase with the
signal, is fed back.
– The result is that the two signals cancel each other
out.
7-3: Power Amplifiers
Switching Power Amplifiers
– A switching amplifier is a transistor that is used as
a switch and is either conducting or
nonconducting.
• A class D amplifier uses a pair of transistors to produce
a square-wave current in a tuned circuit.
• In a class E amplifier, only a single transistor is used.
This amplifier uses a low-pass filter and tuned
impedance-matching circuit to achieve a high level of
efficiency.
7-3: Power Amplifiers
Switching Power Amplifiers
– A class F amplifier is a variation of the E amplifier.
• It contains an additional resonant network which
results in a steeper square waveform.
• This waveform produces faster transistor switching
and better efficiency.
– Class S amplifiers are found primarily in audio
applications but have also been used in low- and
medium-frequency RF amplifiers.
7-3: Power Amplifiers
Linear Broadband Power Amplifiers
– Newer wireless systems require broader
bandwidth than the previously mentioned
amplifiers can accommodate.
– Two common methods of broad-bandwidth
amplification are:
• Feedforward amplification
• Adaptive predistortion amplification
7-3: Power Amplifiers
Linear Broadband Power Amplifiers
– Feedforward Amplification
• With this technique, the distortion produced by the
power amplifier is isolated and subtracted from the
amplified signal, producing a nearly distortion-free
output signal.
• The system is inefficient because two power amplifiers
are required.
• The tradeoff is wide bandwidth and very low distortion.
7-3: Power Amplifiers
Figure 7-34: Feedforward linear power amplifier.
7-3: Power Amplifiers
Linear Broadband Power Amplifiers
– Adaptive Predistortion Amplification
• This method uses digital signal processing (DSP) to
predistort the signal in a way that when amplified, the
amplifier distortion will offset the predistortion
characteristics.
• The result is a a distortion-free output signal.
• The method is complex, but is more efficient than the
feedforward method because only one power amplifier
is needed.
7-3: Power Amplifiers
Figure 7-35: Concept of adaptive predistortion amplification.
7-4: Impedance-Matching Networks
• Matching networks that connect one stage to
another are very important parts of any
transmitter.
• The circuits used to connect one stage to another
are known as impedance-matching networks.
• Typical networks are LC circuits, transformers, or
some combination.
7-4: Impedance-Matching Networks
• The main function of a matching network is to
provide for an optimum transfer of power
through impedance matching techniques.
• Matching networks also provide filtering and
selectivity.
7-4: Impedance-Matching Networks
Figure 8-36: Impedance Matching in RF Circuits
7-4: Impedance-Matching Networks
Networks
– There are three basic types of LC impedancematching networks. They are:
• L network
• T network
• π network
7-4: Impedance-Matching Networks
• L networks consist of an inductor and a
capacitor in various L-shaped configurations.
– They are used as low- and high-pass networks.
– Low-pass networks are preferred because
harmonic frequencies are filtered out.
– The L-matching network is designed so that the
load impedance is matched to the source
impedance.
7-4: Impedance-Matching Networks
Figure 7-37a: L-type impedance-matching network in which ZL < Zi.
7-4: Impedance-Matching Networks
T and π Networks
– To get better control of the Q, or selectivity of a
circuit, matching networks using three reactive
elements can be used.
• A π network is designed by using reactive elements in a
configuration that resembles the Greek letter π
• A T network is designed by using reactive elements in a
configuration that resembles the letter T.
7-4: Impedance-Matching Networks
Figure 7-40(a): π network.
7-4: Impedance-Matching Networks
Figure 7-40(b): T network.
7-4: Impedance-Matching Networks
Transformers and Baluns
– One of the best impedance-matching components
is the transformer.
• Iron-core transformers are widely used at lower
frequencies to match impedances.
• Any load impedance can be made to look like the
desired load impedance by selecting the correct value
of transformer turns ratio.
• A transformer used to connect a balanced source to an
unbalanced load or vice versa, is called a balun
(balanced-unbalanced).
7-4: Impedance-Matching Networks
Transformers and Baluns
– Although air-core transformers are used widely at RFs,
they are less efficient than iron-core transformers.
– The most widely used type of core for RF transformers
is the toroid.
• A toroid is a circular, doughnut-shaped core, usually made of
a special type of powdered iron.
– Single-winding tapped coils called autotransformers
are also used for impedance matching between RF
stages.
7-4: Impedance-Matching Networks
Transformers and Baluns
– Toroid transformers cause the magnetic field
produced by the primary to be completely
contained within the core itself.
– This has two important advantages:
• A toroid does not radiate RF energy.
• Most of the magnetic field produced by the primary
cuts the turns of the secondary winding.
– Thus, the basic turns ratio, input-output voltage, and
impedance formulas for low-frequency transformers apply to
high-frequency toroid transformers.
7-4: Impedance-Matching Networks
Figure 7 - 43: A toroid transformer.
7-4: Impedance-Matching Networks
Transmission Line Transformers and Baluns
– A transmission line or broadband transformer is a
unique type of transformer widely used in power
amplifiers for coupling between stages and
impedance matching.
– It is usually constructed by winding two parallel
wires (or a twisted pair) on a toroid.
7-4: Impedance-Matching Networks
Figure 7-46: A transmission line transformer.
7-5: Typical Transmitter Circuits
• Many transmitters used in recent equipment
designs are a combination of ICs and discrete
component circuits. Two examples are:
– Low-Power FM Transmitter
– Short-Range Wireless Transmitter
7-5: Typical Transmitter Circuits
Low-Power FM Transmitter
– A typical circuit might be made up of:
•
•
•
•
A transmitter chip
Power amplifier
IC voltage regulator
Voltage source.
7-5: Typical Transmitter Circuits
Low-Power FM Transmitter
– The heart of the circuit is the transmitter chip.
– It contains a microphone amplifier with clipping
diodes; an RF oscillator, which is usually crystalcontrolled with an external crystal; and a buffer
amplifier.
– Frequency modulation is produced by a variable
reactance circuit connected to the oscillator.
– It also contains two free transistors that can be
connected with external components as buffer
amplifiers or as multipliers and low-level power
amplifiers.
– This chip is useful up to about 60 to 70 MHz, and is
widely used in cordless telephones.
7-5: Typical Transmitter Circuits
Figure7-51: Freescale MC 2833 IC FM VHF transmitter chip.
7-5: Typical Transmitter Circuits
Figure 7-50: Schematic of sections of the E-Comm transceiver.
7-5: Typical Transmitter Circuits
Short-Range Wireless Transmitter
– There are many short-range wireless applications
that require a transmitter to send data or control
signals to a nearby receiver.
• Examples include:
–
–
–
–
Remote keyless entry (RKE) devices used to open car doors
Tire pressure sensors
Remote-control lights and ceiling fans
Garage door openers
7-5: Typical Transmitter Circuits
Short-Range Wireless Transmitter
– Such transmitters are unlicensed, use very low
power, and operate in the FCC’s industrialscientific-medical (ISM) bands.
– A typical transmitter circuit might be composed
of:
• PLL used as a frequency multiplier
• Output power amplifier
7-5: Typical Transmitter Circuits
Figure 7-52: The Freescale MC 33493D UHF ISM transmitter IC.