Transcript ANALOG COMMUNICATIONS

ANALOG
COMMUNICATIONS
EE721
1
MAIN TOPICS
Introduction to Communication Systems
Radio-Frequency Circuits
Amplitude Modulation
AM Receivers
AM Transmitters
Suppressed-Carrier AM Systems
 Test #1: 4th week;
Test #2: 7th week
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by H Chan, Mohawk College
Elements of a Communication System
Communication involves the transfer of
information or intelligence from a source to
a recipient via a channel or medium.
Basic block diagram of a communication
system:
Source
Transmitter
Receiver
Recipient
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Brief Description
Source: analogue or digital
Transmitter: transducer, amplifier,
modulator, oscillator, power amp., antenna
Channel: e.g. cable, optical fibre, free space
Receiver: antenna, amplifier, demodulator,
oscillator, power amplifier, transducer
Recipient: e.g. person, speaker, computer
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Modulation
Modulation is the process of impressing
information onto a high-frequency carrier
for transmission.
Reasons for modulation:
– to prevent mutual interference between stations
– to reduce the size of the antenna required
Types of modulation: AM, FM, and PM
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by H Chan, Mohawk College
Information and Bandwidth
Bandwidth required by a modulated signal
depends on the baseband frequency range
(or data rate) and the modulation scheme.
Hartley’s Law: I = k t B
where I = amount of information
k = a constant of the system
t = time available
B = channel bandwidth
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Frequency Bands
BAND
ELF
AF
VLF
LF
MF
HF
Hz
30 - 300
300 - 3 k
3 k - 30 k
30 k - 300 k
300 k - 3 M
3 M - 30 M
BAND
VHF
UHF
SHF
EHF
Hz
30M-300M
300M - 3 G
3 G - 30 G
30 G - 300G
•Wavelength, l = c/f
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by H Chan, Mohawk College
Types of Signal Distortion
Types of distortion in communications:
harmonic distortion
intermodulation distortion
nonlinear frequency response
nonlinear phase response
noise
interference
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Time and Frequency Domains
Time domain: an oscilloscope displays the
amplitude versus time
Frequency domain: a spectrum analyzer
displays the amplitude or power versus
frequency
Frequency-domain display provides
information on bandwidth and harmonic
components of a signal
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Non-sinusoidal Waveform
Any well-behaved periodic waveform can be
represented as a series of sine and/or cosine waves
plus (sometimes) a dc offset:
e(t)=Co+SAn cos nw t + SBn sin nw t (Fourier series)
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Effect of Filtering
Theoretically, a non-sinusoidal signal
would require an infinite bandwidth; but
practical considerations would band-limit
the signal.
Channels with too narrow a bandwidth
would remove a significant number of
frequency components, thus causing
distortions in the time-domain.
 A square-wave has only odd harmonics
by H Chan, Mohawk College
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External Noise
Equipment / Man-made Noise is generated
by any equipment that operates with
electricity
Atmospheric Noise is often caused by
lightning
Space Noise is strongest from the sun and,
at a much lesser degree, from other stars
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by H Chan, Mohawk College
Internal Noise
Thermal Noise is produced by the random
motion of electrons in a conductor due to
heat.
Noise power, PN = kTB
where T = absolute temperature in oK
k = Boltzmann’s constant, 1.38x10-23 J/K
B = noise power bandwidth in Hz
Noise voltage, VN  4kTBR
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Internal Noise (cont’d)
Shot Noise is due to random variations in
current flow in active devices.
Partition Noise occurs only in devices
where a single current separates into two or
more paths, e.g. bipolar transistor.
Excess Noise is believed to be caused by
variations in carrier density in components.
Transit-Time Noise occurs only at high f.
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by H Chan, Mohawk College
Noise Spectrum of Electronic Devices
Device
Noise
Transit-Time or
High-Frequency
Effect Noise
Excess or
Flicker Noise
Shot and Thermal Noises
1 kHz
fhc
f
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by H Chan, Mohawk College
Signal-to-Noise Ratio
An important measure in communications
is the signal-to-noise ratio (SNR or S/N). It
is often expressed in dB:
PS
VS
S
(dB)  10 log
 20 log
N
PN
VN
In FM receivers, SINAD = (S+N+D)/(N+D)
is usually used instead of SNR.
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by H Chan, Mohawk College
Noise Figure
Noise Figure is a figure of merit that
indicates how much a component, or a stage
degrades the SNR of a system:
NF = (S/N)i / (S/N)o
where (S/N)i = input SNR (not in dB)
and (S/N)o = output SNR (not in dB)
NF(dB)=10 log NF = (S/N)i (dB) - (S/N)o (dB)
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Equivalent Noise Temperature
and Cascaded Stages
The equivalent noise temperature is very
useful in microwave and satellite receivers.
Teq = (NF - 1)To
where To is a ref. temperature (often 290 oK)
When two or more stages are cascaded:
NF2  1 NF3  1
NFT  NF1 +
+
+ ...
A1
A1A 2
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by H Chan, Mohawk College
High-Frequency Effects
Stray reactances of components (including
the traces on a circuit board) can result in
parasitic oscillations / self resonance and
other unexpected effects in RF circuits.
Care must be given to the layout of
components, wiring, ground plane,
shielding and the use of bypassing or
decoupling circuits.
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Radio-Frequency Amplifiers
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Narrow-band RF Amplifiers
Many RF amplifiers use resonant circuits to limit
their bandwidth. This is to filter off noise and
interference and to increase the amplifier’s gain.
The resonant frequency (fo) , bandwidth (B), and
quality factor (Q), of a parallel resonant circuit
are:
fo
RL
fo 
; B ; Q
Q
XL
2 LC
1
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by H Chan, Mohawk College
Narrowband Amplifier (cont’d)
In the CE amplifier, both the input and
output sections are transformer-coupled to
reduce the Miller effect. They are tapped
for impedance matching purpose. RC and
C2 decouple the RF from the dc supply.
The CB amplifier is quite commonly used
at RF because it provides high input
impedance and also avoids the Miller effect.
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Wideband RF Amplifiers
Wideband / broadband amplifiers are
frequently used for amplifying baseband or
intermediate frequency (IF) signals.
The circuits are similar to those for
narrowband amplifiers except no tuning
circuits are employed.
Another method of designing wideband
amplifiers is by stagger-tuning.
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Stagger-Tuned IF Amplifiers
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Amplifier Classes
An amplifier is classified as:
Class A if it conducts current throughout
the full input cycle (i.e. 360o). It operates
linearly but is very inefficient - about 25%.
Class B if it conducts for half the input
cycle. It is quite efficient (about 60%) but
would create high distortions unless
operated in a push-pull configuration.
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Class B Push-Pull RF Amplifier
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Class C Amplifier
Class C amplifier operates for less than half
of the input cycle. It’s efficiency is about
75% because the active device is biased
beyond cutoff.
It is commonly used in RF circuits where a
resonant circuit must be placed at the output
in order to keep the sine wave going during
the non-conducting portion of the input
cycle.
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Class C Amplifier (cont’d)
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Frequency Multipliers
 One of the applications of class C amplifiers
is in “frequency multiplication”. The basic
block diagram of a frequency multiplier:
Input
fi
High
Distortion
Device +
Amplifier
Tuning
Filter
Circuit
Output
N x fi
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Principle of Frequency Multipliers
A class C amplifier is used as the high
distortion device. Its output is very rich in
harmonics.
A filter circuit at the output of the class C
amplifier is tuned to the second or higher
harmonic of the fundamental component.
Tuning to the 2nd harmonic doubles fi ;
tuning to the 3rd harmonic triples fi ; etc.
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Waveforms for Frequency Multipliers
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Neutralization
At very high frequencies, the junction
capacitance of a transistor could introduce
sufficient feedback from output to input to
cause unwanted oscillations to take place in
an amplifier.
Neutralization is used to cancel the
oscillations by feeding back a portion of the
output that has the opposite phase but same
amplitude as the unwanted feedback.
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Hazeltine Neutralization
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Rice Neutralization
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Transformer-Coupled Neutralization
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Inductive Neutralization
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Oscillators
Barkhausen criteria
for sustained
oscillations:
The closed-loop gain,
|BAV| = 1.
The loop phase shift =
0o or some integer
multiple of 360o at the
operating frequency.
Output
AV
B
AV = open-loop gain
B = feedback factor/fraction
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Hartley Oscillators
L1 + L2
B
L1
1
fo 
; LT  L1 + L2
2 LT C1
L2
B
L1
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Colpitts Oscillator
C1
1
C1C2
B
; fo 
; CT 
C2
C1 + C2
2 LCT
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Clapp Oscillator
C2
1
B
; fo 
C2 + C3
2 LCT
1
CT 
1
1
1
+
+
C2 C3 C4
The Clapp oscillator is a variation of the Colpitts circuit. C4 is
added in series with L in the tank circuit. C2 and C3 are chosen
large enough to “swamp” out the transistor’s junction capacitances
for greater stability. C4 is often chosen to be << either C2 or C3,
thus making C4 the frequency determining element, since CT = C4.
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Voltage-Controlled Oscillator
VCOs are widely used in electronic circuits
for AFC, PLL, frequency tuning, etc.
The basic principle is to vary the
capacitance of a varactor diode in a resonant
circuit by applying a reverse-biased voltage
across the diode whose capacitance is
approximately:
Co
CV 
1+ 2Vb
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Crystals
For high frequency stability in oscillators, a
crystal (such as quartz) has to be used.
Quartz is a piezoelectric material:
deforming it mechanically causes the crystal
to generate a voltage, and applying a
voltage to the crystal causes it to deform.
Externally, the crystal behaves like an
electrical resonant circuit.
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Packaging, symbol, and
characteristic of crystals
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Crystal-Controlled Oscillators
Pierce
Colpitts
by H Chan, Mohawk College
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Mixers
A mixer is a nonlinear circuit that combines
two signals in such a way as to produce the
sum and difference of the two input
frequencies at the output.
A square-law mixer is the simplest type of
mixer and is easily approximated by using a
diode, or a transistor (bipolar, JFET, or
MOSFET).
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by H Chan, Mohawk College
Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p frequencies.
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Balanced Mixers
A balanced mixer is one in which the input
frequencies do not appear at the output.
Ideally, the only frequencies that are
produced are the sum and difference of the
input frequencies.
f1
f1+ f2
Circuit symbol:
f2
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Equations for Balanced Mixer
Let the inputs be v1 = sin w1t and v2 = sin w2t.
A balanced mixer acts like a multiplier. Thus
its output, vo = Av1v2 = A sin w1t sin w2t.
Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]
Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t].
The last equation shows that the output of
the balanced mixer consists of the sum and
difference of the input frequencies.
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Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators.
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Phase-Locked Loop
The PLL is the basis of practically all
modern frequency synthesizer design.
The block diagram of a simple PLL:
fr
Phase
Detector
Vp
LPF
Loop
Amplifier
VCO
fo
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Operation of PLL
Initially, the PLL is unlocked, i.e.,the VCO
is at the free-running frequency, fo.
Since fo is probably not the same as the
reference frequency, fr , the phase detector
will generate an error/control voltage, Vp.
Vp is filtered, amplified, and applied to the
VCO to change its frequency so that fo = fr.
The PLL will then remain in phase lock.
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PLL Frequency Specifications
There is a limit on how far apart the free-running
VCO frequency and the reference frequency can be
for lock to be acquired or maintained.
Lock Range
Capture Range
Free-Running
Frequency
fLL
fLC
fo
fHC
fHL f
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PLL Frequency Synthesizer
For output frequencies in the VHF range and higher,
a prescaler is required. The prescaler is a fixed divider
placed ahead of the programmable divide by N counter.
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AM Waveform
ec = Ec sin wct
em = Em sin wmt
AM signal:
es = (Ec + em) sin wct
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Modulation Index
The amount of amplitude modulation in a
signal is given by its modulation index:
Em
Emax  Emin
m
or
Ec
Emax + Emin
where, Emax = Ec + Em; Emin = Ec - Em (all pk values)
When Em = Ec , m =1 or 100% modulation.
Over-modulation, i.e. Em>Ec , should be avoided
because it will create distortions and splatter.
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Effects of Modulation Index
m=1
m>1
In a practical AM system, it usually contains many
frequency components. When this is the case,
mT  m12 + m22 + ... + mn2
by H Chan, Mohawk College
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AM in Frequency Domain
The expression for the AM signal:
es = (Ec + em) sin wct
can be expanded to:
es = Ec sin wct + ½ mEc[cos (wc-wm)t-cos (wc+wm)t]
The expanded expression shows that the
AM signal consists of the original carrier, a
lower side frequency, flsf = fc - fm, and an
upper side frequency, fusf = fc + fm.
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AM Spectrum
Ec
mEc/2
mEc/2
fm
flsf
fm
fc
fusf
f
fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2
Bandwidth, B = 2fm
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AM Power
Total average (i.e. rms) power of the AM
signal is: PT = Pc + 2Psf , where
Pc = carrier power; and Psf = side-frequency
power
If the signal is across a load resistor, R,
then: Pc = Ec2/(2R); and Psf = m2Pc/4. So,
m2
PT  Pc (1 +
)
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AM Current
The modulation index for an AM station
can be measured by using an RF ammeter
and the following equation:
I  Io
m2
1+
2
where I is the current with modulation and
Io is the current without modulation.
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Complex AM Waveforms
For complex AM signals with many
frequency components, all the formulas
encountered before remain the same, except
that m is replaced by mT. For example:
2
mT
mT
PT  PC (1 +
); I  I o 1 +
2
2
2
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AM Receivers
Basic requirements for receivers:
ability to tune to a specific signal
amplify the signal that is picked up
extract the information by demodulation
amplify the demodulated signal
Two important receiver specifications:
sensitivity and selectivity
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Tuned-Radio-Frequency (TRF) Receiver
The TRF receiver is the simplest receiver
that meets all the basic requirements.
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Drawbacks of TRF Receivers
Difficulty in tuning all the stages to exactly
the same frequency simultaneously.
Very high Q for the tuning coils are required
for good selectivity  BW=fo/Q.
Selectivity is not constant for a wide range
of frequencies due to skin effect which
causes the BW to vary with fo.
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Superheterodyne Receiver
Block diagram of basic superhet receiver:
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Antenna and Front End
The antenna consists of an inductor in the
form of a large number of turns of wire
around a ferrite rod. The inductance forms
part of the input tuning circuit.
Low-cost receivers sometimes omit the RF
amplifier.
Main advantages of having RF amplifier:
improves sensitivity and image frequency
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rejection.
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Mixer and Local Oscillator
The mixer and LO frequency convert the
input frequency, fc, to a fixed fIF:
High-side injection: fLO = fc + fIF
by H Chan, Mohawk College
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Autodyne Converter
Sometimes called a self-excited mixer, the
autodyne converter combines the mixer and LO
into a single circuit:
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IF Amplifier, Detector, & AGC
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IF Amplifier and AGC
Most receivers have two or more IF stages
to provide the bulk of their gain (i.e.
sensitivity) and their selectivity.
Automatic gain control (AGC) is obtained
from the detector stage to adjusts the gain of
the IF (and sometimes the RF) stages
inversely to the input signal level. This
enables the receiver to cope with large
variations in input signal.
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Diode Detector Waveforms
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Diagonal Clipping Distortion
Diagonal clipping distortion is more pronounced at
high modulation index or high modulation frequency.74
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Sensitivity and Selectivity
Sensitivity is expressed as the minimum
input signal required to produce a specified
output level for a given (S+N)/N ratio.
Selectivity is the ability of the receiver to
reject unwanted or interfering signals. It
may be defined by the shape factor of the IF
filter or by the amount of adjacent channel
rejection.
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Shape Factor
B60dB
SF 
B6 dB
by H Chan, Mohawk College
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Image Frequency
One of the problems with the superhet
receiver is that an image frequency signal
could interfere with the reception of the
desired signal. The image frequency is
given by:
fimage = fsig + 2fIF
where fsig = desired signal.
An image signal must be rejected by tuning
circuits prior to mixing.
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Image Frequency Rejection
For a tuned circuit with a quality factor of
Q, then the image frequency rejection is:
IR  1 + Q x
2
x
In dB,
f image
f sig

2
where,
f sig
f image
IR (dB) = 20 log IR
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IF Transformers
The transformers used in the IF stages can
be either single-tuned or double-tuned.
Double-tuned
Single-tuned
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Loose and Tight Couplings
For single-tuned transformers, tighter
coupling means more gain but broader
bandwidth:
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Under, Over, & Critical Coupling
Double-tuned transformers can be over,
under, critically, or optimally coupled:
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Coupling Factors
Critical coupling factor kc is given by:
1
kc 
Q p Qs
where Qp, Qs = prim. & sec. Q, respectively.
IF transformers often use the optimum coupling
factor, kopt = 1.5kc , to obtain a steep skirt and
flat passband. The bandwidth for a double-tuned
IF amplifier with k = kopt is given by B = kfo.
Overcoupling means k>kc; undercoupling, k< kc 82
by H Chan, Mohawk College
Piezoelectric Filters
For narrow bandwidth (e.g. several kHz),
excellent shape factor and stability, a crystal
lattice is used as bandpass filter.
Ceramic filters, because of their lower Q,
are useful for wideband signals (e.g. FM
broadcast).
Surface-acoustic-wave (SAW) filters are
ideal for high frequency usage requiring a
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carefully shaped response.
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Block Diagram of AM TX
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Transmitter Stages
Crystal oscillator generates a very stable
sinewave carrier. Where variable frequency
operation is required, a frequency
synthesizer is used.
Buffer isolates the crystal oscillator from
any load changes in the modulator stage.
Frequency multiplier is required only if HF
or higher frequencies is required.
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Transmitter Stages (cont’d)
RF voltage amplifier boosts the voltage
level of the carrier. It could double as a
modulator if low-level modulation is used.
RF driver supplies input power to later RF
stages.
RF Power amplifier is where modulation is
applied for most high power AM TX. This
is known as high-level modulation.
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Transmitter Stages (cont’d)
High-level modulation is efficient since all
previous RF stages can be operated class C.
Microphone is where the modulating signal
is being applied.
AF amplifier boosts the weak input
modulating signal.
AF driver and power amplifier would not
be required for low-level modulation.
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AM Modulator Circuits
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Impedance Matching Networks
Impedance matching networks at the output
of RF circuits are necessary for efficient
transfer of power. At the same time, they
serve as low-pass filters.
Pi network
T network
by H Chan, Mohawk College
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Trapezoidal Pattern
Instead of using the envelope display to
look at AM signals, an alternative is to use
the trapezoidal pattern display. This is
obtained by connecting the modulating
signal to the x input of the ‘scope and the
modulated AM signal to the y input.
Any distortion, overmodulation, or nonlinearity is easier to observe with this
method.
by H Chan, Mohawk College
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Trapezoidal Pattern (cont’d)
m<1
m=1
m>1
Improper
-Vp>+Vp
phase
Vmax  Vmin
m
Vmax + Vmin
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Suppressed-Carrier AM Systems
Full-carrier AM is simple but not efficient
in terms of transmitted power, bandwidth,
and SNR.
Using single-sideband suppressed-carrier
(SSBSC or SSB) signals, since Psf = m2Pc/4,
and Pt=Pc(1+m2/2 ), then at m=1, Pt= 6 Psf .
SSB also has a bandwidth reduction of half,
which in turn reduces noise by half.
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Generating SSB - Filtering Method
The simplest method of generating an SSB
signal is to generate a double-sideband
suppressed-carrier (DSB-SC) signal first
and then removing one of the sidebands.
Balanced
Modulator DSB-SC
USB
BPF
AF
Input
Carrier
Oscillator
by H Chan, Mohawk College
or
LSB
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Waveforms for Balanced Modulator
V2, fm
Vo
V1, fc
fc-fm fc+fm
f
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by H Chan, Mohawk College
LIC Balanced Modulator 1496
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Filter for SSB
Filters with high Q are needed for
suppressing the unwanted sideband.
fa = f c - f2
fb = fc - f1
fd = fc + f1
fe = f c + f 2
f c anti log( X dB / 20) where X = attenuation of
Q
4f
sideband, and f = fd - fb
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Typical SSB TX using Filter Method
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SSB Waveform
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Generating SSB - Phasing Method
This method is based on the fact that the lsf and
the usf are given by the equations:
cos {(wc - wm)t} = ½(cos wct cos wmt + sin wct sin wmt)
cos {(wc + wm)t} = ½(cos wct cos wmt - sin wct sin wmt)
The RHS of the 1st equation is just the sum of two
products: the product of the carrier and the
modulating signal, and the product of the same
two signals that have been phase shifted by 90o.
The 2nd equation is similar except for the (-) sign.
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Diagram for Phasing Method
Modulating
signal
Em cos wmt
Balanced Modulator 1
Carrier
oscillator
90o phase
shifter
Ec cos wct
90o phase
shifter
+
SSB
output
Balanced Modulator 2
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by H Chan, Mohawk College
Phasing vs Filtering Method
Advantages of phasing method :
No high Q filters are required.
Therefore, lower fm can be used.
SSB at any carrier frequency can be
generated in a single step.
Disadvantage:
Difficult to achieve accurate 90o phase shift
across the whole audio range.
101
by H Chan, Mohawk College
Peak Envelope Power
SSB transmitters are usually rated by the
peak envelope power (PEP) rather than the
carrier power. With voice modulation, the
PEP is about 3 to 4 times the average or rms
2
power.
V
PEP 
p
2 RL
where Vp = peak signal voltage
and RL = load resistance
102
by H Chan, Mohawk College
Block Diagram of SSB RX
103
by H Chan, Mohawk College
SSB Receiver (cont’d)
The input SSB signal is first mixed with the
LO signal (low-side injection is used here).
The filter removes the sum frequency
components and the IF signal is amplified.
Mixing the IF signal with a reinserted
carrier from a beat frequency oscillator
(BFO) and low-pass filtering recovers the
audio information.
104
by H Chan, Mohawk College
SSB RX (cont’d)
The product detector is often just a
balanced modulator operated in reverse.
Frequency accuracy and stability of the
BFO is critical. An error of a little more
than 100 Hz could render the received
signal unintelligible.
In coherent or synchronous detection, a
pilot carrier is transmitted with the SSB
signal to synchronize the BFO.
by H Chan, Mohawk College
105