1 analog communications ee721 by h chan, mohawk college 2 main topics ÊiÊintroduction to...

ANALOG COMMUNICATIONS

by H Chan, Mohawk College

MAIN TOPICS

Introduction to Communication SystemsRadio-Frequency CircuitsAmplitude ModulationAM ReceiversAM TransmittersSuppressed-Carrier AM Systems Test #1: 4th week; Test #2: 7th week

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

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

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

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

Frequency Bands

BAND Hz ELF 30 - 300 AF 300 - 3 k VLF 3 k - 30 k LF 30 k - 300 k MF 300 k - 3 M HF 3 M - 30 M

BAND Hz VHF 30M-300M UHF 300M - 3 G SHF 3 G - 30 G EHF 30 G - 300G

•Wavelength, = c/f

Types of Signal Distortion

Types of distortion in communications:harmonic distortion intermodulation distortionnonlinear frequency responsenonlinear phase responsenoise interference

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

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+An cosnt Bn sin n t (Fourier series)

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

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

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, kTBR4VN

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.

Noise Spectrum of Electronic DevicesDeviceNoise

Shot and Thermal Noises

Excess orFlicker Noise

Transit-Time orHigh-FrequencyEffect Noise

1 kHz fhc

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:

Vlog20

P log 10 dB)(

In FM receivers, SINAD = (S+N+D)/(N+D)is usually used instead of SNR.

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)

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:

1NFNFNF

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.

Radio-Frequency Amplifiers

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:

LCf ;;

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.

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.

Stagger-Tuned IF Amplifiers

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.

Class B Push-Pull RF Amplifier

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.

Class C Amplifier (cont’d)

Frequency Multipliers

One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier:

High DistortionDevice +Amplifier

TuningFilter

Circuit

Inputfi

Output

N x fi

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.

Waveforms for Frequency Multipliers

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.

Hazeltine Neutralization

Rice Neutralization

Transformer-Coupled Neutralization

Inductive Neutralization

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. AV = open-loop gain

B = feedback factor/fraction

Output

Hartley Oscillators

Colpitts Oscillator

Clapp Oscillator

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.

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:

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.

Packaging, symbol, and characteristic of crystals

Crystal-Controlled Oscillators

Pierce Colpitts

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).

Dual-Gate MOSFET Mixer

Good dynamic range and fewer unwanted o/p frequencies.

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.

Circuit symbol:f1

f1+ f2

Equations for Balanced Mixer

Let the inputs be v1 = sin 1t and v2 = sin t.

A balanced mixer acts like a multiplier. Thus

its output, vo = Av1v2 = A sin 1t sin 2t.

Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]

Therefore, vo = A/2[cos(1-2)t-cos(t].The last equation shows that the output of the

balanced mixer consists of the sum and difference of the input frequencies.

Balanced Ring Diode Mixer

Balanced mixers are also called balanced modulators.

Phase-Locked Loop

The PLL is the basis of practically all modern frequency synthesizer design.

The block diagram of a simple PLL:

PhaseDetector

LPFLoop

AmplifierVCO

fr foVp

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.

PLL Frequency Specifications

Free-RunningFrequency

Capture Range

Lock Range

fofLCfLL fHC fHLf

There is a limit on how far apart the free-runningVCO frequency and the reference frequency can be

for lock to be acquired or maintained.

PLL Frequency Synthesizer

For output frequencies in the VHF range and higher,a prescaler is required. The prescaler is a fixed dividerplaced ahead of the programmable divide by N counter.

AM Waveform

ec = Ec sin ctem = Em sin mt

AM signal:es = (Ec + em) sin ct

Modulation Index

The amount of amplitude modulation in a signal is given by its modulation index:

minmax

When Em = Ec , m =1 or 100% modulation.Over-modulation, i.e. Em>Ec , should be avoided

because it will create distortions and splatter.

where, Emax = Ec + Em; Emin = Ec - Em (all pk values)

Effects of Modulation Index

m = 1 m > 1

In a practical AM system, it usually contains manyfrequency components. When this is the case,

21 ... nT mmmm

AM in Frequency Domain

The expression for the AM signal:

es = (Ec + em) sin ct

can be expanded to:es = Ec sin ct + ½ mEc[cos (c-m)t-cos (c+m)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.

AM Spectrum

mEc/2mEc/2

fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2

Bandwidth, B = 2fm

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 = Ec

2/(2R); and Psf = m2Pc/4. So,

AM Current

The modulation index for an AM station can be measured by using an RF ammeter and the following equation:

2mII o

where I is the current with modulation and Io is the current without modulation.

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:

AM Receivers

Basic requirements for receivers:ability to tune to a specific signalamplify the signal that is picked upextract the information by demodulationamplify the demodulated signalTwo important receiver specifications:

sensitivity and selectivity

Tuned-Radio-Frequency (TRF) Receiver

The TRF receiver is the simplest receiver that meets all the basic requirements.

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.

Superheterodyne Receiver

Block diagram of basic superhet receiver:

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 rejection.

Mixer and Local Oscillator

The mixer and LO frequency convert the input frequency, fc, to a fixed fIF:

High-side injection: fLO = fc + fIF

Autodyne Converter

Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit:

IF Amplifier, Detector, & AGC

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.

Diode Detector Waveforms

Diagonal Clipping Distortion

Diagonal clipping distortion is more pronounced athigh modulation index or high modulation frequency.

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.

Shape Factor

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.

Image Frequency Rejection

For a tuned circuit with a quality factor of Q, then the image frequency rejection is:

wherexQIR

In dB, IR (dB) = 20 log IR

IF Transformers

The transformers used in the IF stages can be either single-tuned or double-tuned.

Single-tuned Double-tuned

Loose and Tight Couplings

For single-tuned transformers, tighter coupling means more gain but broader bandwidth:

Under, Over, & Critical Coupling

Double-tuned transformers can be over, under, critically, or optimally coupled:

Coupling Factors

Critical coupling factor kc is given by:

where Qp, Qs = prim. & sec. Q, respectively.IF transformers often use the optimum coupling

factor, kopt = 1.5kc , to obtain a steep skirt andflat passband. The bandwidth for a double-tuned

IF amplifier with k = kopt is given by B = kfo.Overcoupling means k>kc; undercoupling, k< kc

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 carefully shaped response.

Block Diagram of AM TX

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.

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.

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.

AM Modulator Circuits

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

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 non-linearity is easier to observe with this method.

Trapezoidal Pattern (cont’d)

Improperphase

-Vp>+Vp

minmax

m<1 m=1 m>1

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.

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.

BPF orAF

BalancedModulator

CarrierOscillator

DSB-SCUSB

Waveforms for Balanced Modulator

V1, fc

V2, fm Vo

ffc+fm

LIC Balanced Modulator 1496

Filter for SSB

Filters with high Q are needed for suppressing the unwanted sideband.

fa = fc - f2

fb = fc - f1

fd = fc + f1

fe = fc + f2

dBXantifQ c

)20/log( where X = attenuation ofsideband, and f = fd - fb

Typical SSB TX using Filter Method

SSB Waveform

Generating SSB - Phasing Method

This method is based on the fact that the lsf and the usf are given by the equations:

cos {(c - m)t} = ½(cos ct cos mt + sin ct sin mt)

cos {(c + m)t} = ½(cos ct cos mt - sin ct sin mt)

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.

Diagram for Phasing Method

Balanced Modulator 1

Balanced Modulator 2

+90o phaseshifter

90o phaseshifter

Modulatingsignal

Em cos mt

SSBoutput

Ec cos ct

Carrieroscillator

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.

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 power.

where Vp = peak signal voltageand RL = load resistance

Block Diagram of SSB RX

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.

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.

1 analog communications ee721 by h chan, mohawk college 2 main topics ÊiÊintroduction to...

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