ANALOG COMMUNICATIONS

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ANALOG COMMUNICATIONS
EE721

by H Chan, Mohawk College
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 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: Channel Source Transmitter Receiver Recipient by H Chan, Mohawk College

by H Chan, Mohawk College
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 by H Chan, Mohawk College

by H Chan, Mohawk College
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 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 by H Chan, Mohawk College

by H Chan, Mohawk College
Frequency Bands BAND Hz ELF AF k VLF 3 k - 30 k LF 30 k 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, l = c/f 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 by H Chan, Mohawk College

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 by H Chan, Mohawk College

by H Chan, Mohawk College

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) by H Chan, Mohawk College

by H Chan, Mohawk College
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

by H Chan, Mohawk College
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 by H Chan, Mohawk College

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, by H Chan, Mohawk College

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. by H Chan, Mohawk College

Noise Spectrum of Electronic Devices
Transit-Time Noise Excess or Flicker Noise Shot and Thermal Noises f 1 kHz fhc 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: In FM receivers, SINAD = (S+N+D)/(N+D) is usually used instead of SNR. by H Chan, Mohawk College

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) by H Chan, Mohawk College

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: 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. by H Chan, Mohawk College

by H Chan, Mohawk College

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: 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. by H Chan, Mohawk College

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. by H Chan, Mohawk College

Stagger-Tuned IF Amplifiers
by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Class B Push-Pull RF Amplifier
by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Class C Amplifier (cont’d)
by H Chan, Mohawk College

Frequency Multipliers
One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier: High Distortion Device + Amplifier Tuning Filter Circuit Output Input fi N x fi by H Chan, Mohawk College

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. by H Chan, Mohawk College

Waveforms for Frequency Multipliers
by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Hazeltine Neutralization
by H Chan, Mohawk College

by H Chan, Mohawk College
Rice Neutralization by H Chan, Mohawk College

Transformer-Coupled Neutralization
by H Chan, Mohawk College

Inductive Neutralization
by H Chan, Mohawk College

by H Chan, Mohawk College
Oscillators AV 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 B by H Chan, Mohawk College

by H Chan, Mohawk College
Hartley Oscillators by H Chan, Mohawk College

by H Chan, Mohawk College
Colpitts Oscillator by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

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: by H Chan, Mohawk College

by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Packaging, symbol, and characteristic of crystals
by H Chan, Mohawk College

Crystal-Controlled Oscillators
Pierce Colpitts by H Chan, Mohawk College

by H Chan, Mohawk College
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). by H Chan, Mohawk College

Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p frequencies. by H Chan, Mohawk College

by H Chan, Mohawk College
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 f2 by H Chan, Mohawk College

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. by H Chan, Mohawk College

Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators. by H Chan, Mohawk College

by H Chan, Mohawk College
Phase-Locked Loop The PLL is the basis of practically all modern frequency synthesizer design. The block diagram of a simple PLL: Vp fr fo Phase Detector Loop Amplifier LPF VCO by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

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 f fLL fLC fo fHC fHL by H Chan, Mohawk College

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. by H Chan, Mohawk College

by H Chan, Mohawk College
AM Waveform AM signal: es = (Ec + em) sin wct ec = Ec sin wct em = Em sin wmt by H Chan, Mohawk College

Modulation Index The amount of amplitude modulation in a signal is given by its modulation index: 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. by H Chan, Mohawk College

Effects of Modulation Index
In a practical AM system, it usually contains many frequency components. When this is the case, by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

AM Spectrum Ec mEc/2 mEc/2 fm fm f flsf fc fusf
fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2 Bandwidth, B = 2fm by H Chan, Mohawk College

by H Chan, Mohawk College
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, by H Chan, Mohawk College

AM Current The modulation index for an AM station can be measured by using an RF ammeter and the following equation: where I is the current with modulation and Io is the current without modulation. by H Chan, Mohawk College

by H Chan, Mohawk College
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: by H Chan, Mohawk College

by H Chan, Mohawk College
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 by H Chan, Mohawk College

The TRF receiver is the simplest receiver that meets all the basic requirements. by H Chan, Mohawk College

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. by H Chan, Mohawk College

Block diagram of basic superhet receiver: by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

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

by H Chan, Mohawk College
Autodyne Converter Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit: by H Chan, Mohawk College

IF Amplifier, Detector, & AGC
by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Diode Detector Waveforms
by H Chan, Mohawk College

Diagonal Clipping Distortion
Diagonal clipping distortion is more pronounced at high modulation index or high modulation frequency. by H Chan, Mohawk College

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. by H Chan, Mohawk College

by H Chan, Mohawk College
Shape Factor by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Image Frequency Rejection
For a tuned circuit with a quality factor of Q, then the image frequency rejection is: In dB, IR (dB) = 20 log IR by H Chan, Mohawk College

by H Chan, Mohawk College
IF Transformers The transformers used in the IF stages can be either single-tuned or double-tuned. Double-tuned Single-tuned by H Chan, Mohawk College

Loose and Tight Couplings
For single-tuned transformers, tighter coupling means more gain but broader bandwidth: by H Chan, Mohawk College

Under, Over, & Critical Coupling
Double-tuned transformers can be over, under, critically, or optimally coupled: by H Chan, Mohawk College

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 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 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 carefully shaped response. by H Chan, Mohawk College

by H Chan, Mohawk College
Block Diagram of AM TX by H Chan, Mohawk College

by H Chan, Mohawk College
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. by H Chan, Mohawk College

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. by H Chan, Mohawk College

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. by H Chan, Mohawk College

by H Chan, Mohawk College
AM Modulator Circuits by H Chan, Mohawk College

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

by H Chan, Mohawk College
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. by H Chan, Mohawk College

Trapezoidal Pattern (cont’d)
m<1 m=1 m>1 Improper phase -Vp>+Vp by H Chan, Mohawk College

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. by H Chan, Mohawk College

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 USB DSB-SC BPF or AF Input LSB Carrier Oscillator by H Chan, Mohawk College

Waveforms for Balanced Modulator
V2, fm Vo V1, fc f fc-fm fc+fm by H Chan, Mohawk College

LIC Balanced Modulator 1496
by H Chan, Mohawk College

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 where X = attenuation of sideband, and f = fd - fb by H Chan, Mohawk College

Typical SSB TX using Filter Method
by H Chan, Mohawk College

by H Chan, Mohawk College
SSB Waveform by H Chan, Mohawk College

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. by H Chan, Mohawk College

Diagram for Phasing Method
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. 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 power. where Vp = peak signal voltage and RL = load resistance by H Chan, Mohawk College

by H Chan, Mohawk College
Block Diagram of SSB RX by H Chan, Mohawk College

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. by H Chan, Mohawk College

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

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