Presentation on theme: "ANALOG COMMUNICATIONS EE721 by H Chan, Mohawk College MAIN TOPICS ÊIÊIntroduction to Communication Systems ËRËRadio-Frequency Circuits ÌAÌAmplitude Modulation."— Presentation transcript:
ANALOG COMMUNICATIONS EE721
by H Chan, Mohawk College MAIN TOPICS ÊIÊIntroduction to Communication Systems ËRËRadio-Frequency Circuits ÌAÌAmplitude Modulation ÍAÍAM Receivers ÎAÎAM Transmitters ÏSÏSuppressed-Carrier AM Systems T est #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: SourceTransmitterReceiverRecipient
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 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. Hartleys 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 Frequency Bands BAND Hz ELF AF k VLF3 k - 30 k LF30 k k MF300 k - 3 M HF3 M - 30 M BAND Hz VHF30M-300M UHF300M - 3 G SHF3 G - 30 G EHF30 G - 300G Wavelength, = 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
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)=C o + A n cos n t B n sin n t (Fourier series)
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. HA square-wave has only odd harmonics
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 Internal Noise Thermal Noise is produced by the random motion of electrons in a conductor due to heat.Noise power, P N = kTB where T = absolute temperature in o K k = Boltzmanns constant, 1.38x J/K B = noise power bandwidth in Hz Noise voltage,
by H Chan, Mohawk College Internal Noise (contd) 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 Device Noise Shot and Thermal Noises Excess or Flicker Noise Transit-Time Noise 1 kHzf hc f
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 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. T eq = (NF - 1)T o where T o is a ref. temperature (often 290 o K) 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 Radio-Frequency Amplifiers
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 amplifiers gain. The resonant frequency (f o ), bandwidth (B), and quality factor (Q), of a parallel resonant circuit are:
by H Chan, Mohawk College Narrowband Amplifier (contd) 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. R C and C 2 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 Amplifier Classes An amplifier is classified as: Class A if it conducts current throughout the full input cycle (i.e. 360 o ). 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 Class C Amplifier Class C amplifier operates for less than half of the input cycle. Its 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 (contd)
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 Input fifi Output N x f i
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 f i ; tuning to the 3rd harmonic triples f i ; etc.
by H Chan, Mohawk College Waveforms for Frequency Multipliers
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 Rice Neutralization
by H Chan, Mohawk College Transformer-Coupled Neutralization
by H Chan, Mohawk College Inductive Neutralization
by H Chan, Mohawk College Oscillators Barkhausen criteria for sustained oscillations: ¬The closed-loop gain, |BA V | = 1. The loop phase shift = 0 o or some integer multiple of 360 o at the operating frequency. A V = open-loop gain B = feedback factor/fraction AVAV B Output
by H Chan, Mohawk College Hartley Oscillators
by H Chan, Mohawk College Colpitts Oscillator
by H Chan, Mohawk College Clapp Oscillator The Clapp oscillator is a variation of the Colpitts circuit. C 4 is added in series with L in the tank circuit. C 2 and C 3 are chosen large enough to swamp out the transistors junction capacitances for greater stability. C 4 is often chosen to be << either C 2 or C 3, thus making C 4 the frequency determining element, since C T = C 4.
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
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 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 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: f1f1 f2f2 f 1 + f 2
by H Chan, Mohawk College Equations for Balanced Mixer Let the inputs be v 1 = sin 1 t and v 2 = sin t. A balanced mixer acts like a multiplier. Thus its output, v o = Av 1 v 2 = A sin 1 t sin 2 t. Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)] Therefore, v o = A/2[cos( )t-cos( 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 Phase-Locked Loop The PLL is the basis of practically all modern frequency synthesizer design. The block diagram of a simple PLL: Phase Detector LPF Loop Amplifier VCO frfr fofo VpVp
by H Chan, Mohawk College Operation of PLL ¬Initially, the PLL is unlocked, i.e.,the VCO is at the free-running frequency, f o. Since f o is probably not the same as the reference frequency, f r, the phase detector will generate an error/control voltage, V p. ®V p is filtered, amplified, and applied to the VCO to change its frequency so that f o = f r. The PLL will then remain in phase lock.
by H Chan, Mohawk College PLL Frequency Specifications Free-Running Frequency Capture Range Lock Range fofo f LC f LL f HC f HL f 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.
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 AM Waveform e c = E c sin c t e m = E m sin m t AM signal: e s = (E c + e m ) sin c t
by H Chan, Mohawk College Modulation Index The amount of amplitude modulation in a signal is given by its modulation index: When E m = E c, m =1 or 100% modulation. Over-modulation, i.e. E m >E c, should be avoided because it will create distortions and splatter. where, E max = E c + E m ; E min = E c - E m (all pk values)
by H Chan, Mohawk College Effects of Modulation Index m = 1m > 1 In a practical AM system, it usually contains many frequency components. When this is the case,
by H Chan, Mohawk College AM in Frequency Domain The expression for the AM signal: e s = (E c + e m ) sin c t can be expanded to: e s = E c sin c t + ½ mE c [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, f lsf = f c -f m, and an upper side frequency, f usf = f c +f m.
by H Chan, Mohawk College AM Spectrum f fcfc EcEc f usf mE c /2 f lsf fmfm fmfm f usf = f c + f m ; f lsf = f c - f m ; E sf = mE c /2 Bandwidth, B = 2f m
by H Chan, Mohawk College AM Power Total average (i.e. rms) power of the AM signal is: P T = P c + 2P sf, where P c = carrier power; and P sf = side-frequency power If the signal is across a load resistor, R, then: P c = E c 2 /(2R); and P sf = m 2 P c /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 I o is the current without modulation.
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 m T. For example:
by H Chan, Mohawk College AM Receivers Basic requirements for receivers: Êability to tune to a specific signal Ëamplify the signal that is picked up Ìextract the information by demodulation Íamplify the demodulated signal.Two important receiver specifications: sensitivity and selectivity
by H Chan, Mohawk College Tuned-Radio-Frequency (TRF) Receiver The TRF receiver is the simplest receiver that meets all the basic requirements.
by H Chan, Mohawk College Drawbacks of TRF Receivers ÀDifficulty in tuning all the stages to exactly the same frequency simultaneously. ÁVery high Q for the tuning coils are required for good selectivity BW=f o /Q. ÂSelectivity is not constant for a wide range of frequencies due to skin effect which causes the BW to vary with f o.
by H Chan, Mohawk College Superheterodyne Receiver Block diagram of basic superhet receiver:
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, f c, to a fixed f IF : High-side injection: f LO = f c + f IF
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 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 Shape Factor
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:f image = f sig + 2f IF wheref sig = 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 IF Transformers The transformers used in the IF stages can be either single-tuned or double-tuned. Single-tuned Double-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 k c is given by: where Q p, Q s = prim. & sec. Q, respectively..IF transformers often use the optimum coupling factor, k opt = 1.5k c, to obtain a steep skirt and flat passband. The bandwidth for a double-tuned IF amplifier with k = k opt is given by B = kf o..Overcoupling means k>k c ; undercoupling, k< k c
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 Block Diagram of AM TX
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 (contd) 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 (contd) 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 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 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 (contd) Improper phase -V p >+V p m<1m=1m>1
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 P sf = m 2 P c /4, and P t =P c (1+m 2 /2 ), then at m=1, P t = 6 P sf. 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. BPFor AF Input Balanced Modulator Carrier Oscillator DSB-SC USB LSB
by H Chan, Mohawk College Waveforms for Balanced Modulator V 1, f c V 2, f m VoVo f f c +f m f c -f m
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. f a = f c - f 2 f b = f c - f 1 f d = f c + f 1 f e = f c + f 2 where X = attenuation of sideband, and f = f d - f b
by H Chan, Mohawk College Typical SSB TX using Filter Method
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 c t cos m t + sin c t sin m t) cos( c + m )t = ½(cos c t cos m t - sin c t sin m t) 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 90 o. 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 : ÀNo high Q filters are required. ÁTherefore, lower f m can be used. ÂSSB at any carrier frequency can be generated in a single step. Disadvantage: Difficult to achieve accurate 90 o 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 V p = peak signal voltage and R L = load resistance
by H Chan, Mohawk College Block Diagram of SSB RX
by H Chan, Mohawk College SSB Receiver (contd) 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 SSB RX (contd) 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.