Heng Chan; Mohawk College2 Course Content b Introduction & Review b Transmission Line Characteristics b Waveguides & Microwave Devices b Radiowave Propagation b Antennas b Microwave Radio & Radar Systems b Fibre Optic Communications
Heng Chan; Mohawk College3 Introduction & Review b Microwaves are defined as radio waves in the frequency range > 1 GHz. b However, waves > 20 GHz are commonly known as millimeter waves b Distributed, rather than lumped, circuit elements must be used at microwave frequencies because of a phenomenon called skin effect.
Heng Chan; Mohawk College4 Skin Effect b At microwave frequencies current travels on the outer surface, or skin, of the conductor because of the increased inductance created. b The skin depth, (in m), for a conductor with permeability, (in H/m), conductivity, (in S/m), and at a frequency, f (in Hz), is given by:
Heng Chan; Mohawk College5 Skin Effect (contd) b The current density, J, decreases with the distance beneath the surface exponentially. b At a depth, the current density decreases to J o /e. b As f increases, and resistance. J JoJo z J = J o e -z/ conductor surface direction of current
Heng Chan; Mohawk College6 Transverse Electromagnetic Waves x y z Electric Field Magnetic Field Direction of Propagation In free space:
Heng Chan; Mohawk College7 Notes on TEM Waves b The E- and H-fields and the direction of motion of TEM waves are mutually perpendicular to each other. b Velocity of radio waves in free space is c = 3x10 8 m/s, but in a medium with dielectric constant r :
Heng Chan; Mohawk College8 Microwave Materials b Glass epoxy printed circuit boards are unsuitable for microwave use because of high dissipation factor and wide tolerance in thickness and dielectric constant. b Instead, materials such as Teflon fiberglass laminates, alumina substrates, sapphire and quartz substrates must be used (refer to text for details).
Heng Chan; Mohawk College9 Types of Transmission Lines b Differential or balanced lines (where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair. b Single-ended or unbalanced lines (where one conductor is grounded): e.g. concentric or coaxial cable. b Transmission lines for microwave use: e.g. striplines, microstrips, and waveguides.
Heng Chan; Mohawk College10 Transmission Line Equivalent Circuit R L R L C G C G L L C C Lossy Line Lossless Line ZoZo ZoZo
Heng Chan; Mohawk College11 Notes on Transmission Line b Characteristics of a line is determined by its primary electrical constants or distributed parameters: R ( /m), L (H/m), C (F/m), and G (S/m). b Characteristic impedance, Z o, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, Z L = Z o.
Heng Chan; Mohawk College12 Formulas for Some Lines D d D d For parallel two-wire line: For co-axial cable: = o r ; = o r ; o = 4 x10 -7 H/m; o = pF/m
Heng Chan; Mohawk College13 Transmission-Line Wave Propagation Electromagnetic waves travel at < c in a transmission line because of the dielectric separating the conductors. The velocity of propagation is given by: m/s Velocity factor, VF, is defined as:
Heng Chan; Mohawk College14 Propagation Constant b Propagation constant,, determines the variation of V or I with distance along the line: V = V s e -x ; I = I s e -x, where V S, and I S are the voltage and current at the source end, and x = distance from source. b = + j, where = attenuation coefficient (= 0 for lossless line), and = phase shift coefficient = 2 / (rad./m)
Heng Chan; Mohawk College15 Incident & Reflected Waves b For an infinitely long line or a line terminated with a matched load, no incident power is reflected. The line is called a flat or nonresonant line. b For a finite line with no matching termination, part or all of the incident voltage and current will be reflected.
Heng Chan; Mohawk College16 Reflection Coefficient The reflection coefficient is defined as : It can also be shown that: Note that when Z L = Z o, = 0; when Z L = 0, = -1; and when Z L = open circuit, = 1.
Heng Chan; Mohawk College17 Standing Waves V min = E i - E r With a mismatched line, the incident and reflected waves set up an interference pattern on the line known as a standing wave. The standing wave ratio is : V max = E i + E r Voltage
Heng Chan; Mohawk College18 Other Formulas When the load is purely resistive: (whichever gives an SWR > 1) Return Loss, RL = Fraction of power reflected = | | 2, or -20 log | | dB So, P r = | | 2 P i Mismatched Loss, ML = Fraction of power transmitted/absorbed = 1 - | | 2 or -10 log(1-| | 2 ) dB So, P t = P i (1 - | | 2 ) = P i - P r
Heng Chan; Mohawk College19 Time-Domain Reflectometry ZLZL Pulse or Step Generator Oscilloscope Transmission Line TDR is a practical technique for determining the length of the line, the way it is terminated, and the type and location of any impedance discontinuities. The distance to the discontinuity is: d = vt/2, where t = elapsed time of returned reflection. d
Heng Chan; Mohawk College20 Typical TDR Waveform Displays t R L > Z o R L < Z o Z L inductive Z L capacitive ViVi VrVr VrVr ViVi
Heng Chan; Mohawk College21 Transmission-Line Input Impedance The input impedance at a distance l from the load is: When the load is a short circuit, Z i = jZ o tan ( l). For 0 l < /4, shorted line is inductive. For l = /4, shorted line = a parallel resonant circuit. For /4 < l /2, shorted line is capacitive.
Heng Chan; Mohawk College22 T-L Input Impedance (contd) When the load is an open circuit, Z i = -jZ o cot ( l) For 0 < l < /4, open circuited line is capacitive. For l = /4, open-line = series resonant circuit. For /4 < l < /2, open-line is inductive. b A /4 line with characteristic impedance, Z o, can be used as a matching transformer between a resistive load, Z L, and a line with characteristic impedance, Z o, by choosing:
Heng Chan; Mohawk College23 Transmission Line Summary or l < /4 l > /4 is equivalent to: l > /4 or l < /4 is equivalent to: = = /4 ZoZo Z o ZLZL /4-section Matching Transformer l = /4
Heng Chan; Mohawk College24 The Smith Chart b The Smith chart is a graphical aid to solving transmission-line impedance problems. b The coordinates on the chart are based on the intersection of two sets of orthogonal circles. b One set represents the normalized resistive component, r (= R/Z o ), and the other the normalized reactive component, ± jx (= ± jX/Z o ).
Heng Chan; Mohawk College25 Smith Chart Basics r = 0 r = 1 r = 2 +j0.7 -j1.4 j0 z1z1 z2z2 z 1 = 1+j0.7 z 2 = 2-j1.4
Heng Chan; Mohawk College26 Applications of The Smith Chart b Applications to be discussed in this course: Find SWR, | |, RLFind SWR, | |, RL Find Y LFind Y L Find Z i of a shorted or open line of length lFind Z i of a shorted or open line of length l Find Z i of a line terminated with Z LFind Z i of a line terminated with Z L Find distance to V max and V min from Z LFind distance to V max and V min from Z L Solution for quarter-wave transformer matchingSolution for quarter-wave transformer matching Solution for parallel single-stub matchingSolution for parallel single-stub matching
Heng Chan; Mohawk College27 Substrate Lines b Miniaturized microwave circuits use striplines and microstrips rather than coaxial cables as transmission lines for greater flexibility and compactness in design. b The basic stripline structure consists of a flat conductor embedded in a dielectric material and sandwiched between two ground planes.
Heng Chan; Mohawk College28 Basic Stripline Structure Ground Planes Centre Conductor Solid Dielectric b W t r
Heng Chan; Mohawk College29 Notes On Striplines b When properly designed, the E and H fields of the signal are completely confined within the dielectric material between the two ground planes. b The characteristic impedance of the stripline is a function of its line geometry, specifically, the t/b and w/b ratios, and the dielectric constant, r. b Graphs, design formulas, or computer programs are available to determine w for a desired Z o, t, and b.
Heng Chan; Mohawk College30 Microstrip w t b Ground Plane r (dielectric) Circuit Line Microstrip line employs a single ground plane, the conductor pattern on the top surface being open. Graphs, formulas or computer programs would be used to design the conductor line width. However, since the electromagnetic field is partly in the solid dielectric, and partly in the air space, the effective relative permittivity, eff, has to be used in the design instead of r.
Heng Chan; Mohawk College31 Stripline vs Microstrip b Advantages of stripline: signal is shielded from external interferencesignal is shielded from external interference shielding prevents radiation lossshielding prevents radiation loss r and mode of propagation are more predictable for design r and mode of propagation are more predictable for design b Advantages of microstrip: easier to fabricate, therefore less costlyeasier to fabricate, therefore less costly easier to lay, repair/replace componentseasier to lay, repair/replace components
Heng Chan; Mohawk College32 Microstrip Directional Coupler /4 Top View Cross-sectional View Conductor Lines Dielectric Ground Plane Most of the power into port #1 will flow to port #3. Some of the power will be coupled to port #2 but only a minute amount will go to port #4.
Heng Chan; Mohawk College33 Formulas For Directional Coupler The operation of the coupler gives rise to an even mode characteristic impedance, Z oe, and an odd mode characteristic impedance, Z oo, where: For a given coupling factor, C (which is V 2 /V 1 ):
Heng Chan; Mohawk College34 Coupler Applications b Some common applications for couplers: monitoring/measuring the power or frequency at a point in the circuitmonitoring/measuring the power or frequency at a point in the circuit sampling the microwave energy for used in automatic leveling circuits (ALC)sampling the microwave energy for used in automatic leveling circuits (ALC) reflection measurements which indirectly yield information on VSWR, Z L, return loss, etc.reflection measurements which indirectly yield information on VSWR, Z L, return loss, etc.
Heng Chan; Mohawk College35 Branch Coupler Z 1 = Z o Input power at port #1 will divide equally between Ports 2 and 3 and none to port 4. Can provide tighter coupling and can handle higher power than directional coupler. Branches may consist of chokes, filters, or matched load for more design flexibility. ZoZo ZoZo Z1Z1 Z1Z1
Heng Chan; Mohawk College36 Hybrid Ring Coupler Input power at port #1 divides evenly between ports 2 & 4 and none for port 3. Similarly, input at port #2 will divide evenly between ports 1 and 3 and none for port 4. One application: circulator /4
Heng Chan; Mohawk College37 Microstrip & Stripline Filters /4 IN OUT Side-coupled half-wave resonator band-pass filter IN OUT L CCC L L Conventional low-pass filter L
Heng Chan; Mohawk College38 Scattering Parameters b Microwave devices are often characterized by their S-parameters because: measurement of V and I may be difficult at microwave frequencies.measurement of V and I may be difficult at microwave frequencies. Active devices frequently become unstable when open or short-circuit type measurements are made for h, Y or Z parameters.Active devices frequently become unstable when open or short-circuit type measurements are made for h, Y or Z parameters. b An [S] matrix is used to contain all the S- parameters.
Heng Chan; Mohawk College39 S-Variables & S-Parameters a1a1 b1b1 b2b2 a2a2 V1V1 V2V2 For port x: V x = V ix + V rx ; S-variables : P x = P ix - P rx = |a x | 2 -|b x | 2 b 1 = S 11 a 1 + S 12 a 2 b 2 = S 21 a 1 + S 22 a 2 or 2-Port Network
Heng Chan; Mohawk College40 S-Parameters of 2-Port Network Note: when port 2 is terminated with a matched load, a 2 = 0. Similarly, a 1 = 0 when port 1 is matched. S 11, and S 22 are reflection coefficients, i.e., 11, & 22. S 21 represents the forward transmission coefficient. Thus, Insertion Loss/attenuation = -10 log (P o2 /P i1 ) = -20 log |S 21 | dB S 12 is the reversed transmission coefficient.
Heng Chan; Mohawk College41 Properties of S-Parameters b In general, S-parameters have both magnitude and angle. b For matched 2-port reflectionless networks, S 11 = S 22 = 0 b For a reciprocal 2-port network, S 12 = S 21. b For a lossless 2-port network, S 12 = S 21 = 1. b For n-port, [b] = [S] [a]. The n x n [S] matrix characterizes the network.
Heng Chan; Mohawk College42 Microwave Radiation Hazards b The fact that microwaves can be used for cooking purposes and in heating applications suggests that they have the potential for causing biological damage. b Health & Welfare, Canada specifies no limit exposure duration for radiation level of 1 mW/cm 2 or less for frequencies from 10 MHz to 300 GHz. b Avoid being in the direct path of a microwave beam coming out of an antenna or waveguide.
Heng Chan; Mohawk College43 Waveguides b Reasons for using waveguide rather than coaxial cable at microwave frequency: easier to fabricateeasier to fabricate no solid dielectric and I 2 R lossesno solid dielectric and I 2 R losses b Waveguides do not support TEM waves inside because of boundary conditions. b Waves travel zig-zag down the waveguide by bouncing from one side wall to the other.
Heng Chan; Mohawk College44 E-Field Pattern of TE 1 0 Mode a b g /2 End ViewSide View TE mn means there are m number of half-wave variations of the transverse E-field along the a side and n number of half-wave variations along the b side. The magnetic field (not shown) forms closed loops horizontally around the E-field
Heng Chan; Mohawk College45 TE and TM Modes b TE mn mode has the E-field entirely transverse, i.e. perpendicular, to the direction of propagation. b TM mn mode has the H-field entirely transverse to the direction of propagation. b All TE mn and TM mn modes are theoretically permissible except, in a rectangular waveguide, TM mo or TM on modes are not possible since the magnetic field must form a closed loop. b In practice, only the dominant mode, TE 10 is used.
Heng Chan; Mohawk College46 Wavelength for TE & TM Modes Any signal with c will not propagate down the waveguide. For air-filled waveguide, cutoff freq., f c = c/ c Guide wavelength: TE 10 is called the dominant mode since c = 2a is the longest wavelength of any mode. Cutoff wavelength:
Heng Chan; Mohawk College47 Other Formulas for TE & TM Modes Group velocity: Phase velocity: Wave impedance: Z o = 377 for air-filled waveguide
Heng Chan; Mohawk College48 Circular/Cylindrical Waveguides b Differences versus rectangular waveguides : c = 2 r/B mn where r = waveguide radius, and B mn is obtained from table of Bessel functions. c = 2 r/B mn where r = waveguide radius, and B mn is obtained from table of Bessel functions. All TE mn and TM mn modes are supported since m and n subscripts are defined differently.All TE mn and TM mn modes are supported since m and n subscripts are defined differently. Dominant mode is TE 11.Dominant mode is TE 11. b Advantages: higher power-handling capacity, lower attenuation for a given cutoff wavelength. b Disadvantages: larger and heavier.
Heng Chan; Mohawk College49 Waveguide Terminations Dissipative Vane Side ViewEnd View Short-circuit Sliding Short-Circuit g /2 Dissipative vane is coated with a thin film of metal which in turn has a thin dielectric coating for protection. Its impedance is made equal to the wave impedance. The taper minimizes reflection. Sliding short-circuit functions like a shorted stub for impedance matching purpose.
Heng Chan; Mohawk College50 Attenuators Resistive Flap Sliding-vane Type Rotary-vane Type Max. attenuation when flap is fully inside. Slot for flap is chosen to be at a non- radiating position. Max. attenuation when vane is at centre of guide and min. at the side-wall. Atten.(dB) = 10 log (P i /P o ) = -20 log |S 21 | PiPi PoPo PiPi PoPo
Heng Chan; Mohawk College51 Iris Reactors = = = Inductive iris; vanes are vertical Capacitive iris; vanes are horizontal Irises can be used as reactance elements, filters or impedance matching devices.
Heng Chan; Mohawk College52 Tuning Screw s A post or screw can also serve as a reactive element. When the screw is advanced partway into the wave- guide, it acts capacitive. When the screw is advanced all the way into the waveguide, it acts inductive. In between the two positions, one can get a resonant LC circuit. Post Tuning Screws
Heng Chan; Mohawk College53 Waveguide T-Junctions E-Plane JunctionH-Plane Junction Input power at port 2 will split equally between ports 1 and 3 but the outputs will be antiphase for E-plane T and inphase for H-plane T. Input power at ports 1 & 3 will combine and exit from port 1 provided the correct phasing is used.
Heng Chan; Mohawk College54 S-Matrix for T-Junctions For ideal T-junction: Note: + sign is used for H-plane T, and (-) sign for E-plane T. Also note that even though S 22 = 0 (i.e. lossless), S 11 and S 33 are each equal to 1/2, i.e., input power applied to ports 1 and 3 will always suffer from reflection.
Heng Chan; Mohawk College55 Hybrid-T Junction It combines E-plane and H-plane junctions. Note : S 11, S 22, S 33, and S 44 are zero. P in at port 1 or 2 will divide between ports 3 and 4. P in at port 3 or 4 will divide between ports 1 and 2. Under matched & ideal conditions:
Heng Chan; Mohawk College56 Hybrid-T Junction (contd) b If input power of the same phase is applied simultaneously at ports 1 and 2, the combined power exits from port 4. If the input is out-of-phase, the output is at port 3. b Applications: Combining power from two transmitters.Combining power from two transmitters. TX and a RX sharing a common antenna.TX and a RX sharing a common antenna. Low noise mixer circuit.Low noise mixer circuit.
Heng Chan; Mohawk College57 Directional Coupler P1P1 P2P2 P4P4 Termination g /4 P3P3 2-hole Coupler Holes spaced g /4 allow waves travelling toward port 4 to combine. Waves travelling toward port 3, however, will cancel. Therefore, ideally P 3 = 0. To broaden frequency response bandwidth, practical couplers would usually have multi holes. P1P1 P2P2
Heng Chan; Mohawk College59 Cavity Resonators a b L Resonant wavelength for a rectangular cavity: L r For a cylindrical resonator:
Heng Chan; Mohawk College60 Cavity Resonators (contd) b Energy is coupled into the cavity either through a small opening, by a coupling loop or a coupling probe. These methods of coupling also apply for waveguides b Applications of resonators: filtersfilters absorption wavemetersabsorption wavemeters microwave tubesmicrowave tubes
Heng Chan; Mohawk College61 Ferrite Components b Ferrites are compounds of metallic oxides such as those of Fe, Zn, Mn, Mg, Co, Al, and Ni. b They have magnetic properties similar to ferromagnetic metals and at the same time have high resistivity associated with dielectrics. b Their magnetic properties can be controlled by means of an external magnetic field. b They can be transparent, reflective, absorptive, or cause wave rotation depending on the H-field..
Heng Chan; Mohawk College62 Examples of Ferrite Devices Attenuator Isolator Differential Phase Shifter port Circulator
Heng Chan; Mohawk College63 Notes On Ferrite Devices Differential phase shifter - is the phase shift between the two directions of propagation. Differential phase shifter - is the phase shift between the two directions of propagation. b Isolator - permits power flow in one direction only. b Circulator - power entering port 1 will go to port 2 only; power entering port 2 will go to port 3 only; etc. b Most of the above are based on Faraday rotation. b Other usage: filters, resonators, and substrates.
Heng Chan; Mohawk College64 Schottky Barrier Diode Semi- conductor Layer Substrate Contact SiO 2 Dielectric Metal Electrode Metal Electrode Its based on a simple metal- semiconductor interface. There is no p-n junction but a depletion region exists. Current is by majority carriers; therefore, very low in capacitance. Applications: detectors, mixers, and switches.
Heng Chan; Mohawk College65 Varactor Diode Circuit Symbol V CjCj CoCo Junction Capacitance Characteristic Varactors operate under reverse-bias conditions. The junction capacitance is: where V b = barrier potential (0.55 to 0.7 for silicon) and K = constant (often = 1)
Heng Chan; Mohawk College66 Equivalent Circuit for Varactor CjCj RjRj RsRs The series resistance, R s, and diode capacitance, C j, determine the cutoff frequency: The diode quality factor for a given frequency, f, is:
Heng Chan; Mohawk College67 Varactor Applications b Voltage-controlled oscillator (VCO) in AFC and PLL circuits b Variable phase shifter b Harmonic generator in frequency multiplier circuits b Up or down converter circuits b Parametric amplifier circuits - low noise
Heng Chan; Mohawk College68 Parametric Amplifier Circuit Pump signal (f p ) Input signal (f s ) L1 C1 C2 L2 D1 L3 C3 Signal tank (f s ) Idler tank (f i ) Nondegenerative mode: Upconversion - f i = f s + f p Downconversion - f i = f s - f p Power gain, G = f i /f s Regenerative mode: u negative resistance u very low noise u very high gain f p = f s + f i Degenerative Mode : f p = 2f s
Heng Chan; Mohawk College69 PIN Diode P+P+ I N+N+ +V R RFC C1 C2 S1 D1 In Out PIN as shunt switch PIN diode has an intrinsic region between the P + and N + materials. It has a very high resistance in the OFF mode and a very low resistance when forward biased.
Heng Chan; Mohawk College70 PIN Diode Applications b To switch devices such as attenuators, filters, and amplifiers in and out of the circuit. b Voltage-variable attenuator b Amplitude modulator b Transmit-receive (TR) switch b Phase shifter (with section of transmission line)
Heng Chan; Mohawk College71 Tunnel Diode Symbol LsLs CjCj RsRs -R Equivalent Circuit i V VvVv IpIp VpVp Characteristic Curve Heavy doping of the semiconductor material creates a very thin potential barrier in the depletion zone which leads to electron tunneling through the barrier. Note the negative resistance zone between V p and V v. B C A
Heng Chan; Mohawk College72 More Notes On Tunnel Diode The resistive, and self-resonant frequencies are: Tunnel diodes can be used in monostable (A or C), bistable (between A and C), or astable (B) modes. These modes lead to switching, oscillation, and amplification applications. However, the power output levels of the tunnel diode are restricted to a few mW only.
Heng Chan; Mohawk College73 Transferred Electron Devices b TEDs are made of compound semiconductors such as GaAs. b They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded. b The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band.
Heng Chan; Mohawk College74 Gunn Diode The Gunn diode is a transferred electron device that can be used in microwave oscillators or one-port reflection amplifiers. Its basic structure is shown below. N -, the active region, is sandwiched between two heavily doped N + regions. Electrons from the N-N- Metallic Electrode N+N+ Metallic Electrode cathode (K) drifts to the anode (A) in bunched formation called domains. Note that there is no p-n junction. AK l
Heng Chan; Mohawk College75 Gunn Operating Modes b Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect. b Transit Time (TT) Mode: operating frequency, f o = v d / l where v d is the domain velocity, and l is the effective length. Output power < 2 W, and frequency is between 1 GHz to 18 GHz. b Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.
Heng Chan; Mohawk College76 Gunn Diode Circuit and Applications Tuning Screw Diode Resonant Cavity Iris V Gunn diode applications: microwave source for receiver local oscillator, police radars, and microwave communication links. The resonant cavity is shocked excited by current pulses from the Gunn diode and the RF energy is coupled via the iris to the waveguide.
Heng Chan; Mohawk College77 Avalanche Transit-Time Devices b If the reverse-bias potential exceeds a certain threshold, the diode breaks down. b Energetic carriers collide with bound electrons to create more hole-electron pairs. b This multiplies to cause a rapid increase in reverse current. b The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon.
Heng Chan; Mohawk College78 IMPATT Diode A single-drift structure of an IMPATT (impact avalanche transit time) diode is shown below: P+P+ NN+N+ - + l Drift Region Avalanche Region Operating frequency: where v d = drift velocity
Heng Chan; Mohawk College79 Notes On IMPATT Diode b The current build-up and the transit time for the current pulse to cross the drift region cause a 180 o phase delay between V and I; thus, negative R. b IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are possible. b They must operate in conjunction with an external high-Q resonant circuit. b They have relatively high output power (>100 W pulsed) but are very noisy and not very efficient.
Heng Chan; Mohawk College80 Microwave Transistors b Silicon BJTs and GaAsFETs are most widely used. b BJT useful for amplification up to about 6 MHz. b MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz. b FETs have higher input impedance, better efficiency and more frequency stable than BJTs.
Heng Chan; Mohawk College81 Microwave Transistor Power Gain Max. power gain of a unilateral transistor amplifier with conjugate matched input and output: Transistor G o Matching Network G s Matching Network G L ZLZL ZsZs VsVs Note that G o = |S 21 | 2 is the gain of the transistor. For unconditional stability, |S 11 | < 1 and |S 22 | < 1.
Heng Chan; Mohawk College82 Noise Factor & Noise Figure Noise Factor, F n = SNR in /SNR out Noise Figure, NF (dB) = 10 log F n = SNR in (dB) - SNR out (dB) Equivalent noise temperature, T e = (F n -1) T o where T o = 290 o K For amplifiers in cascade, the overall noise factor: where G n = amplifier gain of the nth stage.
Heng Chan; Mohawk College83 Microwave Tubes b Classical vacuum tubes have several factors which limit their upper operating frequency: interelectrode capacitance & lead inductanceinterelectrode capacitance & lead inductance dielectric losses & skin effectdielectric losses & skin effect transit timetransit time b Microwave tubes utilize resonant cavities and the interaction between the electric field, magnetic field and the electrons.
Heng Chan ; Mohawk College84 Magnetrons It consists of a cylindrical cathode surrounded by the anode with a number of resonant cavities. Waveguide Output Coupling Window Cathode Anode Interaction Space Cavity Its a crossed-field device since the E-field is perpendicular to the dc magnetic field. At a critical voltage the electrons from the cathode will just graze the anode.
Heng Chan; Mohawk College85 Magnetron Operation b When an electron cloud sweeps past a cavity, it excites the latter to self oscillation which in turn causes the electrons to bunch up into a spoked wheel formation in the interaction space. b The continuous exchange of energy between the electrons and the cavities sustains oscillations at microwave frequency. b Electrons will eventually lose their energy and fall back into the cathode while new ones are emitted.
Heng Chan; Mohawk College86 More Notes On Magnetrons Alternate cavities are strapped (i.e., shorted) so that adjacent resonators are 180 o out of phase. This enables only the dominant -mode to operate. Alternate cavities are strapped (i.e., shorted) so that adjacent resonators are 180 o out of phase. This enables only the dominant -mode to operate. b Frequency tuning is possible either mechanically (screw tuner) or electrically with voltage. b Magnetrons are used as oscillators for radars, beacons, microwave ovens, etc. b Peak output power is from a few MW at UHF and X-band to 10 kW at 100 GHz.
Heng Chan; Mohawk College87 Klystrons b Klystrons are linear-beam devices since the E-field is parallel to the static magnetic field. b Their operation is based on velocity and density modulation with resonating cavities to create the bunching effect. b They can be employed as oscillators or power amplifiers.
Heng Chan; Mohawk College88 Two-Cavity Klystron Filament RF InRF Out Control Grid Cathode Anode Buncher Cavity Catcher Cavity Collector Gap Drift Region Effect of velocity modulation v Electron Beam
Heng Chan; Mohawk College89 Klystron Operation b RF signal applied to the buncher cavity sets up an alternating field across the buncher gap. b This field alternately accelerates and decelerates the electron beam causing electrons to bunch up in the drift region. b When the electron bundles pass the catcher gap, they excite the catcher cavity into resonance. b RF power is extracted from the catcher cavity by the coupling loop.
Heng Chan; Mohawk College90 Multicavity Klystrons b Gain can be increased by inserting intermediate cavities between the buncher and catcher cavity. b Each additional cavity increases power gain by 15- to 20-dB. b Synchronous tuned klystrons have high gain but very narrow bandwidth, e.g % of f o. b Stagger tuned klystrons have wider bandwidth at the expense of gain. b Can operate as oscillator by positive feedback.
Heng Chan; Mohawk College91 Reflex Klystron Output Anode Filament Cathode Repeller Cavity VrVr Electron Beam Condition for oscillation requires electron transit time to be: where n = an integer and T = period of oscillation
Heng Chan; Mohawk College92 Reflex Klystron Operation b Electron beam is velocity modulated when passing though gridded gap of the cavity. b Repeller decelerates and turns back electrons thus causing bunching. b Electrons are collected on the cavity walls and output power can be extracted. b Repeller voltage, V r, can be used to vary output frequency and power.
Heng Chan; Mohawk College93 Notes On Reflex Klystrons b Only one cavity used. b No external dc magnetic field required. b Compact size. b Can be used as an oscillator only. b Low output power and low efficiency. b Output frequency can be tuned by V r, or by changing the dimensions of the cavity.
Heng Chan; Mohawk College94 Travelling-Wave Tube RF InRF Out Collector Helix Attenuator Electron Beam The TWT is a linear beam device with the magnetic field running parallel to tube lengthwise. The helix is also known as a slow wave structure to slow down the RF field so that its velocity down the the tube is close to the velocity of the electron beam.
Heng Chan; Mohawk College95 TWT Operation b As the RF wave travels along the helix, its positive and negative oscillations velocity modulate the electron beam causing the electrons to bunch up. b The prolonged interaction between the RF wave and electron beam along the TWT results in exponential growth of the RF voltage. b The amplified wave is then extracted at the output. b The attenuator prevents reflected waves that can cause oscillations.
Heng Chan; Mohawk College96 Notes On TWTs b Since interaction between the RF field and the electron beam is over the entire length of the tube, the power gain achievable is very high (> 50 dB). b As TWTs are nonresonant devices, they have wider bandwidths and lower NF than klystrons. b TWTs operate from 0.3 to 50 GHz. b The Twystron tube is a combination of the TWT and klystron. It gives better gain and BW over either the conventional TWT or klystron.
Heng Chan; Mohawk College97 Radio- Wave In Free Space Radio waves propagate as TEM waves in free space. For an isotropic (i.e. omnidirectional) source: where P D = power density (W/m 2 ); E = electric field intensity (V/m); P r = total radiated power (W); and d = distance from source (m). d Point Source
Heng Chan; Mohawk College98 Optical Properties Of Radio Waves b Since light waves and radio waves are part of the electromagnetic spectrum, they behave similarly. b Thus, radio waves can: refract at the boundary between two different mediarefract at the boundary between two different media reflect at the surface of a conductorreflect at the surface of a conductor diffract around the edge of an obstaclediffract around the edge of an obstacle interfere with one and another to degrade performanceinterfere with one and another to degrade performance b Propagation of radio wave in the atmosphere is greatly influenced by the frequency of the wave.
Heng Chan; Mohawk College99 Radio Wave Propagation Modes b In every terrestrial radio system, there are three possible modes of propagation: Ground-wave or surface-wave propagationGround-wave or surface-wave propagation Space-wave or direct-wave propagationSpace-wave or direct-wave propagation Sky-wave propagationSky-wave propagation b At frequencies < 2 MHz, ground wave is best. b Sky waves are used for HF signals. b Space waves are used for VHF and above.
Heng Chan; Mohawk College100 Ground-Wave Propagation Ground waves start out with the electric field being perpendicular to the ground. Due to the gradient density of the earths atmosphere the wavefront tilts progressively. Direction of wave travel Increasing Tilt Earth Wavefront
Heng Chan; Mohawk College101 Notes On Ground Waves b Advantages: Given enough power, can circumnavigate the earth.Given enough power, can circumnavigate the earth. Relatively unaffected by atmospheric conditions.Relatively unaffected by atmospheric conditions. b Disadvantages: Require relatively high transmission power.Require relatively high transmission power. Require large antennas since frequency is low.Require large antennas since frequency is low. Ground losses vary considerably with terrain.Ground losses vary considerably with terrain. b Applications: MF broadcasting; ship-to-ship and ship-to-shore comms; radio navigation; maritime comms.
Heng Chan; Mohawk College102 Space-Wave Propagation Most terrestrial communications in the VHF or higher frequency range use direct, line-of-sight, or tropospheric radio waves. The approximate maximum distance of communication is given by: where d = max. distance in km h T = height of the TX antenna in m h R = height of the RX antenna in m
Heng Chan; Mohawk College103 Notes On Space-Waves b The radio horizon is greater than the optical horizon by about one third due to refraction of the atmosphere. b Reflections from a relatively smooth surface, such as a body of water, could result in partial cancellation of the direct signal - a phenomenon known as fading. Also, large objects, such as buildings and hills, could cause multipath distortion from many reflections.
Heng Chan; Mohawk College104 Sky-Wave Propagation b HF radio waves are returned from the F-layer of the ionosphere by a form of refraction. b The highest frequency that is returned to earth in the vertical direction is called the critical frequency, f c. b The highest frequency that returns to earth over a given path is called the maximum usable frequency (MUF). Because of the general instability of the ionosphere, the optimum working frequency (OWF) = 0.85 MUF, is used instead.
Heng Chan; Mohawk College105 Formulas For Sky Waves b From geometry (assuming flat earth): d = 2h v tan i where h v = virtual height of F-layer b From theory (secant law): MUF = f c sec i i hvhv d F-Layer Earth
Heng Chan; Mohawk College106 Free-Space Path Loss b Defined as the loss incurred by a radio wave as it travels in a straight line through a vacuum with no absorption or reflection of energy from nearby objects. b Formula: L p (dB) = log f + 20log d where f = frequency of radio wave in GHz and d = distance in km. b If f is in MHz, replace 92.4 above by 32.4.
Heng Chan; Mohawk College107 Fade Margin b To account for changes in atmospheric conditions, multipath loss, and terrain sensitivity, a fade margin, F m, must be added to total system loss: F m (dB) = 30log d + 10log(6ABf) - 10log(1-R) -70 where d = distance (km), f = frequency (GHz), R = reliability (decimal value), A = terrain roughness factor (0.25 to 4), and B = factor to convert worst- month probability to annual probability (0.125 to 1 depending on humidity or dryness).
Heng Chan; Mohawk College108 Antenna Basics b An antenna is a passive reciprocal device. b It acts as a transducer to convert electrical oscillations in a transmission line or waveguide to a propagating wave in free space and vice versa. b It functions as an impedance matcher between a transmission line or waveguide and free space. b All antennas have a radiation pattern which is a plot of the field strength or power density at various angular positions relative to the antenna.
Heng Chan; Mohawk College109 Antenna Efficiency An antenna has an equivalent radiation resistance, R r given by: where P r = power radiated and i = antenna current at feedpoint Antenna efficiency: where P d = power dissipated; and R e = effective antenna resistance. All the power supplied to the antenna is not radiated.
Heng Chan; Mohawk College110 Directive Gain & Power Gain Directive gain of an antenna is given by: where P D = power density at some point with a given antenna; P Dr = power density at the same point with a reference antenna. Reference antenna is generally the isotropic source. When antenna efficiency is taken into account directive gain becomes power gain: A p = D. In decibels, power gain is 10 log A p Maximum directive gain is called directivity.
Heng Chan; Mohawk College111 Effective Isotropic Radiated Power EIRP is the equivalent power that an isotropic antenna would have to radiate to achieve the same power density at a given point as another antenna: EIRP = P r A t = P in A p where P r = total radiated power; P in = antenna input power; A t = TX antenna directive gain; and A p = antenna power gain. Therefore, the power density at a distance, d, from an antenna is:
Heng Chan; Mohawk College112 Antenna Miscellany b Power captured by the receiving antenna with an effective area, A eff, is C = P D A eff. Note that A eff includes the gain and efficiency of the antenna. b Antennas can be linearly, elliptically or circularly polarized depending on their E-field radiated. b Antenna beamwidth is the angular separation between the two half-power points on the major lobe of the antennas plane radiation pattern. b Antenna input impedance, Z in = E i /I i
Heng Chan; Mohawk College113 Half-Wave Dipole Balanced Feedline Symbol / 2.Simple and most widely used at f > 2 MHz. Its a resonant antenna since its length is 2 x /4. Z in = 73 approx.; Z max = 2500 approx. at ends.Radiation pattern of dipole in free space has two main lobes perpendicular to the antenna axis..Has a gain of about 2.15 dBi
Heng Chan; Mohawk College114 Free-Space Radiation Pattern of Dipole
Heng Chan; Mohawk College115 Ground & Length Effects On Dipole b Since the ground reflects radio waves, it has a significant effect on the radiation pattern and impedance of the half-wave dipole. b Generally speaking, the closer the dipole is to the ground, the more lobes will form and the lower the radiation impedance. Length also has an effect on the dipole antenna: dipoles shorter than /2 is capacitive while dipoles longer than /2 is inductive. Length also has an effect on the dipole antenna: dipoles shorter than /2 is capacitive while dipoles longer than /2 is inductive.
Heng Chan; Mohawk College116 Marconi/Monopole Antenna Main characteristics: vertical and /4 vertical and /4 good ground plane is required good ground plane is required omnidirectional in the horizontal plane omnidirectional in the horizontal plane 3 dBd power gain 3 dBd power gain impedance: about 36 impedance: about 36
Heng Chan; Mohawk College117 Antenna Impedance Matching b Antennas should be matched to their feedline for maximum power transfer efficiency by using an LC matching network. b A simple but effective technique for matching a short vertical antenna to a feedline is to increase its electrical length by adding an inductance at its base. This inductance, called a loading coil, cancels the capacitive effect of the antenna. b Another method is to use capacitive loading.
Heng Chan; Mohawk College119 Antenna Arrays b Antenna elements can be combined in an array to increase gain and get desired radiation pattern. b Arrays can be classified as broadside or end-fire, according to their direction of maximum radiation. b In a phased array, all elements are fed or driven; i.e. they are connected to the feedline. b Some arrays have only one driven element with several parasitic elements which act to absorb and reradiate power radiated from the driven element.
Heng Chan; Mohawk College120 Yagi-Uda Array b More commonly known as the Yagi array, it has one driven element, one reflector, and one or more directors. Radiation pattern
Heng Chan; Mohawk College121 Characteristics of Yagi Array F unidirectional radiation pattern (one main lobe, some sidelobes and backlobes) F relatively narrow bandwidth since it is resonant F 3-element array has a gain of about 7 dBi F more directors will increase gain and reduce the beamwidth and feedpoint impedance F a folded dipole is generally used for the driven element to widen the bandwidth and increase the feedpoint impedance.
Heng Chan; Mohawk College122 Folded Dipole b Often used - alone or with other elements - for TV and FM broadcast receiving antennas because it has a wider bandwidth and four times the feedpoint resistance of a single dipole. Feed line Z in = 288
Heng Chan; Mohawk College123 Log-Periodic Dipole Array (LPDA) Feed line L6L6 L5L5 L4L4 L3L3 L2L2 D6D6 D5D5 Direction of main lobe Apex
Heng Chan; Mohawk College124 Characteristics of LPDA feedpoint impedance is a periodic function of log f feedpoint impedance is a periodic function of log f unidirectional radiation and wide bandwidth unidirectional radiation and wide bandwidth shortest element is less than or equal to /2 of highest frequency, while longest element is at least /2 of lowest frequency shortest element is less than or equal to /2 of highest frequency, while longest element is at least /2 of lowest frequency reasonable gain, but lower than that of Yagi for the same number of elements reasonable gain, but lower than that of Yagi for the same number of elements design parameter, = L 1 /L 2 = D 1 /D 2 = L 2 /L 3 = …. design parameter, = L 1 /L 2 = D 1 /D 2 = L 2 /L 3 = …. used mainly as HF, VHF, and TV antennas used mainly as HF, VHF, and TV antennas
Heng Chan; Mohawk College125 Turnstile Array omnidirectional radiation in the horizontal plane, with horizontal polarization omnidirectional radiation in the horizontal plane, with horizontal polarization gain of about 3 dB less than that of a single dipole gain of about 3 dB less than that of a single dipole often used for FM broadcast RX and TX often used for FM broadcast RX and TX Half-wave dipoles fed 90 o out-of phase
Heng Chan; Mohawk College126 Collinear Array F all elements lie along a straight line, fed in phase, and often mounted with main axis vertical F result in narrow radiation beam omnidirectional in the horizontal plane when antenna is vertical Half-wave Elements Feed Line Quarter-wave Shorted Stub
Heng Chan; Mohawk College127 Broadside Array all /2 elements are fed in phase and spaced /2 all /2 elements are fed in phase and spaced /2 with axis placed vertically, radiation would have a narrow bidirectional horizontal pattern with axis placed vertically, radiation would have a narrow bidirectional horizontal pattern Feed Line Half-wave Dipoles
Heng Chan; Mohawk College128 End-Fire Array dipole elements are fed 90 o out of phase resulting in a narrow unidirectional radiation pattern off the end of the antenna dipole elements are fed 90 o out of phase resulting in a narrow unidirectional radiation pattern off the end of the antenna Feed Line RadiationPattern Half-wave Dipoles
Heng Chan; Mohawk College129 Non-resonant Antennas Monopole and dipole antennas are classified as resonant type since they operate efficiently only at frequencies that make their elements close to /2. Monopole and dipole antennas are classified as resonant type since they operate efficiently only at frequencies that make their elements close to /2. b Non-resonant antennas do not use dipoles and are usually terminated with a matching load resistor. b They have a broader bandwidth and a radiation pattern that has only one or two main lobes. b Examples of non-resonant antennas are long-wire antennas, vee antennas, and rhombic antennas.
Heng Chan; Mohawk College130 Loop Antenna Main characteristics: very small dimensions very small dimensions bidirectional bidirectional greatest sensitivity in the plane of the loop greatest sensitivity in the plane of the loop very wide bandwidth very wide bandwidth efficient as RX antenna with single or multi-turn loop efficient as RX antenna with single or multi-turn loop Feedline
Heng Chan; Mohawk College131 Helical Antenna S D Ground Plane Coaxial Feedline End-fire Helical Antenna F broadband (+ 20% of f o ) F circularly polarized A p = 15 dB; -3dB = 20 o are typical F when S, D, & # of turns increase: A p increases and decreases F to get higher gainand narrower beamwidth, use an array F applications: V/UHF antenna; satellite tracking antenna
Heng Chan; Mohawk College132 UHF & Microwave Antennas b highly directive and beamwidth of about 1 o or less b antenna dimensions >> wavelength of signal b front-to-back ratio of 20 dB or more b utilize parabolic reflector as secondary antenna for high gain b primary feed is either a dipole or horn antenna b use for point-to-point and satellite communications
Heng Chan; Mohawk College133 Parabolic Reflector Antenna Power gain and -3 dB beamwidth are: where = antenna efficiency (0.55 is typical); D = dish diameter (m); and = wavelength (m)
Heng Chan; Mohawk College134 Hog-horn Antenna The hog-horn antenna, often used for terrestrial microwave links, integrates the feed horn and a parabolic reflecting surface to provide an obstruction-free path for incoming and outgoing signals. Parabolic Section Feed Horn
Heng Chan; Mohawk College135 Microwave Radio Communications b Can be classified as either terrestrial or satellite systems. b Early systems use FDM (frequency division multiplex) technique. b More recent systems use PCM/PSK (pulse code modulation/phase shift keying) technique. b Microwave system capacities range from less than 12 VB (voice-band) channels to > 22,000. b Operate from 24 km to 6,400 km.
Heng Chan; Mohawk College136 Simplified Block Diagram Preemphasized Baseband Input FM Modulator Upconverter Mixer BPF Ch. Combiner IF Oscillator RF Oscillator RF Out Deemphasized Baseband Output FM Detector Downconverter Mixer BPF Ch. Separator RF Oscillator RF In FM Microwave Receiver FM Microwave Transmitter Amp
Heng Chan; Mohawk College137 Notes On FM Microwave Radio System b Baseband signals may comprise one or more of : Frequency-division-multiplexed voice-band channelsFrequency-division-multiplexed voice-band channels Time-division-multiplexed VB channelsTime-division-multiplexed VB channels Broadcast-quality composite video or picturephoneBroadcast-quality composite video or picturephone Wideband dataWideband data b IF carrier is typically 70 MHz b Low-index frequency modulation is used b Common microwave frequencies used: 2-, 4-, 6-, 12-, and 14-GHz bands.
Heng Chan; Mohawk College138 Microwave Radio Systems (contd) b The distance between transmitter and receiver is typically between 24 to 64 km. b Repeaters have to be used for longer distances. b To increase the reliability of microwave links, the following techniques can be used: frequency diversity - two RF carrier frequenciesfrequency diversity - two RF carrier frequencies space diversity - two or more antennas are usedspace diversity - two or more antennas are used polarization diversity - vertical and horizontal polarizationpolarization diversity - vertical and horizontal polarization
Heng Chan; Mohawk College139 System Gain b System gain for microwave radio link is: G s (dB) = P t - C min = F m + L p + L f + L b - A t - A r where P t = transmitter output power (dBm) C min = min. receiver input power (dBm) F m =fade margin for a given reliability objective (dB) L p = free-space path loss between antennas (dB) L f, L b = feeder, coupling, & branching losses (dB) A t, A r = Tx and Rx antenna gain respectively (dB)
Heng Chan; Mohawk College140 Introduction To Pulsed Radar PRT Pulse of energy Pulse Repetition Time Pulse repetition frequency, PRF = 1/PRT Range to target, R = ct/2, where c = speed of light, and t = time between TX pulse and echo return. Dead zone, R dead, and resolution, R, are both = c /2. Duty cycle, D = /PRT Resolution can be improved by pulse compression.
Heng Chan; Mohawk College141 Radar Power & Range Equation Average power, P a = P p (PRF) = P p /PRT = P p D where P p = peak power. Ideal radar range equation: where P R = signal power returned (W) G = antenna gain = wavelength of signal (m) = radar cross section of target (m 2 ) In the real world, losses and noise must be added to above equation.
Heng Chan; Mohawk College142 Pulsed Radar Block Diagram Video Amp Video Detector IF Amp LO Mixer RF Amp T/R Switch Control Section Modulator Timer Transmitter Signal Processor Display Receiver Section Antenna
Heng Chan; Mohawk College143 Radar Display Modes Range Target Elevation N Beam Sweep Targets E-Scan Plan Position Indicator
Heng Chan; Mohawk College144 CW Doppler Radar Microwave Oscillator Doppler Mixer TX RX fdfd The Doppler effect can be used for determining the speed of a moving target. v = f d /2 (m/s) where f d = doppler shift (Hz) = radar wavelength (m) Circulator Basic block diagram of CW Doppler radar
Heng Chan; Mohawk College145 FM Doppler Radar Both distance and velocity can be determined if an FM Doppler radar is used. f d+ f d- t fifi fofo TX RX Range: Velocity: where a = slope of line or rate of change of f i
Heng Chan; Mohawk College146 Optical Fibre Communications b Advantages over metallic/coaxial cable: much wider bandwidth and practically interference-freemuch wider bandwidth and practically interference-free lower loss and light weightlower loss and light weight more resistive to environmental effectsmore resistive to environmental effects safer and easier to installsafer and easier to install almost impossible to tap into a fibre cablealmost impossible to tap into a fibre cable potentially lower in cost over the long termpotentially lower in cost over the long term b Disadvantages: higher initial cost in installation & more expensive to repair/maintainhigher initial cost in installation & more expensive to repair/maintain
Heng Chan; Mohawk College147 Optical Fibre Link Input Signal Coder or Converter Light Source Source-to-fibre Interface Fibre-to-light Interface Light Detector Amplifier/Shaper Decoder Output Fibre-optic Cable Transmitter Receiver
Heng Chan; Mohawk College148 Types Of Optical Fibre Single-mode step-index fibre Multimode step-index fibre Multimode graded-index fibre n 1 core n 2 cladding n o air n 2 cladding n 1 core Variable n n o air Light ray Index porfile
Heng Chan; Mohawk College149 Comparison Of Optical Fibres b Single-mode step-index fibre: minimum signal dispersion; higher TX rate possibleminimum signal dispersion; higher TX rate possible difficult to couple light into fibre; highly directive light source (e.g. laser) required; expensive to manufacturedifficult to couple light into fibre; highly directive light source (e.g. laser) required; expensive to manufacture b Multimode step-index fibres: inexpensive; easy to couple light into fibreinexpensive; easy to couple light into fibre result in higher signal distortion; lower TX rateresult in higher signal distortion; lower TX rate b Multimode graded-index fibre: intermediate between the other two types of fibresintermediate between the other two types of fibres
Heng Chan; Mohawk College150 Acceptance Cone & Numerical Aperture n 2 cladding n 1 core Acceptance Cone Acceptance angle, c, is the maximum angle in which external light rays may strike the air/fibre interface and still propagate down the fibre with <10 dB loss. Numerical aperture: NA = sin c = (n n 2 2 ) C
Heng Chan; Mohawk College151 Losses In Optical Fibre Cables b The predominant losses in optic fibres are: absorption losses due to impurities in the fibre materialabsorption losses due to impurities in the fibre material material or Rayleigh scattering losses due to microscopic irregularities in the fibrematerial or Rayleigh scattering losses due to microscopic irregularities in the fibre chromatic or wavelength dispersion because of the use of a non-monochromatic sourcechromatic or wavelength dispersion because of the use of a non-monochromatic source radiation losses caused by bends and kinks in the fibreradiation losses caused by bends and kinks in the fibre modal dispersion or pulse spreading due to rays taking different paths down the fibremodal dispersion or pulse spreading due to rays taking different paths down the fibre coupling losses caused by misalignment & imperfect surface finishescoupling losses caused by misalignment & imperfect surface finishes
Heng Chan; Mohawk College152 Absorption Losses In Optic Fibre Loss (dB/km) Wavelength ( m) Peaks caused by OH - ions Infrared absorption Rayleigh scattering & ultraviolet absorption
Heng Chan; Mohawk College154 Light Sources b Light-Emitting Diodes (LED) made from material such as AlGaAs or GaAsPmade from material such as AlGaAs or GaAsP light is emitted when electrons and holes recombinelight is emitted when electrons and holes recombine either surface emitting or edge emittingeither surface emitting or edge emitting b Injection Laser Diodes (ILD) similar in construction as LED except ends are highly polished to reflect photons back & forthsimilar in construction as LED except ends are highly polished to reflect photons back & forth
Heng Chan; Mohawk College155 ILD versus LED b Advantages: more focussed radiation pattern; smaller fibremore focussed radiation pattern; smaller fibre much higher radiant power; longer spanmuch higher radiant power; longer span faster ON, OFF time; higher bit rates possiblefaster ON, OFF time; higher bit rates possible monochromatic light; reduces dispersionmonochromatic light; reduces dispersion b Disadvantages: much more expensivemuch more expensive higher temperature; shorter lifespanhigher temperature; shorter lifespan
Heng Chan; Mohawk College156 Optical Transmitter Circuits +V CC Data Input Enable C1C1 R1R1 LED Q1Q1 R2R2 +HV ILD C2C2 Enable C1C1 R1R1 Q1Q1 R2R2 R3R3 Data Input
Heng Chan; Mohawk College157 Light Detectors b PIN Diodes photons are absorbed in the intrinsic layerphotons are absorbed in the intrinsic layer sufficient energy is added to generate carriers in the depletion layer for current to flow through the devicesufficient energy is added to generate carriers in the depletion layer for current to flow through the device b Avalanche Photodiodes (APD) photogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electronsphotogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electrons avalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodesavalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes
Heng Chan; Mohawk College158 Photodetector Circuit +V R1R Threshold adjust Enable Comparator shaper PIN or APD Data Out
Heng Chan; Mohawk College159 Bandwidth & Power Budget The maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d ( s/km) is: The maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d ( s/km) is: R = 1/(5dD) b Power or loss margin, L m (dB) is: L m = P r - P s = P t - M - L sf - (DxL f ) - L c - L fd - P s 0 where P r = received power (dBm), P s = receiver sensitivity(dBm), P t = Tx power (dBm), M = contingency loss allowance (dB), L sf = source-to-fibre loss (dB), L f = fibre loss (dB/km), L c = total connector/splice losses (dB), L fd = fibre-to-detector loss (dB).