Presentation on theme: "H. Chan; Mohawk College1 R. F. Systems EE731. H. Chan; Mohawk College2 Main Topics Transmission Line Characteristics Waveguides and Microwave Devices."— Presentation transcript:
H. Chan; Mohawk College1 R. F. Systems EE731
H. Chan; Mohawk College2 Main Topics Transmission Line Characteristics Waveguides and Microwave Devices Cable Television Systems Test #1 - Week #430 % Final Exam - Week #760 % TLM (Assignments)10 %
H. Chan; Mohawk College3 Types of Transmission Lines Differential or balanced lines (where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair. Single-ended or unbalanced lines (where one conductor is grounded): e.g. concentric or coaxial cable. Transmission lines for microwave use: e.g. striplines, microstrips, and waveguides.
H. Chan; Mohawk College4 Transmission Line Equivalent Circuit R L R L C G C G L L C C Lossy Line Lossless Line ZoZo ZoZo
H. Chan; Mohawk College5 Notes on Transmission Line 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). 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.
H. Chan; Mohawk College6 Formulas for Common Cables 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
H. Chan; Mohawk College7 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:
H. Chan; Mohawk College8 Time Delay & Attenuation A signal will take time to travel down a transmission line. The amount of time delay is given by: (usually in ns/ft or ns/m) For coaxial cable,ns/ft The phase shift coefficient,radians/m Cable attenuation is expressed in dB/100 ft
H. Chan; Mohawk College9 Incident & Reflected Waves 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. For a finite line with no matching termination, part or all of the incident voltage and current will be reflected.
H. Chan; Mohawk College10 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.
H. Chan; Mohawk College11 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
H. Chan; Mohawk College12 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
H. Chan; Mohawk College13 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
H. Chan; Mohawk College14 Typical TDR Waveform Displays t R L > Z o R L < Z o Z L inductive Z L capacitive ViVi VrVr VrVr ViVi
H. Chan; Mohawk College15 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.
H. Chan; Mohawk College16 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. 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:
H. Chan; Mohawk College17 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
H. Chan; Mohawk College18 Substrate Lines Miniaturized microwave circuits use striplines and microstrips rather than coaxial cables as transmission lines for greater flexibility and compactness in design. The basic stripline structure consists of a flat conductor embedded in a dielectric material and sandwiched between two ground planes.
H. Chan; Mohawk College19 Basic Stripline Structure Ground Planes Centre Conductor Solid Dielectric b W t r
H. Chan; Mohawk College20 Notes On Striplines When properly designed, the E and H fields of the signal are completely confined within the dielectric material between the two ground planes. 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. Graphs, design formulas, or computer programs are available to determine w for a desired Z o, t, and b.
H. Chan; Mohawk College21 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.
H. Chan; Mohawk College22 Stripline vs Microstrip Advantages of stripline: –signal is shielded from external interference –shielding prevents radiation loss – r and mode of propagation are more predictable for design Advantages of microstrip: –easier to fabricate, therefore less costly –easier to lay, repair/replace components
H. Chan; Mohawk College23 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.
H. Chan; Mohawk College24 Coupler Applications Some common applications for couplers: –monitoring/measuring the power or frequency at a point in the circuit –sampling the microwave energy for used in automatic leveling circuits (ALC) –reflection measurements which indirectly yield information on VSWR, Z L, return loss, etc.
H. Chan; Mohawk College25 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
H. Chan; Mohawk College26 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
H. Chan; Mohawk College27 Microwave Radiation Hazards The fact that microwaves can be used for cooking purposes and in heating applications suggests that they have the potential for causing biological damage. An exposure limit of 1 mW/cm 2 for a maximum of one hour duration for frequencies from 10 MHz to 300 GHz is generally considered safe. Avoid being in the direct path of a microwave beam coming out of an antenna or waveguide.
H. Chan; Mohawk College28 Waveguides Reasons for using waveguide rather than coaxial cable at microwave frequency: –easier to fabricate –no solid dielectric and I 2 R losses Waveguides do not support TEM waves inside because of boundary conditions. Waves travel zig-zag down the waveguide by bouncing from one side wall to the other.
H. Chan; Mohawk College29 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
H. Chan; Mohawk College30 TE and TM Modes TE mn mode has the E-field entirely transverse, i.e. perpendicular, to the direction of propagation. TM mn mode has the H-field entirely transverse to the direction of propagation. 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. In practice, only the dominant mode, TE 10 is used.
H. Chan; Mohawk College31 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:
H. Chan; Mohawk College32 Other Formulas for TE & TM Modes Group velocity: Phase velocity: Wave impedance: Z o = 377 for air-filled waveguide
H. Chan; Mohawk College33 Circular/Cylindrical Waveguides Differences versus rectangular waveguides : 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. –Dominant mode is TE 11. Advantages: higher power-handling capacity, lower attenuation for a given cutoff wavelength. Disadvantages: larger and heavier.
H. Chan; Mohawk College34 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.
H. Chan; Mohawk College35 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 ) = P i (dBm)-P o (dBm) PiPi PoPo PiPi PoPo
H. Chan; Mohawk College36 Iris Reactors = = = Inductive iris; vanes are vertical Capacitive iris; vanes are horizontal Irises can be used as reactance elements, filters or impedance matching devices.
H. Chan; Mohawk College37 Tuning Screws 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
H. Chan; Mohawk College38 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.
H. Chan; Mohawk College39 Hybrid-T Junction It combines E-plane and H-plane junctions. 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. To antenna Termination Load From TX To RX
H. Chan; Mohawk College40 Hybrid-T Junction (contd) 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. Applications: –Combining power from two transmitters. –TX and a RX sharing a common antenna. –Low noise mixer circuit.
H. Chan; Mohawk College41 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
H. Chan; Mohawk College42 Directional Coupler (contd) Definitions: Coupling Factor, Directivity, Insertion Loss, (I.L.) = 10 log (P 1 /P 2 ) in dB where P 4(fwd) = power out of aux. arm when power in main arm is forward, and P 4(rev) = power out of aux. arm when power in main arm is reversed.
H. Chan; Mohawk College43 Cavity Resonators a b L Resonant wavelength for a rectangular cavity: L r For a cylindrical resonator:
H. Chan; Mohawk College44 Cavity Resonators (contd) 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 Applications of resonators: –filters –absorption wavemeters –microwave tubes
H. Chan; Mohawk College45 Ferrite Components Ferrites are compounds of metallic oxides such as those of Fe, Zn, Mn, Mg, Co, Al, and Ni. They have magnetic properties similar to ferromagnetic metals and at the same time have high resistivity associated with dielectrics. Their magnetic properties can be controlled by means of an external magnetic field. They can be transparent, reflective, absorptive, or cause wave rotation depending on the H-field..
H. Chan; Mohawk College46 Examples of Ferrite Devices Attenuator Isolator Differential Phase Shifter port Circulator
H. Chan; Mohawk College47 Notes On Ferrite Devices Differential phase shifter - is the phase shift between the two directions of propagation. Isolator - permits power flow in one direction only. Circulator - power entering port 1 will go to port 2 only; power entering port 2 will go to port 3 only; etc. Most of the above are based on Faraday rotation. Other usage: filters, resonators, and substrates.
H. Chan; Mohawk College48 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.
H. Chan; Mohawk College49 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)
H. Chan; Mohawk College50 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:
H. Chan; Mohawk College51 Varactor Applications Voltage-controlled oscillator (VCO) in AFC and PLL circuits Variable phase shifter Harmonic generator in frequency multiplier circuits Up or down converter circuits Parametric amplifier circuits - low noise
H. Chan; Mohawk College52 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
H. Chan; Mohawk College53 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.
H. Chan; Mohawk College54 PIN Diode Applications To switch devices such as attenuators, filters, and amplifiers in and out of the circuit. Voltage-variable attenuator Amplitude modulator Transmit-receive (TR) switch Phase shifter (with section of transmission line)
H. Chan; Mohawk College55 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
H. Chan; Mohawk College56 More Notes On Tunnel Diode 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.
H. Chan; Mohawk College57 Transferred Electron Devices TEDs are made of compound semiconductors such as GaAs. They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded. 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.
H. Chan; Mohawk College58 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
H. Chan; Mohawk College59 Gunn Operating Modes Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect. 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. Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.
H. Chan; Mohawk College60 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.
H. Chan; Mohawk College61 Avalanche Transit-Time Devices If the reverse-bias potential exceeds a certain threshold, the diode breaks down. Energetic carriers collide with bound electrons to create more hole-electron pairs. This multiplies to cause a rapid increase in reverse current. 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.
H. Chan; Mohawk College62 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
H. Chan; Mohawk College63 Notes On IMPATT Diode 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. IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are possible. They must operate in conjunction with an external high-Q resonant circuit. They have relatively high output power (>100 W pulsed) but are very noisy and not very efficient.
H. Chan; Mohawk College64 Microwave Transistors Silicon BJTs and GaAsFETs are most widely used. BJT useful for amplification up to about 6 MHz. MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz. FETs have higher input impedance, better efficiency and more frequency stable than BJTs.
H. Chan; Mohawk College65 SAW Devices Surface Acoustic Wave is an ultrasonic wave that traverses the polished surface of a piezoelectric substrate such as quartz and lithium niobate. Examples of SAW devices: filters, resonators, delay lines, and oscillators. Applications of SAW devices: mobile telephone, DBS receiver, pager, CATV converter, cordless phone, UHF radio, measuring equipment, etc.
H. Chan; Mohawk College66 SAW Filter InputOutput Absorber Piezoelectric substrate Comb electrode Centre frequency v = propagation velocity Comb electrodes for exciting and receiving waves are metallic deposit on a piezoelectric substrate.
H. Chan; Mohawk College67 SAW Resonator The frequency of the resonator depends upon the pitch between the teeth of the comb electrodes. One-port resonators have high Q factors and are primarily used as oscillators. Input Output 1-port resonator
H. Chan; Mohawk College68 Microwave Tubes Classical vacuum tubes have several factors which limit their upper operating frequency: –interelectrode capacitance & lead inductance –dielectric losses & skin effect –transit time Microwave tubes utilize resonant cavities and the interaction between the electric field, magnetic field and the electrons.
Heng Chan ; Mohawk College69 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.
H. Chan; Mohawk College70 Magnetron Operation 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. The continuous exchange of energy between the electrons and the cavities sustains oscillations at microwave frequency. Electrons will eventually lose their energy and fall back into the cathode while new ones are emitted.
H. Chan; Mohawk College71 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. Frequency tuning is possible either mechanically (screw tuner) or electrically with voltage. Magnetrons are used as oscillators for radars, beacons, microwave ovens, etc. Peak output power is from a few MW at UHF and X-band to 10 kW at 100 GHz.
H. Chan; Mohawk College72 Klystrons Klystrons are linear-beam devices since the E-field is parallel to the static magnetic field. Their operation is based on velocity and density modulation with resonating cavities to create the bunching effect. They can be employed as oscillators or power amplifiers.
H. Chan; Mohawk College73 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
H. Chan; Mohawk College74 Klystron Operation RF signal applied to the buncher cavity sets up an alternating field across the buncher gap. This field alternately accelerates and decelerates the electron beam causing electrons to bunch up in the drift region. When the electron bundles pass the catcher gap, they excite the catcher cavity into resonance. RF power is extracted from the catcher cavity by the coupling loop.
H. Chan; Mohawk College75 Multicavity Klystrons Gain can be increased by inserting intermediate cavities between the buncher and catcher cavity. Each additional cavity increases power gain by 15- to 20-dB. Synchronous tuned klystrons have high gain but very narrow bandwidth, e.g % of f o. Stagger tuned klystrons have wider bandwidth at the expense of gain. Can operate as oscillator by positive feedback.
H. Chan; Mohawk College76 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
H. Chan; Mohawk College77 Reflex Klystron Operation Electron beam is velocity modulated when passing though gridded gap of the cavity. Repeller decelerates and turns back electrons thus causing bunching. Electrons are collected on the cavity walls and output power can be extracted. Repeller voltage, V r, can be used to vary output frequency and power.
H. Chan; Mohawk College78 Notes On Reflex Klystrons Only one cavity used. No external dc magnetic field required. Compact size. Can be used as an oscillator only. Low output power and low efficiency. Output frequency can be tuned by V r, or by changing the dimensions of the cavity.
H. Chan; Mohawk College79 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.
H. Chan; Mohawk College80 TWT Operation As the RF wave travels along the helix, its positive and negative oscillations velocity modulate the electron beam causing the electrons to bunch up. The prolonged interaction between the RF wave and electron beam along the TWT results in exponential growth of the RF voltage. The amplified wave is then extracted at the output. The attenuator prevents reflected waves that can cause oscillations.
H. Chan; Mohawk College81 Notes On TWTs 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). As TWTs are nonresonant devices, they have wider bandwidths and lower NF than klystrons. TWTs operate from 0.3 to 50 GHz. The Twystron tube is a combination of the TWT and klystron. It gives better gain and BW over either the conventional TWT or klystron.
H. Chan; Mohawk College82 Master Antenna TV Systems For apartments and condos, a watered down form of cable TV, called MATV system can be used. The basic MATV system consists of a single broadband antenna mounted on the roof, broadband amplifiers, distribution cables, splitters, and subscriber outlets. It eliminates antennas cluttering the roof of the apartment building but reception is limited to local TV stations.
H. Chan; Mohawk College83 Cable TV Systems Today, the majority of homes receive cable TV where signals from antennas, satellites, studio, and other sources go to the headend first. The signals are processed, scrambled where necessary, and combined or frequency multiplexed onto a single cable for distribution. In addition to TV signals, cable also provide other services such as FM stations, pay TV, specialized programming, internet, distance education, etc.
H. Chan; Mohawk College84 Parts Of A CATV System Headend Satellite TV Stations Microwave Link FM Radio Processor Combiner Trunk Amplifier Trunk Cable Distribution Amps Feeder Cable Line Extender Amps Drop Cable Splitter Cable Box TV Set
H. Chan; Mohawk College85 Signal Processing Heterodyne processing is used to translate each signal to a different frequency at the headend. This prevents interference with local TV channels and allows satellite signals to be converted to a lower frequency for the cable. LO RF Amp Mixer IF Amp LO Mixer Input From Other Heterodyne Processors Combiner or Multiplexer Directional Coupler
H. Chan; Mohawk College86 Cable TV Channels Low Band VHF: Ch. 2 to Ch. 6; 54 MHz to 88 MHz FM Channels: 88 MHz to 108 MHz Mid Band VHF: Ch. A1 to Ch. I; 108 MHz to 174 MHz High Band VHF: Ch. 7 to Ch. 13; 174 MHz to 216 MHz Super Band: Ch. J to Ch. W; 216 MHz to 300 MHz Hyper Band: Ch. AA to Ch. RR; 300 MHz to 408 MHz
H. Chan; Mohawk College87 Cable TV Spectrum 5460 f MHz 66 Video Carrier Audio Carrier 1.25 MHz 4.5 MHz Channel 2Channel 3 Each TV channel occupies a bandwidth of 6 MHz. Audio info occupies a bandwidth of about 80 kHz. Video info occupies the rest of the channel.
H. Chan; Mohawk College88 Trunk Cable After amplification, the combined signals are sent to one or more trunk cables. Each trunk cable, constructed out of a large, low- loss coaxial cable, carries the signals to a series of distribution points. Booster amplifiers (max ) spaced at about 1 km intervals are usually required to restore the signal strength. Fibre-optic cables are now replacing coaxial cables as trunks since their losses are much lower.
H. Chan; Mohawk College89 Feeder & Drop Cables Feeder cables branch out from trunks to serve local neighbourhoods. A maximum of 2 line extender amplifiers are allowed per feed. Feeder cables are tapped at periodic locations for connection by co-ax drop cables to customers premises. Drop cables are limited in length to about 50 m.
H. Chan; Mohawk College90 Passive CATV Devices Splitters: They are used mainly for dividing RF energy equally to their output ports. Directional Couplers: They allow a portion of the RF energy in the main cable to be fed to a distribution or feeder cable. Taps: They are used to tap off RF energy from the feeder cable to the subscriber. They possess the combined features of the splitter and the directional coupler.
H. Chan; Mohawk College91 CATV Graphic Symbols -3.5 dB -7 dB 2-way splitter 4-way splitter Tap output InputOutput Directional Coupler 2-port tap 4-port tap 8-port tap
H. Chan; Mohawk College92 Equalization The differential in transmission loss through a length of co-axial cable between the lowest frequency of 50 MHz and the upper frequency of 400 MHz is significant. Equalization must be applied at spaced distances of the cable to correct the tilt of the signal spectrum. Equalizer MHz MHz Incoming signal tiltEqualized output
H. Chan; Mohawk College93 Noise & Distortions In the CATV system, noise may be generated in amplifiers or picked up from external sources. Since a large number of channels are combined in the system, second and higher order intermodulation distortions can be a serious problem. All devices used in the CATV system must be impedance-matched to avoid reflections and echoes.
H. Chan; Mohawk College94 Amplifiers and AGC Since the resistance of co-ax cables varies with temperature and there are hundreds of km of cable, CATV amplifiers must have automatic gain control (AGC) to compensate for the variations in cable loss. Cascading lower-gain amplifiers would give the highest quality of transmission in terms of noise and intermodulation distortion for a given distance, but will incur higher initial & operating costs.
H. Chan; Mohawk College95 Elements of System Design dB dB dB Signal level (dBmV) Drop input (dBmV) : Tap insert loss (dB) : Standard tap values are (in dB): 8, 11, 14, 17, 20, 23, 26, 29, 32. Tap insertion loss ranges from 0.4 dB to 2.8 dB. The desired signal level to the drop cable is about 10 dBmV.
H. Chan; Mohawk College96 Two-Way Amplifier HPF LPF MHz MHz 5-30 MHz 5-30 MHz Amp Two-way amplifiers permit the cable subscriber to transmit data (e.g. from a modem) to the headend.
H. Chan; Mohawk College97 Cable Modem Click Web ProForums for tutorial on cable modems.Web ProForums
H. Chan; Mohawk College98 Cableless TV Systems Direct Broadcasting Satellites (DBS) enable consumers to receive multi-channel TV signals with a pizza-sized dish and a set-top box. Another alternative is to use a Multichannel Multipoint Distribution System (MMDS) where TV signals are received via a microwave beam at about 2.5 GHz.
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