# R. F. Systems EE731 H. Chan; Mohawk College.

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R. F. Systems EE731 H. Chan; Mohawk College

Main Topics Transmission Line Characteristics
Waveguides and Microwave Devices Cable Television Systems Test #1 - Week #4 30 % Final Exam - Week #7 60 % TLM (Assignments) 10 % H. Chan; Mohawk College

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 College

Transmission Line Equivalent Circuit
Zo Zo C G C C C G “Lossy” Line Lossless Line H. Chan; Mohawk College

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, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo. H. Chan; Mohawk College

Formulas for Common Cables
For parallel two-wire line: D d m = momr; e = eoer; mo = 4px10-7 H/m; eo = pF/m For co-axial cable: D d H. Chan; Mohawk College

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 College

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 College

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 College

Reflection Coefficient
The reflection coefficient is defined as: It can also be shown that: Note that when ZL = Zo,  = 0; when ZL = 0,  = -1; and when ZL = open circuit,  = 1. H. Chan; Mohawk College

Standing Waves With a mismatched line, the incident and reflected
Voltage Vmax = Ei + Er Vmin = Ei - Er l 2 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 : H. Chan; Mohawk College

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, Pr = ||2Pi Mismatched Loss, ML = Fraction of power transmitted/absorbed = 1 - ||2 or -10 log(1-||2) dB So, Pt = Pi (1 - ||2) = Pi - Pr H. Chan; Mohawk College

Time-Domain Reflectometry
ZL Transmission Line Oscilloscope Pulse or Step Generator 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. H. Chan; Mohawk College

Typical TDR Waveform Displays
Vr Vi Vr t Vi RL > Zo RL < Zo ZL inductive ZL capacitive H. Chan; Mohawk College

Transmission-Line Input Impedance
The input impedance at a distance l from the load is: When the load is a short circuit, Zi = jZo 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 College

T-L Input Impedance (cont’d)
When the load is an open circuit, Zi = -jZo 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, Zo’, can be used as a matching transformer between a resistive load, ZL, and a line with characteristic impedance, Zo, by choosing: H. Chan; Mohawk College

Transmission Line Summary
or is equivalent to: l < /4 l > /4 or is equivalent to: l > /4 l < /4 /4 = Zo ZL Zo’ l = /4 /4-section Matching Transformer = H. Chan; Mohawk College

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 College

Basic Stripline Structure
Ground Planes W b t er Solid Dielectric Centre Conductor H. Chan; Mohawk College

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 Zo, t, and b. H. Chan; Mohawk College

Microstrip w Circuit Line t r (dielectric) b Ground Plane
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 College

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 College

Microstrip Directional Coupler
2 4 Conductor Lines /4 Dielectric Ground Plane Top View Cross-sectional View 1 3 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 College

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, ZL, return loss, etc. H. Chan; Mohawk College

Hybrid Ring Coupler Input power at port #1 divides evenly
between ports 2 & 4 and none for port 3. 3l/4 4 1 l/4 l/4 Similarly, input at port #2 will divide evenly between ports 1 and 3 and none for port 4. l/4 3 2 One application: circulator. H. Chan; Mohawk College

Microstrip & Stripline Filters
/4 OUT Side-coupled half-wave resonator band-pass filter IN L L L L OUT C C C Conventional low-pass filter H. Chan; Mohawk College

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/cm2 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 College

Waveguides Reasons for using waveguide rather than coaxial cable at microwave frequency: easier to fabricate no solid dielectric and I2R 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 College

E-Field Pattern of TE1 0 Mode
b a g/2 End View Side View TEmn 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 College

TE and TM Modes TEmn mode has the E-field entirely transverse, i.e. perpendicular, to the direction of propagation. TMmn mode has the H-field entirely transverse to the direction of propagation. All TEmn and TMmn modes are theoretically permissible except, in a rectangular waveguide, TMmo or TMon modes are not possible since the magnetic field must form a closed loop. In practice, only the dominant mode, TE10 is used. H. Chan; Mohawk College

Wavelength for TE & TM Modes
Cutoff wavelength: Any signal with l  lc will not propagate down the waveguide. For air-filled waveguide, cutoff freq., fc = c/lc TE10 is called the dominant mode since lc = 2a is the longest wavelength of any mode. Guide wavelength: H. Chan; Mohawk College

Other Formulas for TE & TM Modes
Group velocity: Phase velocity: Wave impedance: Zo = 377 W for air-filled waveguide H. Chan; Mohawk College

Circular/Cylindrical Waveguides
Differences versus rectangular waveguides : lc = 2pr/Bmn where r = waveguide radius, and Bmn is obtained from table of Bessel functions. All TEmn and TMmn modes are supported since m and n subscripts are defined differently. Dominant mode is TE11. Advantages: higher power-handling capacity, lower attenuation for a given cutoff wavelength. Disadvantages: larger and heavier. H. Chan; Mohawk College

Waveguide Terminations
lg/2 Dissipative Vane Short-circuit Sliding Short-Circuit Side View End View 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 College

Attenuators Resistive Flap Max. attenuation when flap
is fully inside. Slot for flap is chosen to be at a non- radiating position. Pi Po Rotary-vane Type Atten.(dB) = 10 log (Pi/Po) = Pi (dBm)-Po(dBm) Max. attenuation when vane is at centre of guide and min. at the side-wall. Pi Po Sliding-vane Type H. Chan; Mohawk College

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 College

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. H. Chan; Mohawk College

Waveguide T-Junctions
2 3 3 1 2 1 E-Plane Junction H-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 College

Hybrid-T Junction It combines E-plane and H-plane junctions.
To RX To antenna 2 3 1 4 Termination Load From TX It combines E-plane and H-plane junctions. Pin at port 1 or 2 will divide between ports 3 and 4. Pin at port 3 or 4 will divide between ports 1 and 2. H. Chan; Mohawk College

Hybrid-T Junction (cont’d)
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 College

Directional Coupler Holes spaced lg/4 allow waves travelling toward
Termination P3 P1 P2 P1 P2 2-hole Coupler Holes spaced lg/4 allow waves travelling toward port 4 to combine. Waves travelling toward port 3, however, will cancel. Therefore, ideally P3 = 0. To broaden frequency response bandwidth, practical couplers would usually have multi holes. H. Chan; Mohawk College

Directional Coupler (cont’d)
Definitions: Coupling Factor, Directivity, where P4(fwd) = power out of aux. arm when power in main arm is forward, and P4(rev) = power out of aux. arm when power in main arm is reversed. Insertion Loss, (I.L.) = 10 log (P1/P2) in dB H. Chan; Mohawk College

Cavity Resonators Resonant wavelength for a rectangular cavity:
b L a For a cylindrical resonator: r L H. Chan; Mohawk College

Cavity Resonators (cont’d)
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 College

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 College

Examples of Ferrite Devices
Isolator Attenuator 2 q 1 3 Differential Phase Shifter 4-port Circulator 4 H. Chan; Mohawk College

Notes On Ferrite Devices
Differential phase shifter - q 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 College

Schottky Barrier Diode
Metal Electrode Contact It’s 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. Semi- conductor Layer SiO2 Dielectric Substrate Metal Electrode Applications: detectors, mixers, and switches. H. Chan; Mohawk College

Junction Capacitance Characteristic
Varactor Diode Cj Co Circuit Symbol V Junction Capacitance Characteristic Varactors operate under reverse-bias conditions. The junction capacitance is: where Vb = barrier potential (0.55 to 0.7 for silicon) and K = constant (often = 1) H. Chan; Mohawk College

Equivalent Circuit for Varactor
The series resistance, Rs, and diode capacitance, Cj, determine the cutoff frequency: Cj Rj Rs The diode quality factor for a given frequency, f, is: H. Chan; Mohawk College

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 College

Parametric Amplifier Circuit
Pump signal (fp) Degenerative Mode: fp = 2fs Nondegenerative mode: L2 Upconversion - fi = fs + fp Downconversion - fi = fs - fp Power gain, G = fi /fs C2 C1 Input signal (fs) Regenerative mode: negative resistance very low noise very high gain fp = fs + fi C3 L1 D1 L3 Signal tank (fs) Idler tank (fi) H. Chan; Mohawk College

PIN Diode PIN diode has an intrinsic region between the P+
RFC R +V P+ C2 C1 I In Out N+ D1 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 College

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 College

Tunnel Diode Heavy doping of the semiconductor material creates
Ls Ip Cj -R A B C Rs V Symbol Equivalent Circuit Vp Vv 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 Vp and Vv. H. Chan; Mohawk College

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 College

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 College

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 l cathode (K) drifts to the anode (A) in bunched formation called domains. Note that there is no p-n junction. K N- A Metallic Electrode N+ Metallic Electrode H. Chan; Mohawk College

Gunn Operating Modes Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect. Transit Time (TT) Mode: operating frequency, fo = vd / l where vd 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 College

Gunn Diode Circuit and Applications
Resonant Cavity Tuning Screw 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. Iris Diode V Gunn diode applications: microwave source for receiver local oscillator, police radars, and microwave communication links. H. Chan; Mohawk College

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 College

IMPATT Diode - A single-drift structure of an IMPATT (impact
avalanche transit time) diode is shown below: - + P+ N N+ l Avalanche Region Drift Region Operating frequency: where vd = drift velocity H. Chan; Mohawk College

Notes On IMPATT Diode The current build-up and the transit time for the current pulse to cross the drift region cause a 180o 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 College

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 College

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 College

SAW Filter Input Output l Centre frequency v = propagation velocity
Comb electrode Absorber Piezoelectric substrate Comb electrodes for exciting and receiving waves are metallic deposit on a piezoelectric substrate. H. Chan; Mohawk College

SAW Resonator Input 1-port resonator Output 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. H. Chan; Mohawk College

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. H. Chan; Mohawk College

Heng Chan ; Mohawk College
Magnetrons It consists of a cylindrical cathode surrounded by the anode with a number of resonant cavities. It’s a crossed-field device since the E-field is perpendicular to the dc magnetic field. Interaction Space Waveguide Output At a critical voltage the electrons from the cathode will just graze the anode. Cavity Coupling Window Anode Cathode Heng Chan ; Mohawk College

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 College

More Notes On Magnetrons
Alternate cavities are strapped (i.e., shorted) so that adjacent resonators are 180o out of phase. This enables only the dominant p-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 College

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 College

Effect of velocity modulation
Two-Cavity Klystron RF In RF Out Control Grid Gap Filament Collector Cathode Drift Region Buncher Cavity Catcher Cavity Anode v Electron Beam Effect of velocity modulation H. Chan; Mohawk College

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 College

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 fo. Stagger tuned klystrons have wider bandwidth at the expense of gain. Can operate as oscillator by positive feedback. H. Chan; Mohawk College

Reflex Klystron Condition for oscillation requires electron transit
Output Anode Cavity Cathode Repeller Filament Electron Beam Vr Condition for oscillation requires electron transit time to be: where n = an integer and T = period of oscillation H. Chan; Mohawk College

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, Vr, can be used to vary output frequency and power. H. Chan; Mohawk College

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 Vr , or by changing the dimensions of the cavity. H. Chan; Mohawk College

Travelling-Wave Tube The TWT is a linear beam device with the magnetic
RF In RF Out Helix Collector Electron Beam Attenuator 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 College

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 College

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 College

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 College

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 College

Parts Of A CATV System Trunk Amplifier Satellite Trunk Cable
Microwave Link Processor Combiner TV Stations FM Radio Headend Distribution Amps Feeder Cable Splitter Drop Cable Cable Box TV Set Line Extender Amps H. Chan; Mohawk College

Heterodyne Processors
Signal Processing Directional Coupler Input From Other Heterodyne Processors Mixer Mixer RF Amp IF Amp LO LO Combiner or Multiplexer 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. H. Chan; Mohawk College

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 College

Cable TV Spectrum Each TV channel occupies a bandwidth of 6 MHz.
Video Carrier Audio Carrier f MHz 54 60 66 Channel 2 Channel 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 College

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 College

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 customer’s premises. Drop cables are limited in length to about 50 m. H. Chan; Mohawk College

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 College

CATV Graphic Symbols -3.5 dB Input Output 2-way splitter -3.5 dB
Tap output -7 dB Directional Coupler -7 dB 26 2-port tap -7 dB 20 4-port tap 4-way splitter -7 dB 14 8-port tap H. Chan; Mohawk College

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 400 50 400 50 MHz MHz Incoming signal “tilt” Equalized output H. Chan; Mohawk College

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 College

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 College

Elements of System Design
Signal level (dBmV) 8 40 39 38.6 32.6 32 27 26.3 100’ 600’ 500’ 29 20 17 -1 dB -6 dB -5 dB Drop input (dBmV) : 10 12.6 10 Tap insert loss (dB) : 0.4 0.6 0.7 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 College

Two-Way Amplifier 50-400 MHz 50-400 MHz HPF Amp HPF 5-30 MHz LPF LPF
Two-way amplifiers permit the cable subscriber to transmit data (e.g. from a modem) to the headend. H. Chan; Mohawk College

Cable Modem Click Web ProForums for tutorial on cable modems.
H. Chan; Mohawk College

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. H. Chan; Mohawk College