Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

Transmission-Line Basics Transmission lines in communication carry: Telephone signals, Computer data in LANs, TV signals in cable TV systems, Signals from a transmitter to an antenna or from an antenna to a receiver. Transmission lines are also circuits.

Transmission-Line Basics The two primary requirements of a transmission line are: The line should introduce minimum attenuation to the signal. The line should not radiate any of the signal as radio energy.

Transmission-Line Basics Characteristic Impedance When the length of transmission line is longer than several wavelengths at the signal frequency, the two parallel conductors of the transmission line appear as a complex impedance. An RF generator connected to a considerable length of transmission line sees an impedance that is a function of the inductance, resistance, and capacitance in the circuit—the characteristic or surge impedance (Z0).

Transmission-Line Basics Velocity Factor The speed of the signal in the transmission line is slower than the speed of a signal in free space. The velocity of propagation of a signal in a cable is less than the velocity of propagation of light in free space by a fraction called the velocity factor (VF). VF = VC/VL

Transmission-Line Basics Time Delay Because the velocity of propagation of a transmission line is less than the velocity of propagation in free space, any line will slow down or delay any signal applied to it. A signal applied at one end of a line appears some time later at the other end of the line. This is called the time delay or transit time. A transmission line used specifically for the purpose of achieving delay is called a delay line.

Transmission-Line Basics The effect of the time delay of a transmission line on signals. (a) Sine wave delay causes a lagging phase shift. (b) Pulse delay.

Transmission-Line Basics Transmission-Line Specifications Attenuation is directly proportional to cable length and increases with frequency. A transmission line is a low-pass filter whose cutoff frequency depends on distributed inductance and capacitance along the line and on length. It is important to use larger, low-loss cables for longer runs despite cost and handling inconvenience. A gain antenna can be used to offset cable loss.

Transmission-Line Basics Attenuation versus length for RG-58A/U coaxial cable. Note that both scales on the graph are logarithmic.

Standing Waves If the load on the line is an antenna, the signal is converted into electromagnetic energy and radiated into space. If the load at the end of the line is an open or a short circuit or has an impedance other than the characteristic impedance of the line, the signal is not fully absorbed by the load.

Standing Waves When a line is not terminated properly, some of the energy is reflected and moves back up the line, toward the generator. This reflected voltage adds to the forward or incident generator voltage and forms a composite voltage that is distributed along the line. The pattern of voltage and its related current constitute what is called a standing wave. Standing waves are not desirable.

Standing Waves A transmission line must be terminated in its characteristic impedance for proper operation.

Standing Waves Calculating the Standing Wave Ratio The magnitude of the standing waves on a transmission line is determined by the ratio of the maximum current to the minimum current, or the ratio of the maximum voltage to the minimum voltage, along the line. These ratios are referred to as the standing wave ratio (SWR).

The Smith Chart The mathematics required to design and analyze transmission lines is complex, whether the line is a physical cable connecting a transceiver to an antenna or is being used as a filter or impedance-matching network. This is because the impedances involved are complex ones, involving both resistive and reactive elements. The impedances are in the familiar rectangular form, R + jX.

The Smith Chart The Smith Chart is a sophisticated graph that permits visual solutions to transmission line calculations. Despite the availability of the computing options today, this format provides a more or less standardized way of viewing and solving transmission-line and related problems. ZO ZIN ZL

The Smith Chart The horizontal axis is the pure resistance or zero-reactance line. The point at the far left end of the line represents zero resistance, and the point at the far right represents infinite resistance. The resistance circles are centered on and pass through this pure resistance line. The circles are all tangent to one another at the infinite resistance point, and the centers of all the circles fall on the resistance line.

The Smith Chart Any point on the outer circle represents a resistance of 0 Ω. The R = 1 circle passes through the exact center of the resistance line and is known as the prime center. Values of pure resistance and the characteristic impedance of transmission line are plotted on this line. The linear scales printed at the bottom of Smith charts are used to find the SWR, dB loss, and reflection coefficient.

The Smith Chart The Smith chart.

Optical Communication

Optical Principles Optical communication systems use light to transmit information from one place to another. Light is a type of electromagnetic radiation like radio waves. Today, infrared light is being used increasingly as the carrier for information in communication systems. The transmission medium is either free space or a light-carrying cable called a fiber-optic cable. Because the frequency of light is extremely high, it can accommodate very high rates of data transmission with excellent reliability.

Optical Principles Light Light, radio waves, and microwaves are all forms of electromagnetic radiation. Light frequencies fall between microwaves and x-rays. The optical spectrum is made up of infrared, visible, and ultraviolet light.

Optical Principles The optical spectrum. (a) Electromagnetic frequency spectrum showing the optical spectrum.

Optical Principles The optical spectrum. (b) Optical spectrum details.

Optical Principles Light Light waves are very short and are usually expressed in nanometers or micrometers. Visible light is in the 400- to 700-nm range. Another unit of measure for light wavelength is the angstrom (Ǻ). One angstrom is equal to 10-10 m.

Optical Principles Light: Speed of Light Light waves travel in a straight line as microwaves do. The speed of light is approximately 300,000,000 m/s, or about 186,000 mi/s, in free space (in air or a vacuum). The speed of light depends upon the medium through which the light passes.

Optical Principles Physical Optics Physical optics refers to the ways that light can be processed. Light can be processed or manipulated in many ways. Lenses are widely used to focus, enlarge, or decrease the size of light waves from some source.

Optical Principles Physical Optics: Reflection The simplest way of manipulating light is to reflect it. When light rays strike a reflective surface, the light waves are thrown back or reflected. By using mirrors, the direction of a light beam can be changed.

Optical Principles Physical Optics: Reflection The law of reflection states that if the light ray strikes a mirror at some angle A from the normal, the reflected light ray will leave the mirror at the same angle B to the normal. In other words, the angle of incidence is equal to the angle of reflection. A light ray from the light source is called an incident ray.

Optical Principles n=c/v Sin A/Sin C=(n2/n1) Illustrating reflection and refraction at the interface of two optical materials.

Optical Principles Physical Optics: Refraction The direction of the light ray can also be changed by refraction, which is the bending of a light ray that occurs when the light rays pass from one medium to another. Refraction occurs when light passes through transparent material such as air, water, and glass. Refraction takes place at the point where two different substances come together. Refraction occurs because light travels at different speeds in different materials.

Optical Principles Examples of the effect of refraction.

Optical Principles Physical Optics: Refraction The amount of refraction of the light of a material is usually expressed in terms of the index of refraction n. This is the ratio of the speed of light in air to the speed of light in the substance. It is also a function of the light wavelength.

Optical Communication Systems Optical communication systems use light as the carrier of the information to be transmitted. The medium may be free space as with radio waves or a special light “pipe” or waveguide known as fiber-optic cable. Using light as a transmission medium provides vastly increased bandwidths.

Optical Communication Systems Light Wave Communication in Free Space An optical communication system consists of: A light source modulated by the signal to be transmitted. A photodetector to pick up the light and convert it back into an electrical signal. An amplifier. A demodulator to recover the original information signal.

Optical Communication Systems Free-space optical communication system.

Optical Communication Systems Basic elements of a fiber-optic communication system.

Optical Communication Systems Benefits of fiber-optic cables over conventional electrical cables.

Fiber-Optic Cables Fiber-Optic Cable Specifications The most important specifications of a fiber-optic cable are: Size Attenuation Bandwidth

Fiber-Optic Cables Fiber-Optic Cable Specifications: Cable Size Fiber-optic cable comes in a variety of sizes and configurations. Size is normally specified as the diameter of the core, and cladding is given in micrometers (μm). Cables come in two common varieties, simplex and duplex. Simplex cable is a single-fiber core cable. In a common duplex cable, two cables are combined within a single outer cladding.

Fiber-Optic Cables Fiber-Optic Cable Specifications: Attenuation The most important specification of a fiber-optic cable is its attenuation. Attenuation refers to the loss of light energy as the light pulse travels from one end of the cable to the other. Absorption refers to how light energy is converted to heat in the core material because of the impurity of the glass or plastic. Scattering refers to the light lost due to light waves entering at the wrong angle and being lost in the cladding because of refraction.