Principles of Electronic Communication Systems

1. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr.

2. 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.

3. 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.

4. 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).

5. 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

6. 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 delayor transit time. • A transmission line used specifically for the purpose of achieving delay is called a delay line.

7. 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.

8. 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.

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

10. 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.

11. 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.

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

13. 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).

14. 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.

15. 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

16. 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.

17. 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.

18. The Smith Chart The Smith chart.