Principles of Electronic Communication Systems

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

2. Chapter 16 Microwave Communication

3. Topics Covered in Chapter 16 • 16-1: Microwave Concepts • 16-2: Microwave Lines and Devices • 16-3: Waveguides and Cavity Resonators • 16-4: Microwave Semiconductor Diodes • 16-5: Microwave Tubes • 16-6: Microwave Antennas • 16-7: Microwave Applications

4. 16-1: Microwave Concepts • Microwaves are the ultrahigh, superhigh, and extremely high frequencies directly above the lower frequency ranges where most radio communication now takes place and below the optical frequencies that cover infrared, visible, and ultraviolet light.

5. 16-1: Microwave Concepts Microwave Frequencies and Bands • The practical microwave region is generally considered to extend from 1 to 30 GHz, although frequencies could include up to 300 GHz. • Microwave signals in the 1- to 30-GHz have wavelengths of 30 cm to 1 cm. • The microwave frequency spectrum is divided up into groups of frequencies, or bands. • Frequencies above 40 GHz are referred to as millimeter (mm) waves and those above 300 GHz are in the submillimeter band.

6. 16-1: Microwave Concepts Figure 16-1: Microwave frequency bands.

7. 16-1: Microwave Concepts Benefits of Microwaves • Moving into higher frequency ranges has helped to solve the problem of spectrum crowding. • Today, most new communication services are assigned to the microwave region. • At higher frequencies there is a greater bandwidth available for the transmission of information. • Wide bandwidths make it possible to use various multiplexing techniques to transmit more information. • Transmission of high-speed binary information requires wide bandwidths and these are easily transmitted on microwave frequencies.

8. 16-1: Microwave Concepts Disadvantages of Microwaves • The higher the frequency, the more difficult it becomes to analyze electronic circuits. • At microwave frequencies, conventional components become difficult to implement. • Microwave signals, like light waves, travel in perfectly straight lines. Therefore, communication distance is limited to line-of-sight range. • Microwave signals penetrate the ionosphere, so multiple-hop communication is not possible.

9. 16-1: Microwave Concepts Microwave Communication Systems • Like any other communication system, a microwave communication system uses transmitters, receivers, and antennas. • The same modulation and multiplexing techniques used at lower frequencies are also used in the microwave range. • The RF part of the equipment, however, is physically different because of the special circuits and components that are used to implement the components.

10. 16-1: Microwave Concepts Microwave Communication Systems: Transmitters • Like any other transmitter, a microwave transmitter starts with a carrier generator and a series of amplifiers. • It also includes a modulator followed by more stages of power amplification. • The final power amplifier applies the signal to the transmission line and antenna. • A transmitter arrangement could have a mixer used to up-convert an initial carrier signal with or without modulation to the final microwave frequency.

11. 16-1: Microwave Concepts Figure 16-3: Microwave transmitters. (a) Microwave transmitter using frequency multipliers to reach the microwave frequency. The shaded stages operate in the microwave region.

12. 16-1: Microwave Concepts Figure 16-3: Microwave transmitters. (b) Microwave transmitter using up-conversion with a mixer to achieve an output in the microwave range.

13. 16-1: Microwave Concepts Microwave Communication Systems: Receivers • Microwave receivers, like low-frequency receivers, are the superheterodyne type. • Their front ends are made up of microwave components. • Most receivers use double conversion.

14. 16-1: Microwave Concepts Microwave Communication Systems: Receivers • The antenna is connected to a tuned circuit, which could be a cavity resonator or microstrip or stripline tuned circuit. • The signal is then applied to a special RF amplifier known as a low-noise amplifier (LNA). • Another tuned circuit connects the amplified input signal to the mixer. • The local oscillator signal is applied to the mixer. • The mixer output is usually in the UHF or VHF range. • The remainder of the receiver is typical of other superheterodynes.

15. 16-1: Microwave Concepts Figure 16-4: A microwave receiver. The shaded areas denote microwave circuits.

16. 16-1: Microwave Concepts Microwave Communication Systems: Transmission Lines • Coaxial cable, most commonly used in lower-frequency communication has very high attenuation at microwave frequencies and conventional cable is unsuitable for carrying microwave signals. • Special microwave coaxial cable that can be used on bands L, S, and C is made of hard tubing. This low-loss coaxial cable is known as hard line cable. • At higher microwave frequencies, a special hollow rectangular or circular pipe called waveguide is used for the transmission line.

17. 16-1: Microwave Concepts Microwave Communication Systems: Antennas • At low microwave frequencies, standard antenna types, including the simple dipole and one-quarter wavelength vertical antenna, are still used. • At these frequencies antennas are very small; for example, a half-wave dipole at 2 GHz is about 3 in. • At higher microwave frequencies, special antennas are generally used.

18. 16-2: Microwave Lines and Devices • Although vacuum and microwave tubes like the klystron and magnetron are still used, most microwave systems use transistor amplifiers. • Special geometries are used to make bipolar transistors that provide voltage and power gain at frequencies up to 10 GHz. • Microwave FET transistors have also been created. • Monolithic microwave integrated circuits (MMICs) are widely used.

19. 16-2: Microwave Lines and Devices Microstrip Tuned Circuits • At higher frequencies, standard techniques for implementing lumped components such as coils and capacitors are not possible. • At microwave frequencies, transmission lines, specifically microstrip, are used. • Microstrip is preferred for reactive circuits at the higher frequencies because it is simpler and less expensive than stripline. • Stripline is used where shielding is necessary.

20. 16-2: Microwave Lines and Devices Figure 16-6: Microstrip transmission line used for reactive circuits. (a) Perspective view. (b) Edge or end view. (c) Side view (open line). (d) Side view (shorted line).

21. 16-2: Microwave Lines and Devices Figure 16-7: Equivalent circuits of open and shorted microstrip lines.

22. 16-2: Microwave Lines and Devices Microstrip Tuned Circuits • An important characteristic of microstrip is its impedance. • The characteristic impedance of a transmission line depends on its physical characteristics. • The dielectric constant of the insulating material is also a factor. • Most characteristic impedances are less than 100 Ω. • One-quarter wavelength transmission line can be used to make one type of component look like another.

23. 16-2: Microwave Lines and Devices Figure 16-8: How a one-quarter wavelength microstrip can transform impedances and reactances.

24. 16-2: Microwave Lines and Devices Microstrip Tuned Circuits • Microstrip can also be used to realize coupling from one circuit. • One microstrip line is simply placed parallel to another segment of microstrip. • The degree of coupling between the two depends on the distance of separation and the length of the parallel segment. • The closer the spacing and the longer the parallel run, the greater the coupling. • Microstrip patterns are made directly onto printed-circuit boards.

25. 16-2: Microwave Lines and Devices Microstrip Tuned Circuits • A special form of microstrip is the hybrid ring. • The unique operation of the hybrid ring makes it very useful for splitting signals or combining them. • Microstrip can be used to create almost any tuned circuit necessary in an amplifier, including resonant circuits, filters, and impedance-matching networks.

26. 16-2: Microwave Lines and Devices Figure 16-12: A microstrip hybrid ring.

27. 16-2: Microwave Lines and Devices Microwave Transistors • The primary differences between standard lower-frequency transistors and microwave types are internal geometry and packaging. • To reduce internal inductances and capacitances of transistor elements, special chip configurations known as geometries are used. • Geometries permit the transistor to operate at higher power levels and at the same time minimize distributed and stray inductances and capacitances.

28. 16-2: Microwave Lines and Devices Microwave Transistors • The GaAs MESFET, a type of JFET using a Schottky barrier junction, can operate at frequencies above 5 GHz. • A high electron mobility transistor (HEMT) is a variant of the MESFET and extends the range beyond 20 GHz by adding an extra layer of semiconductor material such as AlGaAs. • A popular device known as a heterojunction bipolar transistor (HBT) is making even higher-frequency amplification possible in discrete form and in integrated circuits.

29. 16-2: Microwave Lines and Devices Figure 16-14: Microwave transistors. (a) and (b) Low-power small signal. (c) FET power. (d) NPN bipolar power.

30. 16-2: Microwave Lines and Devices Small-Signal Amplifiers • A small-signal microwave amplifier can be made up of a single transistor or multiple transistors combined with a biasing circuit and any microstrip circuits or components as required. • Most microwave amplifiers are of the tuned variety. • Another type of small-signal microwave amplifier is a multistage integrated circuit, a variety of MMIC.

31. 16-2: Microwave Lines and Devices Small-Signal Amplifiers: Transistor Amplifiers • A low-noise transistor with a gain of about 10 to 25 dB is typically used as a microwave amplifier. • Most microwave amplifiers are designed to have input and output impedances of 50 Ω. • The transistor is biased into the linear region for class A operation. • RFCs are used in the supply leads to keep the RF out of the supply and to prevent feedback paths that can cause oscillation and instability in multistage circuits. • Ferrite beads (FB) are used in the collector supply lead for further decoupling.

32. 16-2: Microwave Lines and Devices Small-Signal Amplifiers: MMIC Amplifiers • A common monolithic microwave integrated circuit (MMIC) amplifier is one that incorporates two or more stages of FET or bipolar transistors made on a common chip to form a multistage amplifier. • The chip also incorporates resistors for biasing and small bypass capacitors. • Physically, these devices look like transistors. • Another form of MMIC is the hybrid circuit, which combines an amplifier IC connected to microstrip circuits and discrete components.

33. 16-2: Microwave Lines and Devices Figure 16-15: A single-stage class A RF microwave amplifier.

34. 16-2: Microwave Lines and Devices Small-Signal Amplifiers: Power Amplifiers • A typical class A microwave power amplifier is designed with microstrip lines used for impedance matching and tuning. • Input and output impedances are 50 Ω. • Typical power-supply voltages are 12, 24, and 28 volts. • Most power amplifiers obtain their bias from constant-current sources. • A single-stage FET power amplifier can achieve a power output of 100 W in the high UHF and low microwave region.

35. 16-2: Microwave Lines and Devices Figure 16-16: A class A microwave power amplifier.

36. 16-2: Microwave Lines and Devices Figure 16-17: A constant-current bias supply for a linear power amplifier.

37. 16-2: Microwave Lines and Devices Figure 16-18: An FET power amplifier.

38. 16-3: Waveguides and Cavity Resonators Waveguides • Most microwave energy transmission above 6 GHz is handled by waveguides. • Waveguides are hollow metal conducting pipes designed to carry and constrain the electromagnetic waves of a microwave signal. • Most waveguides are rectangular. • Waveguides are made from copper, aluminum or brass. • Often the insides of waveguides are plated with silver to reduce resistance and transmission losses.

39. 16-3: Waveguides and Cavity Resonators Waveguides: Signal Injection and Extraction • A microwave signal to be carried by a waveguide is introduced into one end of the waveguide with an antennalike probe. • The probe creates an electromagnetic wave that propagates through the waveguide. • The electric and magnetic fields associated with the signal bounce off the inside walls back and forth as the signal progresses down the waveguide. • The waveguide totally contains the signal so that none escapes by radiation.

40. 16-3: Waveguides and Cavity Resonators Figure 16-19: Injecting a sine wave into a waveguide and extracting a signal.

41. 16-3: Waveguides and Cavity Resonators Waveguides: Signal Injection and Extraction • Probes and loops can be used to extract a signal from a waveguide. • When the signal strikes a probe or a loop, a signal is induced which can then be fed to other circuitry through a short coaxial cable.

42. 16-3: Waveguides and Cavity Resonators Waveguides: Waveguide Size and Frequency. • The frequency of operation of a waveguide is determined by the inside width of the pipe (dimension (a) in the figure following). • This dimension is usually made equal to one-half wavelength, a bit below the lowest frequency of operation. This frequency is known as the waveguide cutoff frequency. • At its cutoff frequency and below, a waveguide will not transmit energy. • Above the cutoff frequency, a waveguide will propagate electromagnetic energy.

43. 16-3: Waveguides and Cavity Resonators Figure 16-20: The dimensions of a waveguide determine its operating frequency range.

44. 16-3: Waveguides and Cavity Resonators Waveguides: Signal Propagation • In a waveguide, when the electric field is at a right angle to the direction of wave propagation, it is called a transverse electric (TE) field. • Whenthe magnetic field is transverse to the direction of propagation, it is called a transverse magnetic (TM) field.

45. 16-3: Waveguides and Cavity Resonators Waveguides: Signal Propagation • The angles of incidence and reflection depend on the operating frequency. • At high frequencies, the angle is large and the path between the opposite walls is relatively long. • As the operating frequency decreases, the angle also decreases and the path between the sides shortens. • When the operating frequency reaches the cutoff frequency of the waveguide, the signal bounces back and forth between the sidewalls of the waveguide. No energy is propagated.

46. 16-3: Waveguides and Cavity Resonators Figure 16-22: Wave paths in a waveguide at various frequencies. • High frequency. • Medium frequency. • Low frequency. • Cutoff frequency.

47. 16-3: Waveguides and Cavity Resonators Waveguides: Signal Propagation • When a microwave signal is launched into a waveguide by a probe or loop, electric and magnetic fields are created in various patterns depending upon the method of energy coupling, frequency of operation, and size of waveguide. • The pattern of the electromagnetic fields within a waveguide takes many forms. Each form is called an operating mode.

48. 16-3: Waveguides and Cavity Resonators Figure 16-23: Electric (E ) and magnetic (H) fields in a rectangular waveguide.

49. 16-3: Waveguides and Cavity Resonators Waveguide Hardware and Accessories • Waveguides have a variety of special parts, such as couplers, turns, joints, rotary connections, and terminations. • Most waveguides and their fittings are precision-made so that the dimensions match perfectly. • A choke jointis used to connect two sections of waveguide. It consists of two flanges connected to the waveguide at the center. • A T section or T junction is used to split or combine two or more sources of microwave power.

50. 16-3: Waveguides and Cavity Resonators Figure 16-25: A choke joint permits sections of waveguide to be interconnected with minimum loss and radiation.