Microwave Engineering Viva Questions and Answer 2023 | Part 2

This is a 2nd set of Microwave Engineering Viva Questions with Answers

21. What is single stub matching?
Answer: Stub matching is used in order to match any complex load to a transmission line. Stub matching is a group of shorted or opened segments of the line, which are connected in parallel or in series with the line at an appropriate distance from the load.

22. Define Phase velocity and Group velocity?
Answer: The velocity having a wave packet that travels is referred as group velocity. Whereas, the velocity having the phase of a wave that travels is referred as phase velocity.

23. Give the speed of Electromagnetic waves in free space?
Answer: In free space, the electromagnetic waves has a speed same as that of light, c = 3 x 10⁸ meter/second.

24. What are ‘S parameters’ ?
Answer: S-Parameters are used to describe the relationship between different ports, when it becomes especially important to describe a network in terms of amplitude and phase versus frequencies, rather than voltage and current.

It is not possible to have open circuit and short circuit at microwave frequency.

Equipment are not available to measure total voltage and total current for microwave frequency.

Active devices like tunnel diodes, IMPATT diodes, Microwave transistor etc., have a problem of stability at microwave frequency.

25. Explain Lossy Transmission Line.
Answer: Lossy Transmission Line: Each component in a two-port network that represents a lossy waveguide, or cable, through which a microwave signal propagates. Each transmission line parameter is infinitesimally small, and the connections between the elements are not treated as perfect conductors (that is, they have impedance).

26. Explain Normalized Impedance, Admittance and Reflection Coefficient.

Answer:

Reflection Coefficient: It is the ratio of reflected voltage to incident voltage on standing waves.
Normalized Impedance: Impedance divided by the characteristic impedance of the air.
Admittance: the reciprocal of the impedance of a circuit (Y = 1/Z)

27. Define wave guide and working principle.
Answer: It is hollow metallic tube through which waves travel. It is also called as single conductor waveguide since it has no inner conductor like co-axial cable.

28. What are types of Waveguides ?
Answer:

• Rectangular Waveguides
• Circular Waveguide
• Elliptical Waveguide
• Single-ridged waveguide
• Double-ridged waveguide

29. What are various modes of waveguides ?

Answer: There are three modes of waveguides as follow: Transverse Magnetic (TM), Transverse Electric and Magnetic(TEM), Transverse Electric (TE) Modes.

30. What is the dominating mode of Rectangular/Circular Waveguides, TM, TEM, TE modes ?
Answer: Transverse Electric Mode: The TE10 mode is the dominant mode of a rectangular waveguide with a>b, since it has the lowest attenuation of all modes. Either m or n can be zero, but not both.
Transverse Magnetic Mode: For TM modes, m=0 and n=0 are not possible, thus, TM11 is the lowest possible TM mode.

31. Explain passive Microwave Devices .
Answer: Isolator, Circulator,Gyrator.

32. Explain Attenuator, Phase Shifter, Resonator, Power divider and Directional coupler.
Answer: Attenuator: Attenuator is Passive element used to control the amount of microwave power transferred from one point to another on a microwave transmission line. An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform . Attenuator prodes loss or gain less than 1.
Resonator: Microwave Resonators are tunable circuits used in oscillators, amplifiers, wave meters, filters.
At the tuned frequency: Average energy stored in electric field = Average energy stored in magnetic field. Total energy is maximum at resonant frequency.

33. What are the Ferrite devices ?
Answer:

34. What is Impedance Matching ? How it s achieved ?
Answer: : Impedance matching is very desirable with RF transmission lines. Standing waves lead to increased losses and frequency cause the transmitter to malfunction.

• Conjugate Matching The matching of load impedance to generator for maximum transfer of power.
• Z0 Matching Matching a load impedance to the characteristic impedance of transmission line.

35. Explain Microwave Tubes working/operation of Two Cavity Klystron, Magnetron, TWT, Gyrotron etc.
Answer: Microwave Tube: A microwave tube works on the principle of velocity modulation. A velocity modulation principle generally avoids the problem of frequency limitation that often occurs in microwave tubes. The size of microwave tubes should range from 0.25mm to 200mm.
Two cavity klystron: Klystron are a special type of vacuum tubes that finds applications as amplifiers and oscillators at microwave frequencies. Basically the operating principle of Klystron is such that the kinetic energy of a moving electron beam is utilized for amplifying and generating microwave signals. Thus the device used for the amplifying microwave signal is known as Two-cavity Klystron.
Magnetron: The magnetron is a high-powered vacuum tube that works as a self-exited microwave oscillator. Crossed electron and magnetic fields are used in the magnetron to produce the high-power required in radar equipment.

36. Explain Microwave Transistor.
Answer: Compare to small low frequency, microwave transistors need to reduce the device and package parasitic reactance and transit time of the device should be reduced because frequency of oscillation microwave region is quite high. Commonly used semiconductor are silicon at this microwave transistor also Gallium Arsenide is heavily used. Fabrication of Microwave Transistors are done by using Molecular beam epitaxial technique. High Electron Mobility Transistors that is called HEMT can handle upto 100GHz frequency.

37. Explain Microwave diodes like Veracter diode, Tunnel diode, Gun diode, IMPATT diode.
Answer:

38. Explain MMIC.
Answer: In Monolithic Microwave Integrated Circuits all active and passive components are fabricated on single crystal. So, the transmission line, Active components and Passive components everything on single monolithic crystal that is why name is Monolithic Microwave Integrated Circuit.

Applications of MMIC:

Power amplifiers

Mixers

Transmit/Receive module

Cellular telephones

Direct broadcast satellite

Radar transmission and detection

Automobile collision avoidance system

39. Advantages of MMIC
Answer:

• Due to its construction, component density is higher. Hence thousands of devices can easily be fabricated in a single MMIC simultaneously during fabrication.
• Due to higher component density during fabrication, cost of fabrication is lower. Hence overall MMIC cost is less.
• There are minimal mismatches in the MMIC between components.
• The signal delay is minimum due to shorter distances between the components on a MMIC.
• MMICs do not have any wire bond reliability issues.

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40. Manufacturing process of MMIC.
Answer: Designing an MMIC requires extensive use of CAD software for circuit design and optimization, as well as for mask generation. Careful consideration must be given to the circuit design to allow for component variations and tolerance.

Masks are generated after finalization of circuit design. One or more masks are generally required for each processing step.

Active layer in the semiconductor substrate is formed for the active devices. This can be done by ion implantation or by epitaxial techniques. Then, active areas are isolated by etching or additional implantation.

Ohmic contacts are made to the active device areas by alloying a gold or gold/germanium layer onto the substrate. FET gates are then formed with a titanium/platinum/gold compound deposited between the source and drain areas.

Next step is to deposit the first layer of metallization for contacts, transmission lines, inductors and other conducting areas. So, context to the active element, transmission line connecting different parts or different components this are the done in the first layer of metalization.

Then resistors are formed by depositing resistive films, and the dielectric films required for capacitors and overlays are deposited.

A second layer of metallization is done to complete the formation of capacitors and any remaining interconnections.

The final processing steps nvolve the bottom, or back of the substrate. It is first lapped to the required thickness. Via holes are then formed by etching and plating. Via holes provide ground connections to the circuitry on the top side of the substrate, and also a heat dissipation path from the active devices to the ground plane.

After completing the processing, the individual circuits can be cut from the wafer and tested.