MICROWAVE ENGINEERING LAB VIVA QUESTIONS AND ANSWERS

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Stella Norma Watts
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1 MICROWAVE ENGINEERING LAB VIVA QUESTIONS AND ANSWERS. Why can t conventional tubes be used at microwave frequencies? Conventional tubes can t be used at microwave frequencies because of transit time effect. Lead inductance and inter electrode capacitance of the devices will finally limit the output which may even be zero. 2. What is transit time? It is the time taken by the electrons to travel from cathode to anode. 3. What is the operating principle of reflex klystron? It works on the principle of velocity modul ation and current modulation. 4. What is velocity and current modulation in a reflex klystron? Variation in velocities of the electrons in the electron beam is called velocity modulation; variation in the current density of the electron beam is referred to as current modulation. 5. How does bunching occur in a reflex klystron? A reference electron passing the gap when the gap voltage is zero travels with no change in velocity. An electron leaving the gap earlier during slightly positive voltage would travel further into repeller space and hence would take longer time then the reference electron to return to the gap. An electron leaving the gap later will face slightly negative voltage and gets retarded. So it returns back after a shorter travel in the repeller space. Thus all the electrons would arrive back to the gap in bunches. Bunching around reference electron takes place once per cycle of RF oscillations. 6. Why do different modes of operation exist for a reflex klystron? There are several combinations of repeller voltage and anode voltage that provide favorable conditions for bunching. Accordingly there are several modes of operation, expressed by N + ¾ where N is an integer.

2 7. What modes are generally used in a reflex klystron? ¾ and 2 ¾ are the most commonly used modes in a practical reflex klystron. 8. What is the operating frequency and power output of a reflex klystron? Frequency range: 4GHz 2GHz Output power: maximum 3W in X-band to mw at 22GHz 9. What is the maximum theoretical efficiency of the reflex klystron? 22.7% for ¾ mode.. What is mechanical tuning and electronic tuning in a reflex klystron? Variation in frequency of resonance of cavity by varying its dimension by a mechanical method like adjusting screws is called mechanical tuning. Variation of frequency by adjusting repeller voltage is called electronic tuning.. What is the range of frequency variation of reflex klystron in the X-band by the above two methods? Mechanical tuning: +2MHz. Electronic tuning: +8MHz 2. What are applications of reflex klystron? They are used as ) signal source in microwave generators. 2) Local oscillators in receivers. 3) Pump oscillators in parametric amplifiers. 3. Leaving reflex klystron, what other devices are used as microwave signal sources? ) Magnetron oscillator 2) Backward wave oscillator 3) Gunn oscillator 4) IMPATT diode oscillator 5) TRAPPAT diode oscillator 4. Is Gunn diode a p-n junction? No. It isn t. 5. What is Gunn Effect? When the voltage gradient across a slice of GaAs exceeds about 33V/cm, negative resistance will develop and oscillations occur if the slice is connected to a suitable tank circuit. 6. What are the materials used in the construction of Gunn diode? GaAs (Gallium Arsenide), InP (Indium Phosphate) doped in Si.

3 7. What is negative resistance? A device is said to have negative resistance characteristics if, during its operation an increase in the voltage across it leads to decrease in its current. 8. Can any negative resistance device be used for amplification? Yes. 9. What are the other names of Gunn diode and Gunn oscillator? Transferred electron device and transferred electron oscillator. 2. Draw the equivalent circuit of an X-band GaAs Gunn diode. Equivalent circuit is a parallel combination of resistance (~Ω) and a capacitance (~.6pF). 2. Mention the characteristics of an X-band GaAs Gunn diode. Typical values are: Bias voltage: 9V Output power: 3mW Operating current: 95mA Efficiency: 3.5% 22. Can a Gunn diode be used as an amplifier? Yes. 23. Mention some applications of Gunn diode. It is used as ) low and medium power oscillators in microwave receivers. 2) Pump sources in parametric amplifiers 3) Power output oscillators in radars 24. Expand the terms IMPATT, TRAPATT, & PIN referring to the types of diodes. IMPATT Impact Avalanche & Transit Time Diode TRAPATT Trapped Plasma Avalanche Triggered Transit Diode PIN Positive Intrinsic Negative 25. What is the use of crystal diode in a microwave bench setup? To detect the microwave signal picked up by the probe and to provide the rectified current. 26. What is a square law device? A device whose output current is proportional to the square of the input voltage applied. 27. Mention conditions for a crystal diode to function as a square law device. The rectified current should be less than 2µA or input power should be less than mw.

4 28. Name the types of crystal diodes used. Point contact type, barrier type & back type are commonly used types of crystal diodes. 29. Draw the equivalent circuit of a microwave crystal detector. Equivalent circuit consists of a parallel combination of: ) Lead inductance (L) in series with lead resistance (R) 2) Junction capacitance (C) 3. Define current sensitivity and voltage sensitivity of a crystal detector. Current sensitivity is the maximum short circuit current for unit input power (short circuit power/input power). Voltage sensitivity is the maximum open circuit voltage for unit input power (open circuit power/input power). 3. Why is a tuning element provided with a crystal mount? To eliminate reflections from crystal mount. 32. Apart from signal detection what are the other applications of crystal diode? Used as ) frequency mixers in superhetrodyne receivers 2) Harmonic generators 3) frequency converters 33. Which crystal diode is commonly used for X-band operations? IN Give the relation between output current and input voltage for a crystal diode. I = KE P where K is a constant due to detector and probe coupling. P is a constant depending on the individual crystal & magnitude of operating voltage. P = 2 for square law operation. 35. What is a wave guide? It is a hollow metallic pipe of either rectangular or circular cross section, used to guide electromagnetic waves at microwave frequencies. 36. Define guide wavelength. It is the distance between two identical points on standing waves in a waveguide. 37. Define cutoff wavelength for a waveguide. It is the longest wavelength beyond which there is no propagation of EM waves within the waveguide.

5 38. Define dominant mode with respect to a waveguide. Mention dominant mode for rectangular and circular waveguides. The mode with the least cutoff frequency is called dominant mode. For rectangular waveguide the dominant mode is TE & for circular waveguide it s TE. 39. If the broader dimension of a rectangular waveguide is 2.2cms, what is the cutoff frequency and wavelength for dominant mode? For rectangular waveguide the dominant mode is TE. The cutoff wavelength λc = 2a = 4.4cms. The cutoff frequency fc = c/ λc = 6.8GHz. 4. Write the relation between guide wavelength, cutoff wavelength and free space wavelength. / λo 2 = / λc 2 + / λg 2 or λo = λc λg/ (λg 2 + λc 2 ) /2 Where λo free space wavelength, λc - cutoff wavelength, λg guide wavelength 4. What are the various power losses in a waveguide? ) Losses in guide walls 2) losses in dielectric (if any other than air) 42. How are modes excited in a rectangular waveguide? Modes are excited using a probe, a hole or a loop. Commonly used techniques are electric excitation, magnetic excitation and aperture excitation. 43. What is the preferred frequency range for waveguide operation? Why? Frequency range: 3GHz GHz as the dimensions of the waveguide are convenient in size, easy to fabricate. 44. Is a waveguide equivalent to a high pass filter or a low pass filter? Explain Waveguide is equivalent to a high pass filter. It passes frequencies only above the cutoff frequency fc, but attenuates those below fc. 45. What is TE mode and TM mode? TE mode is the one in which the electric field is perpendicular the direction of propagation of the EM wave. TM mode is the one in which the magnetic field is perpendicular the direction of propagation of the EM wave. 46. In the notation TEmn and TMmn what do m and n represent? m denotes the half wave periodicity in the x direction while n represents the half wave periodicity in the y direction; assuming the wave is traveling in the z direction.

6 47. Define wave impedance and characteristic impedance. Wave impedance Zz looking in the z direction is the ratio of the transverse electric field strength to the total magnetic field strength. Characteristic impedance Zo of a line is the impedance offered by the line of infinite length. 48. Write the formula for wave impedance of a TE and TM wave considering propagation in z direction. TE mode: Zz (TE) = η/ ( (fc/f) 2 ) /2 TM mode: Zz (TM) = η/ ( (fc/f) 2 ) /2 Where f = signal frequency and fc = cutoff frequency. 49. What are the applications of waveguide bends, corners and twists? Waveguide bends are used to change the direction of energy flow at microwave frequencies. Corners are used to change are used to change the direction of energy flow at larger wavelengths. Twists are employed to change the plane of polarization of the EM wave. 5. What is a mitered corner? A 9 corner would produce large reflections, so a part of it is cut which is called mitered corner. 5. Define VSWR. VSWR (voltage standing wave ratio) is the ratio of the maximum field strength to the minimum field strength of standing waves. 52. What is reflection coefficient? It is the ratio of reflected voltage to incident voltage on standing waves. 53. Write the relation between VSWR and reflection coefficient. S = ( + K)/ ( K) Where S standing wave ratio; K Reflection coefficient. 54. Write the values of VSWR for open circuit, short circuit & matched load. Open/closed circuit: VSWR = Matched load: VSWR = 55. What is the range of VSWR and inverse VSWR? VSWR: -. Inverse VSWR: What is a matched load? A load that offers the characteristic impedance to a transmission line is called a matched load.

7 57. Define attenuation. Attenuation is the reduction in power level, generally expressed in db. For any network attenuation α = log (p/p2) db where p2 is the output power and p is the input power to the network. 58. Define insertion loss. Insertion loss is the attenuation introduced due to addition of any component in any circuit. If p is the output power without the component and p2 is the output power with the component, then the insertion loss in db is given by log (p/p2) db. 59. Mention the different types of attenuators. ) Fixed 2) Step adjustable 3) Continuously variable attenuator 6. Why are attenuators used? To ) control the microwave power in the circuit 2) Reduce the power load to a desired value. 6. Define directivity and coupling factor of a directional coupler. Directivity = log (p3/p4) db. Indicates how well the forward traveling wave in main guide couples only to desired terminals of secondary guide. Coupling factor = log (p/p3) db. P4 P3 SECONDARY LINE P P2 MAIN LINE 62. What are the properties of directional coupler? Power fed to port is coupled to port 2 and port 3 but not to port 4. Power fed to port 2 is coupled to port and port 4 but not to port 3. The degree of coupling between ports, 3 and ports 2, 4 depends on the structure of the unit. There is a free transfer of power between ports and Mention the different types of directional couplers. ) Two hole coupler 2) multi hole coupler 3) sloth type coupler 4) Bethe type coupler 5) cross waveguide coupler.

8 64. What should be the value of directivity? High or low? Higher the value of directivity higher is the quality of the directional coupler. For a perfect directional coupler directivity would be infinite. 65. Mention some applications of directional coupler. ) Power measurements 2) used in reflectometer 3) as a fixed attenuator 4) to arrange balanced duplexer 66. What is Uniline? A uniline or isolator is a 2 port device that allows free transmission of power in forward direction and no transmission in the opposite direction. 67. Why is an isolator used in a microwave bench? Isolator is connected between load and source. It allows free transmission of power from source to load. But if there is any mismatch and reflections arise, these reflections are prevented to travel to the source side. 68. What are the types of ferrite isolators? ) Faraday rotation isolators for power up to a few hundred watts. 2) Resonant absorption isolators for higher power. 69. What is Faraday rotation? Rotation of polarized wave traveling through ferrite. 7. What are the commonly used ferrite materials for isolators? ) Yitrium iron garnet (YIG) 2) Manganese ferrite 3) Zinc ferrite 7. What is meant by line width of ferrite? It is the range of magnetic field strengths over which absorption will take place. It is defined between the half power points for absorption. 72. What is Curie temperature? It is the temperature at which a magnetic material looses its magnetic properties. 73. What is a circulator? It is a ferrite multiport (generally 4) device where each port is connected only to the next clockwise terminal.

9 74. What are the main applications of circulators? ) In radar work to isolate transmitter and the receiver connected to the same antenna 2) To isolate input and output in parametric amplifier. 75. Mention some types of circulators. Faraday rotation circulator (4 port), Wye circulator ( 3 port) How can you construct a 4 port circulator with magic tees and a gyrator? To arrange a four port circulator we need two magic tees and a gyrator such that it provides a phase shift of 8 in the direction of arrow only. A wave incident on port splits equally into 2 in phase waves in arms b, d. They reach ports a and c of magic tee, add up and appear as output at port 2 (adder arms). A wave incident on port 2 splits into two in phase equal amplitude waves at a and c. They arrive at b and d of magic tee 2 with a phase difference of 8 and emerge out of the subtractor arm (port 3). Similar analysis shows that wave incident on port 3 comes out of port 4 and that incident on port 4 emerges out of port only. 4 a Magic Tee E arm c b 3 E arm d 2 H arm GYRATOR π Magic Tee 2 H arm

10 77. If an isolator is not available explain how a 3 port circulator can be used to serve the function of isolator? SOURCE 2 3 PORT CIRCULATOR LOAD 3 MATCHED TERMINATION There will free flow of power from source to load (port to 2). Any reflected energy due to mismatch from port 2 will be absorbed by matched termination at port Why is a matched termination needed at port 3 in the above case? If matched termination is not connected reflections from port will be incident on port 3. Any mismatch at this port will cause reflections which will be transmitted to source side. To avoid this, port 3 is connected with matched load. 79. Is it necessary to connect source to port of a circulator only always? No. If source is connected to port 2,then connect load at port 3 and matched load to port, or connect source to port 3, load to port and matched termination to port What are scattering parameters? They are the relations between reflection coefficients and transmission coefficients of a microwave network. 8. What is scattering matrix? At microwave frequencies the measurable quantities are power, reflection coefficient, transmission coefficient and standing wave ratio. A linear relation between input and output of a microwave network in terms of precisely measurable parameters is called scattering description (or scattering matrix is a matrix made up of scattering coefficients).

11 82. Write the S matrix for magic tee. S = Write S matrix for a 4 port ideal circulator. S = 84. Write the S matrix for an ideal directional coupler. S = j j j j Choosing a reference plane such that α is positive real and β is positive imaginary. 85. What is magic tee? It is a combination of E plane and H plane tee. It is a 4 port device.

12 86. What are the properties of magic tee? E ARM ARM 4 ARM ARM 2 ARM 3 H ARM Power fed to arm 3 (H arm) divides equally between arms and 2. The electric field in the 2 arms will be in phase. No coupling to arm 4. Similarly power fed to arm 4 is not coupled to arm 3. It is divided equally between arms & 2 but the electric fields in these differ in phase by 8. Powers fed in arms & 2 are added in arm 3 (H arm) and subtracted in arm 4 ( E arm). 87. What are the applications of magic tee? Used as ) An antenna duplexer, to use as a single antenna for transmission and reception 2) balanced microwave mixer 3) frequency discriminator 4) impedance measuring bridges. REFER- ENCE ZR MATCHED DETECTOR ZX RX ANTENNA TX SOURCE MATCHED LOAD Adjust the reference impedance till there is no signal in the detector. Use the equation (Zr )/(Zr + ) = (zx )/(Zx + ) to find Zx, because at balance, reflection coefficients of reference and unknown impedance are the same.

13 88. What is pyramidal horn? It is a EH horn where both sides of a waveguide are flared. 89. Mention different types of horn antennas. ) Sectoral H plane 2) Sectoral E plane 3) Pyramidal plane 4) conical plane 5) biconical plane 9. What is half power beam width? The angular separation between half power points in the radiation pattern of an antenna. 9. Define directivity if an antenna. It is the ratio of the maximum radiation intensity to the average radiation intensity of an antenna. 92. How are far and near fields identical for a horn antenna? Near field is up to a distance 2D 2 /λ from the antenna. Any point at a distance greater than this is considered to be in the far field, where D largest dimension of horn mouth 93. What are the other names for the above fields? Near field Fresnel region Far field Fraunhofer region 94. Define antenna. Antenna is defined as the transition region between guided and free space wave. 95. What are the devices used for microwave power measurements/ )Bolometer 2)Calorimeter 96. What are the types of Bolometer elements? Thermisters and Barreters. 97. What is the characteristic of Barreter bolometer? Barrators have positive temperature coefficient of resistance.

14 98. What is the principle of calorimetry? Fluid, generally water absorbs the microwave power, resulting in temperature rise. Noting this temperature rise, power is calculated. 99. Which method is preferred for low power measurements? Bolometer.. When is calorimeter used for power measurements? For medium power measurement.. Mention the main application of IMPATT diode. Used as transmitter for microwave communication systems. 2. Mention the application of TRAPATT diode. Used as pulsed transmitter in s-band phased array radar. 3. What is the S-band frequency range? As per IEEE microwave band designation s-band range: 2-4 GHz 4. where is BARITT diode used? As local oscillator in radar receiver. 5. npn structure is preferred to pnp in high frequency transistor. Why? Mobility of electrons is greater than that of holes 6. What are the commonly used ways of fabricating microwave transistors? Diffusion and ion implantation. 7. What are the applications of tunnel diode? Used as microwave oscillator and amplifier. 8. What is cavity resonator? It is a metallic enclosure that confines the EM energy. 9. What is quality factor of a cavity resonator? Measure of frequency selectivity of the resonator.. The power measured in auxillary arm of a 2 db directional coupler is mw.what is the power input to the main arm? mw.

St.MARTIN S ENGINEERING COLLEGE Dhulapally, Secunderabad

St.MARTIN S ENGINEERING COLLEGE Dhulapally, Secunderabad 500014. Department of Electronics and Communication Engineering SUB: MICROWAVE ENGINEERING SECTION: ECE IV A & B NAME OF THE FACULTY: S RAVI KUMAR,T.SUDHEER