Microwave Engineering Notes Questions and Answers part 1

Tuesday, 12 November 2013

Microwave Engineering Notes Questions and Answers part 1


Here i am sharing 2 marks microwave engineering notes.

1. Define Microwave.

Microwaves are generally described as electromagnetic waves with frequencies that range from approximately 1GHz to 1000 GHz. Therefore, microwave signals, because of their inherently high frequencies, have relatively short wavelengths, hence the name “micro” waves.

2. Define a microwave junction.

The point of interconnection of two or more microwave devices is called microwave junction.

3. Define scattering matrix.

In a microwave junction there is intersection of three or more Components. There will be an output port; in addition there may be reflection from the junction of other ports. Totally there may be many combinations, these are represented easily using a matrix called S matrix.

4. What are the properties of s-matrix?

1. It possess symmetric property Sij =Sji

2. It possess unitary property

   [S][S]*= [I]

3. Under perfect matched conditions, the diagonal elements of [S] are zero.

5. Write the unitary property for a lossless junction.

          For any lossless network the sum of the products of each term of any one row or of any column of the S-matrix multiplied by its complex conjugate is unity.

6. Define tee-junction.

          In microwave circuits a waveguide or co-axial line with three independent ports is commonly referred to as a tee-junction.

7. What is E-plane Tee?

          An E-plane tee is a waveguide tee in which the axis of its side arm is parallel to the E-field of the main guide.

8. What is H-plane Tee?

          An H-plane tee is a waveguide tee in which the axis of its side arm is shunting the E-field or parallel to the H-field of the main guide.

9. Define difference arm.

          In E-plane tee, the power out of port 3 is proportional to the difference between instantaneous powers entering from port 1 and port 2. Therefore, this third port is called as difference arm.

10. Define difference arm.

          In H-plane tee, if two input waves are fed into port 1 and port 2 of the collinear arm, the output wave at port 3 will be in phase and additive. Because of this, this third port is called as sum arm.

11. What do you mean by hybrid junction?

          A hybrid junction is a four port network in which a signal incident on any one of the ports divides between two output ports with the remaining port being isolated.

12. Why bends are used?

          Bends are used to alter the direction of propagation in a waveguide system.

13. What is hybrid ring?

Hybrid ring consists of an annular line of proper electrical length to sustain standing waves, to which four arms are connected at proper intervals by means of series or parallel junctions.

14. Define isolator.

          An isolator is a two port non-reciprocal device which produces a minimum attenuation to wave in one direction and very high attenuation in opposite direction.

15. Why isolators are called uniline?

An ideal isolator completely absorbs the power for propagation in one direction and provides lossless transmission in the opposite direction. Thus isolators are called uniline.

16. Give the applications of directional coupler

1. Unidirectional power measurement

2. SWR measurement

3. Unidirectional wave launching

4. Reflectometer

5. Balanced duplexer

17. Give a note on directional couplers.

          Directional couplers are transmission line devices that couple together two circuits in one direction, while providing a great degree of isolation in the opposite direction.

18. What are the different types of Directional coupler?

1. Two hole directional coupler

2. Be the hole directional coupler

3. Four hole directional coupler

19. Define Coupling factor.

          The coupling factor of a directional coupler is defined as the ratio of the incident power Pi to the forward power Pf measured in dB.

20. Define directivity of directional coupler.

          The directivity of a directional coupler is defined as the ratio of the forward power Pf to the back power Pb measured in dB.

21. What is Faraday’s rotation law?

If a circularly polarized wave is made to pass through a ferrite rod which has been influenced by an axial magnetic field B, then the axis of polarization gets tilted in clockwise direction and amount of tilt depends upon the strength of magnetic field and geometry of the ferrite.

22. List two microwave devices using Faraday rotation principles

Isolator, Circulator

23. Give some coupling parameters of directional coupler?

Coupling coefficient, Directivity, Insertion loss, Isolation

24. What are ferrites and give its properties?

Ferrites are ceramic like materials. These are made by sintering a mixture of metallic oxides.

Properties:

1. Specific resistivities may be used as much as 1014 greater than that of metals.

2. Dielectric constants around 10to 15 or greater.

3. Relative permeability is 1000.

25. Give some examples of ferrite devices?

        Isolator, Circulator, Phase shifters, Modulators, Power limiters.

26. What are the advantages of microwave transistors?

          Microwave transistors are miniaturized designs to reduce device and package parasitic capacitances and inductances and to overcome the finite transit time of the charge carriers in the semiconductor materials.

27. What is bipolar transistor?

          Bipolar is three semiconductor region structures where charge carriers of both negative and positive polarities are involved in transistor operation.

28. Write the applications of bipolar transistors.

Bipolar transistors are suitable for oscillator and power amplifier applications in addition to small signal amplifiers.

29. What is MESFET?

If the field effect transistor is constructed with metal semiconductor schottky barrier diode, the device is called metal-semiconductor field effect transistor.

30. What is negative resistance?

Negative resistance is defined as that property of a device which causes the current through it to be 180 degree out of phase with the voltage across it.

31.  Define Gunn Effect.

Guneffect was first observed by GUNN in n_type GaAs bulk diode.according to GUNN,above some critical voltage corresponding to an electric field of 2000-4000v/cm,the current in every specimen became a fluctuating fuction of time.The frequency of oscillation was determined mainly by the specimen and not by the external circuit.

32. What are the various modes of operation of Gunn diode?

          1. Gunn oscillation mode.

          2. Stable amplification mode.

          3. LSA oscillation mode.

          4. Bias circuit oscillation mode.

33. Mention the name of domain modes available in Gunn oscillation mode.

          1. Transit-time domain mode.

          2. Delayed domain mode.

          3. Quenched domain mode.

34. What are the applications of GaAs MESFET?

1. Used in microwave integrated circuits for high power, low noise applications.

2. Used in broadband amplifier application.

35. What is negative resistance in Gunn diode?

The carrier drift velocity increases linearly from 0 to maximum,when the electric field is increased from 0 to threshold value in Gunn diodes. When the electric field is beyond the threshold value of 3000v/cm the drift velocity is decreased and the diode exhibit negative resistance.

36. What is Transferred electron effect?

Some materials like GaAs exhibit negative differentialmobility, when biased above a threshold value of the electric field. This behaviour is called transferred electron effect.

37. What are time parameters for TED’S

1. Domain growth time constant

2. Dielectric relaxation time

3. Transit time.

38. Define Avalanche transit time devices.

          Avalanche transit time devices are p-n junction diode with the highly doped p and n regions. They could produce a negative resistance at microwave frequencies by using a carrier impact ionization avalanche breakdown and carriers drift in the high field intensity region under reverse biased condition.

39. What are modes available in avalanche device?

          There are three modes avalanche device

          1. IMPATT- Impact Ionization Avalanche Transit Time Device.

          2. TRAPATT- Trapped Plasma Avalanche Triggered Transit Device and

          3.  BARITT- Barrier Injected Transit Time Device.

40. What are the factors reducing efficiency of IMPATT diode?

1. Space charge effect

2. Reverse saturation current effect

3. High frequency skin effect

4. Ionization saturation effect.

41. List the type of circuit used for IMPATT diode circuits.

1. Broadly tunable circuits

2. Low ‘Q’circuits

3. High ‘Q’circuits

42. What are the applications of low Q-oscillators and amplifier circuits?

1. Final output stage of FM telecommunication transmitter

2. Up converter pump

3. CW Doppler radar transmitter

43. What are the Key phenomenon taking places in TRAPATT diode?

The Key phenomena are

1. Carrier generation by impact ionization producing a current pulse of phase delay of 90 degree.

2. An additional phase shift introduced by the drift of carriers.

44. What is the operating frequency of TRAPATT devices?

TRAPATT devices operate at frequencies from 400 MHz to about 12GHz.

45. Explain plasma formation in TRAPATT diode.

During the operation of the diode a high field avalanche zone propagates through the depletion region and fills the layer with a dense plasma of electrons and holes which get trapped in the low field region behind the zone.

46. What are the applications of TRAPATT devices?

The applications are

1. Phased-array Radar systems

2. Intermediate frequency transmitters.

3. Proxity fuse sources

4. Radio altimeters

5. Microwave landing systems.

47. What is the main advantage of TRAPATT over IMPATT?

TRAPATT diode has much greater efficiency than IMPATT.

48. Write the properties of parametric up converter.

 1. The output frequency is equal to the sum of the signal frequency and the pump frequency.

 2. There is no power flow in the parametric device at frequencies other than the signal, pump and output frequencies.

49. What is the parametric device?

          A parametric device is one that uses a non-linear reactance or time varying reactance. The word parametric is derived from the parametric excitation, which is a reactive parameter, can be used to produce capacitance or inductive excitation.

50. Give the applications of parametric amplifier.

          1. Space communication systems.

          2. Radio telescopes and

          3. Tropo-receivers.


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