Microwave Engineering Questions and Answers – Noise in Micro – Wave Circuits

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This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Noise in Micro – Wave Circuits”.

1. The type of noise caused by vibration of bound charges is called:
a) Thermal noise
b) Shot noise
c) Flicker noise
d) None of the mentioned

Explanation: Thermal noise is the most basic type of noise, being caused by thermal vibrations of bound charges in a material. When an electron bound to atom gains energy and vibrates, thermal noise is produced. It is also called as Johnson noise.

2. ________ noise occurs due to the random fluctuation of charge in an electron tube.
a) Flicker noise
b) Shot noise
c) Thermal noise
d) White noise

Explanation: Shot noise or Poisson noise is a type of electronic noise that can be modeled by Poisson process. This type of noise occurs due to the discrete nature of electric charge.

3. Flicker noise occurs in solid-state components and vacuum tubes.
a) True
b) False

Explanation: Flicker noise is a form of noise that exhibits an inverse frequency power density curve. It has a pink noise power density spectrum. Since this noise is inversely proportional to the operating frequency, it is called 1/f noise.
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4.________ noise is caused by random motion of charges in ionized gas.
a) Plasma noise
b) Quantum noise
c) Thermal noise
d) Flicker noise

Explanation: Plasma noise is caused by random motion of charges in an ionized gas such as a plasma, ionosphere or sparking electrical contacts. A material medium is required to produce this type of noise.

5. The most insignificant form of noise is:
a) Plasma noise
b) Quantum noise
c) Shot noise
d) Flicker noise

Explanation: Quantum originates due to the quantized nature of charge carriers and photons. This noise does not pose any problem in microwave circuits and also does not affect the signal strength. Hence they are often significant.

6. An X- band amplifier has a gain of 20 dB and a gain of 1GHz bandwidth. Noise figure of the amplifier is -62 dBm at 290 K and -64.7 dBm at 77 K. then the Y factor of the amplifier is :
a) 3 dB
b) 6.4 dB
c) 2.7 dB
d) 5.6 dB

Explanation: The Y factor of an amplifier is given by the difference in noise figure of the amplifier measured at two different temperatures. Taking the difference of the two values, the Y factor is 2.7 dB.

7. The Y factor of an amplifier obtained by measuring the noise figure at the temperatures 77 K and 290 K is 2.7 db. Then the equivalent noise temperature of the amplifier is:
a) 100 K
b) 150 K
c) 170 K
d) None of the mentioned

Explanation: The equivalent noise temperature of the amplifier is given by the relation (T1-YT2)/(Y-1). Substituting the given values in the above equation, equivalent noise temperature is 170 K.

8. An amplifier has a noise equivalent temperature of 170 K. If the amplifier is used with a source having an equivalent noise temperature of TS=450 K, the output noise power of the amplifier is:
a) -50 dBm
b) -60 dBm
c) -60.7 dBm
d) -55 dBm

Explanation: Output noise power of an amplifier is given by the expression GkTSB + GkTeB Substituting the given values in the above expression, the output noise power of the amplifier is -60.7 dBm.

9. The noise power associated with a two port network modeled as thevinin equivalent is:
a) kTB
b) k/TB
c) TB/ K
d) None of the mentioned

Explanation: Noise associated with a 2 port network modeled as thevinin equivalent is kTB) K is the Boltzmann constant, T is the temperature and B is the operating bandwidth of the circuit.

10. Excess noise ratio is defined as the ratio of generator noise to the noise associated with room temperature
a) True
b) False

Explanation: Excess noise ratio is defined as the ratio of the difference in noise power of the generator and noise power associated with the room temperature to the noise power associated with room temperature.

11. Equivalent noise temperature associated with an arbitrary white noise source is:
a) N0/GKB
b) N0
c) N0/ kB
d) None of the mentioned

Explanation: Equivalent noise temperature associated with an arbitrary white noise source is N0/GKb) Here N0 is the noise power delivered to the load resistor by the source, B is the operating bandwidth G is the gain of the circuit.

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