# Microwave Engineering Questions and Answers – Noise Figure

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This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Noise Figure”.

1. ___________ is defined as the ratio of desired signal power to undesired noise power.
a) Signal to noise ratio
b) Noise to signal ratio
c) Noise figure
d) Noise temperature

Explanation: SNR is defined as the ratio of desired signal power to undesired noise power, and so is dependent on the signal power. When noise and a desired signal are applied to the input of a noise less network, both noise and signal will be attenuated or amplified by the same factor, so that the signal to noise ratio will be unchanged.

2. __________ is defined as the ratio of input signal to noise ratio to the output signal to noise ratio.
a) Noise figure
b) Noise temperature
c) SNRo
d) None of the mentioned

Explanation: Noise figure is defined as the ratio of input signal to noise ratio to the output signal to noise ratio of a system or a receiver. SNRi is the signal to noise ratio measured at the input terminals of the device. SNR0 is the output signal to noise ratio measured at the output terminals of the device.

3. The equivalent noise temperature of a network given the noise figure of the network or system is:
a) T0(F-1)
b) T0(F+1)
c) T0(F)
d) T0/F

Explanation: The equivalent noise temperature of a network given the noise figure of the network or system is given by T0(F-1). In this expression, F is the noise figure of the system. T0 has the value 290 K. T0 is the standard temperature considered.
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4. Noise figure can be defined for any microwave network irrespective of any other constraints.
a) True
b) False

Explanation: Noise figure is defined only for a matched input source and for a noise source equivalent to a matched load at a temperature T0= 290 K. noise figure and noise temperature are interchangeable noise properties.

5. Expression for noise of a two port network considering the noise due to transmission line and other lossy components is:
b) GkTB
d) None of the mentioned

Explanation: Expression for noise of a two port network considering the noise due to transmission line and other lossy components is GkTB + GNadded. Here, G is the gain of the system. Nadded is the noise generated by the transmission line, as if it appeared at the input terminals of the line.

6. Noise equivalent temperature of a transmission line that adds noise to the noise of a device is:
a) T (L-1)
b) T (L+1)
c) T (L)
d) T/L

Explanation: Noise equivalent temperature of a transmission line that adds noise to the noise of a device is given by T (L-1). Here L is the loss factor of the line and T is the temperature at which the system is thermal equilibrium.

7. If the noise figures of the first stage of a two stage cascade network is 8 dB and the noise figure of the second stage is 7 dB and the gain of the first stage is 10, then the noise figure of the cascade is:
a) 8. 6 dB
b) 7.6 dB
c) 5.6 dB
d) 8.9 dB

Explanation: Noise figure of a two stage cascade network is given by F1+ (F2-1)/G1. Here F1, F2 are the noise figure of the first and the second stage respectively. G1 is the gain of the first stage. Substituting the given values in the above equation, noise figure of the cascade is 8.6 dB.

8. Noise equivalent temperature of a 2 stage cascade network is given by:
a) Te1 + Te2/ G1
b) Te1 + Te1
c) Te1 / Te1
d) None of the mentioned

Explanation: Noise equivalent temperature of a 2 stage cascade network is given by Te1 + Te1/ G1. Here, Te1 is the noise equivalent temperature of stage 1 and Te1 is the noise equivalent temperature of stage 2. G1 is the gain of the first stage of the amplifier.

9. When a network is matched to its external circuitry, the gain of the two port network is given by:
a) │S212
b) │S222
c) │S122
d) │S112

Explanation: The gain of a two port network is given by the product of SS21 of the network and reflection co-efficient at the source end. But when the two port network is matched to the external circuitry, reflection coefficient becomes zero and gain reduces to │S212.

10. For a Wilkinson power divider of insertion loss L and the coupler is matched to the external circuitry, and then the gain of the coupler in terms of insertion loss is:
a) 2L
b) 1/2L
c) L
d) 1/L

Explanation: To evaluate the noise figure of the coupler, third port is terminated with known impedance. Then the coupler becomes a two port device. Since the coupler is matched, ГS=0 and Гout=S22=0. So the available gain is │S212. This is equal to 1/2L from the available data.

11. Noise equivalent temperature of Wilkinson coupler having a gain of 1/2L is given as:
a) T (2L-1)
b) T (2L+1)
c) T (2L*1)
d) T / (2L-1)

Explanation: Noise equivalent temperature of the Wilkinson coupler is found using the relation
T (1-G21)/G21. Substituting for G21 in the above expression, equivalent noise temperature is T (2L-1).

12. Expression for over all noise figure of a mismatched amplifier is:
a) 1+ (F-1)/ (1 -│Г│2)
b) 1
c) 1+ (F-1)
d) (F-1)/ (1 -│Г│2)

Explanation: The overall noise figure of a mismatched amplifier is given by 1+ (F-1)/ (1 -│Г│2). Here F is the noise figure of the amplifier, when there is an impedance mismatch at the input of the amplifier; this impedance mismatch is given by Г.

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