Noise in Analog Systems Questions and Answers

Q: Which factor decides whether flicker noise matters in a given circuit?

Flicker noise rises as frequency falls, so it matters where the circuit must work down to very low frequencies, and is negligible at radio frequencies. That is why direct-coupled instrumentation amplifiers fight it while a radio front end can ignore it. Impedance level governs thermal noise voltage. Bandwidth scales thermal and shot noise. And temperature drives thermal noise, whereas flicker noise depends chiefly on the device's material and processing. Knowing which parameter each mechanism responds to is what lets a measurement identify the one present.

Q: Why does the chopper technique help a low-level direct-coupled measurement?

Chopping translates the slowly varying signal up to the chopping frequency, where flicker noise has fallen away and only the flat thermal and shot contributions remain, and the signal is recovered by synchronous detection afterwards. It does nothing to the input resistor's thermal noise, which is flat anyway and so cannot be escaped by moving in frequency. It does not itself narrow the bandwidth, though the synchronous detector that follows it does. And shot noise is likewise flat. The whole technique is an application of the one-over-f spectrum, and it makes sense only once that shape is understood.

Q: What does a receiver's quieting measurement determine?

Quieting sensitivity is the input level that suppresses the output noise by a stated number of decibels, typically twenty, and it is the standard FM sensitivity measure because it captures exactly the behaviour a listener notices as a station comes up out of the hiss. It does not yield the noise figure directly, though the two are related. It says nothing about where the noise spectrum is flat. And while limiter action underlies the effect, the measurement reports an input level rather than a threshold. Each scheme has acquired the sensitivity measure that matches its own failure mode, and quieting is FM's.

Q: The main difference between the thermal noise and shot noise is that :

The keyed difference is that shot noise has a non-uniform spectrum in the high microwave frequency range.Both thermal noise and shot noise are white over ordinary frequencies: their mean values are zero and their mean-square values are proportional to the noise bandwidth (\(4kTRB\) and \(2qIB\)). Those are similarities, not differences. Shot noise, however, arises from the discrete arrival of charge carriers, and at microwave frequencies, where the period approaches the carriers' transit time, its spectrum is no longer flat; transit-time effects make it rise with frequency.The last option is false because shot noise depends on the current, not on temperature; only thermal noise is proportional to temperature.

Q: The correct statement for shot noise and Johnson noise is that :(a) Shot noise occurs because the carriers do not cross the barrier simultaneously(b) Shot noise is a random fluctuation that accompanies any direct current crossing a potential barrier(c) Johnson noise arises from thermal cause wherein electron in conductor possess kinetic energy(d) Mean noise voltage across the conductor is zero but the root-mean-square value is infinite and cannot be measured

Statements a, b and c are correct, so the answer is (a), (b) and (c).Shot noise arises because current is carried by discrete charges that cross a potential barrier at random times rather than simultaneously (a); it is a random fluctuation that accompanies any direct current crossing a barrier, as in diodes and transistors (b). Johnson (thermal) noise arises from the random thermal motion of electrons, which have kinetic energy, in any conductor (c).Statement d is false. The mean of thermal noise voltage is zero, but its root-mean-square value is finite and measurable:\[ V_{rms} = \sqrt{4kTRB} \]so the option 'd only' and any option omitting b or c are wrong.

Q: The equivalent noise temperature of a network given the noise figure of the network or system is :

The equivalent noise temperature is \(T_{0}(F - 1)\).The noise figure \(F\) is defined with the source at the reference temperature \(T_{0}\) (290 K). The noise the network adds, referred to its input, can be expressed as an extra temperature \(T_{e}\), so\[ \begin{aligned} F &= \frac{T_{0} + T_{e}}{T_{0}} = 1 + \frac{T_{e}}{T_{0}} \\ \Rightarrow T_{e} &= T_{0}(F - 1) \end{aligned} \]A noiseless network has \(F = 1\) and \(T_{e} = 0\), as it should. \(T_{0}(F + 1)\) and \(T_{0}F\) would give a noiseless network a non-zero noise temperature, and \(T_{0}/F\) would make a noisier network appear cooler.

111.

Which factor decides whether flicker noise matters in a given circuit?

A.The lowest frequency the circuit must handle
B.The impedance level at the circuit's input
C.The bandwidth over which the circuit operates
D.The physical temperature of the semiconductor

Answer: A

Flicker noise rises as frequency falls, so it matters where the circuit must work down to very low frequencies, and is negligible at radio frequencies. That is why direct-coupled instrumentation amplifiers fight it while a radio front end can ignore it. Impedance level governs thermal noise voltage. Bandwidth scales thermal and shot noise. And temperature drives thermal noise, whereas flicker noise depends chiefly on the device's material and processing. Knowing which parameter each mechanism responds to is what lets a measurement identify the one present.

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112.

Why does the chopper technique help a low-level direct-coupled measurement?

A.It cancels shot noise by averaging the current
B.It moves the signal above the flicker noise region
C.It reduces the thermal noise of the input resistor
D.It narrows the amplifier's noise bandwidth

Answer: B

Chopping translates the slowly varying signal up to the chopping frequency, where flicker noise has fallen away and only the flat thermal and shot contributions remain, and the signal is recovered by synchronous detection afterwards. It does nothing to the input resistor's thermal noise, which is flat anyway and so cannot be escaped by moving in frequency. It does not itself narrow the bandwidth, though the synchronous detector that follows it does. And shot noise is likewise flat. The whole technique is an application of the one-over-f spectrum, and it makes sense only once that shape is understood.

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113.

Which single figure summarises a receiving system's weak-signal capability?

A.Audio output power in watts
B.Image rejection in decibels
C.Sensitivity in microvolts or dBm
D.Intermediate frequency in megahertz

Answer: C

Sensitivity, quoted as a power or a voltage for a stated output ratio, is the composite figure, and it collapses the noise figure, the noise bandwidth and the required output ratio into one number a user can compare across receivers. The intermediate frequency is an architectural choice. Audio power is about loudness. And image rejection describes unwanted-response suppression rather than weak-signal performance. A sensitivity figure quoted without its bandwidth and its required ratio is incomplete, which is why reputable specifications always state all three together.

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114.

What does a receiver's quieting measurement determine?

A.The noise figure of the receiver front end directly
B.The bandwidth at which the output noise is flat
C.The threshold at which the limiter begins to act
D.The input level that reduces the output noise by a stated amount

Answer: D

Quieting sensitivity is the input level that suppresses the output noise by a stated number of decibels, typically twenty, and it is the standard FM sensitivity measure because it captures exactly the behaviour a listener notices as a station comes up out of the hiss. It does not yield the noise figure directly, though the two are related. It says nothing about where the noise spectrum is flat. And while limiter action underlies the effect, the measurement reports an input level rather than a threshold. Each scheme has acquired the sensitivity measure that matches its own failure mode, and quieting is FM's.

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115.

The main difference between the thermal noise and shot noise is that :

A.Mean square noise component is dependent noise bandwidth in shot and thermal noise
B.Mean noise component is zero in both
C.Shot noise non-uniform spectrum in high microwave frequency range
D.Shot noise and thermal noise is temperature dependent

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The keyed difference is that shot noise has a non-uniform spectrum in the high microwave frequency range.

Both thermal noise and shot noise are white over ordinary frequencies: their mean values are zero and their mean-square values are proportional to the noise bandwidth (\(4kTRB\) and \(2qIB\)). Those are similarities, not differences. Shot noise, however, arises from the discrete arrival of charge carriers, and at microwave frequencies, where the period approaches the carriers' transit time, its spectrum is no longer flat; transit-time effects make it rise with frequency.

The last option is false because shot noise depends on the current, not on temperature; only thermal noise is proportional to temperature.

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116.

Assertion (A) : Thermal noise is sometimes referred to as white noise.
Reason (R) : When the white noise is passed through a network, the spectral density will be altered by the shape of the network frequency response.

A.Both (A) and (R) are true and (R) is the correct explanation of (A)
B.Both (A) and (R) are true but (R) is not the correct explanation of (A)
C.(A) is true but (R) is false
D.(A) is false and (R) is true

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

Both (A) and (R) are true but (R) is not the correct explanation of (A).

Thermal (Johnson) noise has a power spectral density of 4kTR that is flat over all frequencies used in electronics. A flat spectrum, like white light containing all colours, is why it is called white noise, so the assertion is true.

The reason is also true: passing white noise through a network shapes its output spectrum as \(S_{out}(f) = |H(f)|^{2} S_{in}(f)\), giving coloured noise. But that describes what happens after filtering, not why thermal noise is called white, so it does not explain the assertion.

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117.

The correct statement for shot noise and Johnson noise is that :

(a) Shot noise occurs because the carriers do not cross the barrier simultaneously
(b) Shot noise is a random fluctuation that accompanies any direct current crossing a potential barrier
(c) Johnson noise arises from thermal cause wherein electron in conductor possess kinetic energy
(d) Mean noise voltage across the conductor is zero but the root-mean-square value is infinite and cannot be measured

A.(a) and (b)
B.(a), (b) and (c)
C.(d) only
D.(a) and (c)

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

Statements a, b and c are correct, so the answer is (a), (b) and (c).

Shot noise arises because current is carried by discrete charges that cross a potential barrier at random times rather than simultaneously (a); it is a random fluctuation that accompanies any direct current crossing a barrier, as in diodes and transistors (b). Johnson (thermal) noise arises from the random thermal motion of electrons, which have kinetic energy, in any conductor (c).

Statement d is false. The mean of thermal noise voltage is zero, but its root-mean-square value is finite and measurable:

\[ V_{rms} = \sqrt{4kTRB} \]

so the option 'd only' and any option omitting b or c are wrong.

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118.

The equivalent noise temperature of a network given the noise figure of the network or system is :

A.\(T_{0}\,(F - 1)\)
B.\(T_{0}\,(F + 1)\)
C.\(T_{0}\,(F)\)
D.\(T_{0}/F\)

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

The equivalent noise temperature is \(T_{0}(F - 1)\).

The noise figure \(F\) is defined with the source at the reference temperature \(T_{0}\) (290 K). The noise the network adds, referred to its input, can be expressed as an extra temperature \(T_{e}\), so

\[ \begin{aligned} F &= \frac{T_{0} + T_{e}}{T_{0}} = 1 + \frac{T_{e}}{T_{0}} \\ \Rightarrow T_{e} &= T_{0}(F - 1) \end{aligned} \]

A noiseless network has \(F = 1\) and \(T_{e} = 0\), as it should. \(T_{0}(F + 1)\) and \(T_{0}F\) would give a noiseless network a non-zero noise temperature, and \(T_{0}/F\) would make a noisier network appear cooler.

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119.

Indicate the false statement.

(i) HF mixers are generally noiser than HF amplifier.
(ii) Impulse noise voltage is independent of bandwidth.
(iii) In thermal noise, the noise power generated by a resistor, is proportional to the bandwidth.
(iv) Industrial noise is not an impulse type.

A.(i) and (ii) are correct.
B.(ii) and (iii) are correct.
C.(iii) and (iv) are correct.
D.(i) and (iii) are correct.

Exam Year(s)

UGC NET - 2012,

Answer: D

The statements that are correct are (i) and (iii), and the options are worded as 'are correct', so the key picks them. Statement (i), that HF mixers are generally noisier than HF amplifiers, is true because a mixer is a non-linear device that adds more noise than an amplifier. Statement (iii), that the thermal noise power from a resistor is proportional to the bandwidth, is true from the relation P = kTB. Statement (ii) is false: impulse noise voltage is proportional to the bandwidth, so it is not independent of it. Statement (iv) is also false, because industrial noise, from ignition, motors and switches, is of the impulse type. The answer is (i) and (iii).

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120.

One of the following types of noise becomes of great importance at high frequencies. It is the

A.Shot noise
B.Random noise
C.Impulse noise
D.Transit-time noise

Exam Year(s)

UGC NET - 2012,

Answer: D

Transit-time noise becomes important at high frequencies: when the time a carrier takes to cross a device becomes comparable to the period of the signal, the carriers do not respond in step with it and extra noise appears in transistors and tubes. Shot noise and random (thermal) noise exist at all frequencies and do not rise sharply at high frequency, and impulse noise comes from sudden disturbances and is not a high-frequency effect of the device. The answer is option D.

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