Noise in Analog Systems Questions and Answers

Q: Consider the following pairs of scheme and figure of merit. AM at unity index - One third DSB-SC - One SSB - One Wideband FM at index 5 - 450 How many of the pairs given above are correctly matched?

All four pairings are correct, and together they are the table worth committing to memory, since almost every comparative question about analogue noise reduces to reading it. The two unity entries are the ones most often got wrong, on the assumption that single sideband must beat double sideband; it does not, because its halved power is matched by its halved bandwidth. And the FM figure follows from the cubic dependence on index. Learning the table alongside the bandwidth each scheme occupies makes the trade visible at a glance.

Q: A cascade is specified as follows. StageGainNoise figure Amplifier15 dB2 dBMixer-6 dB8 dBIF amplifier40 dB6 dB Which stage most limits the system noise figure?

The amplifier's own 2 dB dominates, because its 15 dB of gain divides everything after it - the mixer's 8 dB contributes only a modest amount and the IF amplifier's 6 dB is divided by the net gain of nine decibels ahead of it. The mixer matters more than it would with more gain in front, which is exactly why the amplifier precedes it. Reading a cascade table means working left to right and tracking the accumulated gain, since that denominator is what decides each stage's weight.

Q: The noise power available from a resistor is ______, and it is independent of the resistance value.

The available power is \( kTB \), with no resistance in it, while \( 4kTBR \) is the mean-square open-circuit voltage and does depend on resistance. \( 2qIB \) is the shot noise density times bandwidth. And dividing by bandwidth inverts the dependence that makes noise power grow with the band admitted. The pair of expressions is worth holding together: the same physics stated as a power delivered to a matched load and as a voltage across an open circuit, which is why one carries \( R \) and the other does not.

Q: In an FM receiver the noise power spectral density at the discriminator output rises with the square of the ______ frequency.

It rises with the square of the baseband frequency, because the discriminator differentiates and differentiation weights a component in proportion to its frequency. That produces the triangular noise spectrum, and pre-emphasis and de-emphasis exist precisely to exploit it. Carrier, intermediate and local oscillator frequencies are all far above the baseband and play no part in the shape of the output noise. Tracing a spectrum back to the operation that produced it is the reliable way to remember which frequency variable governs it.

Q: Which of the following does not belong with the others? Thermal noise Shot noise Flicker noise Adjacent channel interference

Adjacent channel interference is another transmitter's signal falling in the passband, so it is interference rather than noise: it is deterministic, it originates outside, and the remedy is filtering or channel planning rather than device design. The other three are random processes generated within the receiver's own devices. Keeping noise and interference separate matters because they are measured differently, specified differently, and cured differently, and a complaint of poor reception should be sorted into one or the other before anything else is done.

Q: Which of the following is not a way of reducing the effect of noise on a link? Increasing the receiver's audio gain Cooling the receiver's first stage Narrowing the receiver bandwidth to the signal Raising the modulation index within the allocated bandwidth

Increasing the audio gain raises signal and noise together and improves nothing, which is the most common misconception about receiver noise. Cooling reduces thermal noise in the stage that dominates. Narrowing the bandwidth admits less noise power. And a higher index buys demodulation gain where the bandwidth allows it. Each genuine remedy acts on the noise itself or on the demodulation, and any proposal that scales signal and noise equally can be dismissed without calculation.

Q: Which statement about noise bandwidth is correct?

Noise bandwidth is the width of an ideal rectangular filter that would admit the same noise power as the real filter, which is what makes a single number usable in the \( kTB \) calculation. It is close to but generally a little wider than the 3 dB bandwidth, because a real filter's skirts admit noise beyond its nominal edge. It is a property of the receiver's filtering wherever that sits. And it is independent of the modulation index, though the index does dictate what bandwidth the receiver must provide. Replacing an awkward shape with an equivalent rectangle is a standard move, and knowing it is an equivalence rather than an identity keeps the small discrepancy from being puzzling.

Q: What does the term noise floor mean in a receiver specification?

The noise floor is the noise power present with no input signal, computed from the thermal reference, the bandwidth and the noise figure. Sensitivity is the closely related but distinct figure: the weakest usable signal, which sits a required ratio above the floor. The noise figure alone is a degradation rather than an absolute power. And limiter operation is an FM detail unrelated to the definition. The floor is a power, sensitivity is a power, and the noise figure is a ratio, so a specification that mixes them is worth reading twice.

Q: Why is the equivalent noise temperature preferred to noise figure for very quiet amplifiers?

Because noise figures of 0.3 and 0.5 dB look almost identical while the corresponding noise temperatures of 21 K and 35 K differ by nearly half, the temperature scale discriminates far better in the low-noise regime that satellite and radio astronomy work occupies. It is defined against the same 290 kelvin reference rather than being independent of it. Both measures apply to passive components. And both need a calibrated source to measure. Choosing the scale that spreads out the range of interest is a general principle of good specification, and this is a clean example of it.

101.

Statement I: A noise figure of 0 dB describes a noise-free device.

Statement II: A noise figure of 0 dB describes a device at absolute zero.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are incorrect
B.Statement I is incorrect but Statement II is correct
C.Statement I is correct but Statement II is incorrect
D.Both Statement I and Statement II are correct

Answer: C

The first is true by definition and the second confuses two ideas: a noise figure of 0 dB means the device adds nothing to the noise already present at its input, which is an ideal rather than a temperature. Its equivalent noise temperature would be zero kelvin, but that describes the added noise, not the device's physical temperature. A cooled amplifier at 20 kelvin still has a small but non-zero noise figure. Distinguishing added noise from ambient noise, and equivalent temperature from physical temperature, resolves most confusion about this measure.

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102.

Consider the following pairs of scheme and figure of merit.

AM at unity index - One third

DSB-SC - One

SSB - One

Wideband FM at index 5 - 450

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.Only three
D.All four

Answer: D

All four pairings are correct, and together they are the table worth committing to memory, since almost every comparative question about analogue noise reduces to reading it. The two unity entries are the ones most often got wrong, on the assumption that single sideband must beat double sideband; it does not, because its halved power is matched by its halved bandwidth. And the FM figure follows from the cubic dependence on index. Learning the table alongside the bandwidth each scheme occupies makes the trade visible at a glance.

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103.

A cascade is specified as follows.

Stage	Gain	Noise figure
Amplifier	15 dB	2 dB
Mixer	-6 dB	8 dB
IF amplifier	40 dB	6 dB

Which stage most limits the system noise figure?

A.The amplifier
B.The mixer
C.The IF amplifier
D.All three equally

Answer: A

The amplifier's own 2 dB dominates, because its 15 dB of gain divides everything after it - the mixer's 8 dB contributes only a modest amount and the IF amplifier's 6 dB is divided by the net gain of nine decibels ahead of it. The mixer matters more than it would with more gain in front, which is exactly why the amplifier precedes it. Reading a cascade table means working left to right and tracking the accumulated gain, since that denominator is what decides each stage's weight.

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104.

The noise power available from a resistor is ______, and it is independent of the resistance value.

A.k T over B
B.k T B
C.4 k T B R
D.2 q I B

Answer: B

The available power is \( kTB \), with no resistance in it, while \( 4kTBR \) is the mean-square open-circuit voltage and does depend on resistance. \( 2qIB \) is the shot noise density times bandwidth. And dividing by bandwidth inverts the dependence that makes noise power grow with the band admitted. The pair of expressions is worth holding together: the same physics stated as a power delivered to a matched load and as a voltage across an open circuit, which is why one carries \( R \) and the other does not.

Discuss in Forum

105.

In an FM receiver the noise power spectral density at the discriminator output rises with the square of the ______ frequency.

A.intermediate
B.local oscillator
C.baseband
D.carrier

Answer: C

It rises with the square of the baseband frequency, because the discriminator differentiates and differentiation weights a component in proportion to its frequency. That produces the triangular noise spectrum, and pre-emphasis and de-emphasis exist precisely to exploit it. Carrier, intermediate and local oscillator frequencies are all far above the baseband and play no part in the shape of the output noise. Tracing a spectrum back to the operation that produced it is the reliable way to remember which frequency variable governs it.

Discuss in Forum

106.

Which of the following does not belong with the others?

Thermal noise

Shot noise

Flicker noise

Adjacent channel interference

A.Shot noise
B.Flicker noise
C.Adjacent channel interference
D.Thermal noise

Answer: D

Adjacent channel interference is another transmitter's signal falling in the passband, so it is interference rather than noise: it is deterministic, it originates outside, and the remedy is filtering or channel planning rather than device design. The other three are random processes generated within the receiver's own devices. Keeping noise and interference separate matters because they are measured differently, specified differently, and cured differently, and a complaint of poor reception should be sorted into one or the other before anything else is done.

Discuss in Forum

107.

Which of the following is not a way of reducing the effect of noise on a link?

Increasing the receiver's audio gain

Cooling the receiver's first stage

Narrowing the receiver bandwidth to the signal

Raising the modulation index within the allocated bandwidth

A.Increasing the receiver's audio gain
B.Cooling the receiver's first stage
C.Narrowing the receiver bandwidth to the signal
D.Raising the modulation index within the allocated bandwidth

Answer: A

Increasing the audio gain raises signal and noise together and improves nothing, which is the most common misconception about receiver noise. Cooling reduces thermal noise in the stage that dominates. Narrowing the bandwidth admits less noise power. And a higher index buys demodulation gain where the bandwidth allows it. Each genuine remedy acts on the noise itself or on the demodulation, and any proposal that scales signal and noise equally can be dismissed without calculation.

Discuss in Forum

108.

Which statement about noise bandwidth is correct?

A.It depends on the signal's modulation index
B.It is the width of an ideal filter admitting the same noise power
C.It is always equal to the 3 dB bandwidth of the filter
D.It is measured at the antenna rather than at the detector

Answer: B

Noise bandwidth is the width of an ideal rectangular filter that would admit the same noise power as the real filter, which is what makes a single number usable in the \( kTB \) calculation. It is close to but generally a little wider than the 3 dB bandwidth, because a real filter's skirts admit noise beyond its nominal edge. It is a property of the receiver's filtering wherever that sits. And it is independent of the modulation index, though the index does dictate what bandwidth the receiver must provide. Replacing an awkward shape with an equivalent rectangle is a standard move, and knowing it is an equivalence rather than an identity keeps the small discrepancy from being puzzling.

Discuss in Forum

109.

What does the term noise floor mean in a receiver specification?

A.The noise figure expressed in absolute terms
B.The noise power that causes the limiter to operate
C.The output noise power with no signal present
D.The weakest signal the receiver can detect reliably

Answer: C

The noise floor is the noise power present with no input signal, computed from the thermal reference, the bandwidth and the noise figure. Sensitivity is the closely related but distinct figure: the weakest usable signal, which sits a required ratio above the floor. The noise figure alone is a degradation rather than an absolute power. And limiter operation is an FM detail unrelated to the definition. The floor is a power, sensitivity is a power, and the noise figure is a ratio, so a specification that mixes them is worth reading twice.

Discuss in Forum

110.

Why is the equivalent noise temperature preferred to noise figure for very quiet amplifiers?

A.It is independent of the reference temperature
B.It applies to passive components as well as active ones
C.It can be measured without a calibrated source
D.It resolves small differences more clearly

Answer: D

Because noise figures of 0.3 and 0.5 dB look almost identical while the corresponding noise temperatures of 21 K and 35 K differ by nearly half, the temperature scale discriminates far better in the low-noise regime that satellite and radio astronomy work occupies. It is defined against the same 290 kelvin reference rather than being independent of it. Both measures apply to passive components. And both need a calibrated source to measure. Choosing the scale that spreads out the range of interest is a general principle of good specification, and this is a clean example of it.

Discuss in Forum

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