Noise in Analog Systems Questions and Answers

Q: Why is a mixer often a significant noise contributor?

A passive mixer has conversion loss, so the Friis formula divides the following stage's noise by less than unity and the mixer's own contribution is undiminished. Frequency alone does not determine noise contribution. Local oscillator noise matters but is a separate mechanism. And signal handling concerns intermodulation rather than noise. Wherever a chain loses gain, the stages after it become more important to the noise budget. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which receiver specification does the noise figure most directly determine?

Sensitivity follows from noise figure and noise bandwidth together, since the weakest usable signal is the one that exceeds the noise floor by the required ratio. Selectivity and image rejection are filtering properties. Capture ratio is a limiter and discriminator property peculiar to FM. Each figure of merit traces back to a different part of the receiver, and the noise figure's territory is the weak-signal end. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: What does the term ultimate signal-to-noise ratio describe?

It is the ceiling the receiver imposes once the signal is ample, set by the demodulation and the audio stages rather than by external noise. The threshold ratio is the other end of the scale. The antenna ratio is what arrives rather than what emerges. And de-emphasis is one contributor among several. Quoting both the sensitivity and the ultimate ratio describes a receiver at both ends of its operating range, which one figure cannot do. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Which impairment does the threshold effect describe?

It describes the sudden collapse below a critical carrier-to-noise ratio, which is what makes FM coverage boundaries sharp. Gradual degradation is AM's behaviour. Suppression of a weaker signal is capture. And rising noise with baseband frequency is the triangular spectrum. Four distinct FM phenomena, each with its own name, and using the right name is half of describing a fault correctly.

Q: What would happen to FM's advantage if the limiter were removed?

Without limiting, amplitude noise reaches the discriminator and appears at the output, so most of the advantage disappears and the receiver behaves much like an AM set. The demodulation gain remains in principle but is swamped. Nothing about removing the limiter improves matters. And the index would still matter but could not deliver its benefit. Identifying which single stage a scheme's advantage depends on is a good test of whether the mechanism is understood.

Q: Which quantity must exceed a threshold for FM's advantage to appear?

The carrier-to-noise ratio at the detector must be high enough, typically some ten decibels, before the improvement materialises. The index sets how large the improvement is once the threshold is passed, but it cannot substitute for signal. Audio level and deviation ratio are transmitter parameters. Separating the condition for a gain from the magnitude of that gain is the key to reading any statement about FM performance. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Why does a satellite earth station mount its low-noise amplifier at the dish?

Because any feeder loss ahead of the first amplifier adds straight to the system noise figure, the amplifier is mounted at the feed and the long cable run then follows the gain, where its loss is harmless. Local oscillator cabling is a separate concern. Transmit power is a different link. And directivity is fixed by the dish geometry. The principle is general: put the gain before the loss.

Q: Which system quantity does the ratio of gain to noise temperature describe?

The gain-over-temperature ratio is the receiving station's figure of merit, combining the antenna's gain with the system's noise temperature into the single number a link budget needs. Spectral purity concerns phase noise and spurious outputs. Transmitted power density is the satellite's effective isotropic radiated power. And linearity is a modulator property. Each end of a link is characterised by one composite figure, and confusing them makes a budget nonsense.

Q: Why does a radio telescope cool its receiver front end?

Cooling reduces the physical temperature and so the thermal noise of the first stage, which is the whole point when the signals are far below any ordinary noise floor. Oscillator stability benefits incidentally but is not the reason. Effective area is geometry. And bandwidth is set by filtering. Where the source is extremely weak the only remaining lever is the receiver's own noise, and temperature is the way to pull it down. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

61.

Why is a mixer often a significant noise contributor?

A.It generates flicker noise in its local oscillator port
B.It handles the largest signals in the receiver
C.It has loss rather than gain
D.It operates at the highest frequency in the chain

Answer: C

A passive mixer has conversion loss, so the Friis formula divides the following stage's noise by less than unity and the mixer's own contribution is undiminished. Frequency alone does not determine noise contribution. Local oscillator noise matters but is a separate mechanism. And signal handling concerns intermodulation rather than noise. Wherever a chain loses gain, the stages after it become more important to the noise budget. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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62.

Which receiver specification does the noise figure most directly determine?

A.Selectivity
B.Capture ratio
C.Image rejection
D.Sensitivity

Answer: D

Sensitivity follows from noise figure and noise bandwidth together, since the weakest usable signal is the one that exceeds the noise floor by the required ratio. Selectivity and image rejection are filtering properties. Capture ratio is a limiter and discriminator property peculiar to FM. Each figure of merit traces back to a different part of the receiver, and the noise figure's territory is the weak-signal end. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

63.

What does the term ultimate signal-to-noise ratio describe?

A.The best obtainable on a very strong signal
B.The ratio at the threshold of usability
C.The ratio at the antenna terminals
D.The ratio after de-emphasis alone

Answer: A

It is the ceiling the receiver imposes once the signal is ample, set by the demodulation and the audio stages rather than by external noise. The threshold ratio is the other end of the scale. The antenna ratio is what arrives rather than what emerges. And de-emphasis is one contributor among several. Quoting both the sensitivity and the ultimate ratio describes a receiver at both ends of its operating range, which one figure cannot do. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

64.

Which impairment does the threshold effect describe?

A.Rise of noise with baseband frequency at the detector
B.Abrupt collapse of FM output below a signal level
C.Gradual increase of noise as a signal weakens
D.Suppression of the weaker of two co-channel signals

Answer: B

It describes the sudden collapse below a critical carrier-to-noise ratio, which is what makes FM coverage boundaries sharp. Gradual degradation is AM's behaviour. Suppression of a weaker signal is capture. And rising noise with baseband frequency is the triangular spectrum. Four distinct FM phenomena, each with its own name, and using the right name is half of describing a fault correctly.

Discuss in Forum

65.

Which two effects together give FM its practical noise advantage?

A.Capture and image rejection
B.Threshold and de-emphasis
C.Limiting and demodulation gain
D.Pre-emphasis and channel spacing

Answer: C

Limiting discards amplitude noise, and the demodulation gain that grows with the index improves what remains, which between them account for the large figure of merit. Pre-emphasis adds a further few decibels but is a refinement. Capture concerns interference rather than noise. And threshold is a limitation rather than an advantage. Distinguishing the principal mechanisms from the refinements is what makes an explanation proportionate. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

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66.

What would happen to FM's advantage if the limiter were removed?

A.It would be unaffected
B.It would improve at high signal levels
C.It would depend only on the index thereafter
D.Most of it would be lost

Answer: D

Without limiting, amplitude noise reaches the discriminator and appears at the output, so most of the advantage disappears and the receiver behaves much like an AM set. The demodulation gain remains in principle but is swamped. Nothing about removing the limiter improves matters. And the index would still matter but could not deliver its benefit. Identifying which single stage a scheme's advantage depends on is a good test of whether the mechanism is understood.

Discuss in Forum

67.

Which quantity must exceed a threshold for FM's advantage to appear?

A.Carrier-to-noise ratio at the detector
B.Modulation index at the transmitter
C.Audio level at the modulator input
D.Deviation ratio of the system as designed

Answer: A

The carrier-to-noise ratio at the detector must be high enough, typically some ten decibels, before the improvement materialises. The index sets how large the improvement is once the threshold is passed, but it cannot substitute for signal. Audio level and deviation ratio are transmitter parameters. Separating the condition for a gain from the magnitude of that gain is the key to reading any statement about FM performance. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

68.

Why does a satellite earth station mount its low-noise amplifier at the dish?

A.To improve the antenna's directivity
B.To keep feeder loss out of the noise figure
C.To shorten the cable to the local oscillator
D.To reduce the power the transmitter must deliver

Answer: B

Because any feeder loss ahead of the first amplifier adds straight to the system noise figure, the amplifier is mounted at the feed and the long cable run then follows the gain, where its loss is harmless. Local oscillator cabling is a separate concern. Transmit power is a different link. And directivity is fixed by the dish geometry. The principle is general: put the gain before the loss.

Discuss in Forum

69.

Which system quantity does the ratio of gain to noise temperature describe?

A.A satellite's transmitted power density
B.A modulator's linearity
C.An earth station's receiving figure of merit
D.A transmitter's spectral purity

Answer: C

The gain-over-temperature ratio is the receiving station's figure of merit, combining the antenna's gain with the system's noise temperature into the single number a link budget needs. Spectral purity concerns phase noise and spurious outputs. Transmitted power density is the satellite's effective isotropic radiated power. And linearity is a modulator property. Each end of a link is characterised by one composite figure, and confusing them makes a budget nonsense.

Discuss in Forum

70.

Why does a radio telescope cool its receiver front end?

A.To stabilise the local oscillator frequency
B.To increase the antenna's effective area
C.To widen the receiver's usable bandwidth
D.To reduce thermal noise in the first stage

Answer: D

Cooling reduces the physical temperature and so the thermal noise of the first stage, which is the whole point when the signals are far below any ordinary noise floor. Oscillator stability benefits incidentally but is not the reason. Effective area is geometry. And bandwidth is set by filtering. Where the source is extremely weak the only remaining lever is the receiver's own noise, and temperature is the way to pull it down. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Discuss in Forum

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