Noise in Analog Systems Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.