Noise in Analog Systems Questions and Answers

Answer: D

A directional antenna can null an external interferer arriving from a particular bearing, which is why it is the first remedy for local electrical noise. The other three are generated inside the receiver and no antenna can influence them. Separating external from internal noise decides whether the remedy lies in siting and antennas or in devices and circuit design, and it is the first question to ask about any noise complaint. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Noise power is proportional to bandwidth, so doubling it raises the floor by 3 dB and halves the signal-to-noise ratio for the same signal. Six decibels would correspond to a fourfold increase. The floor certainly changes. And the noise figure is a device property independent of bandwidth, so it does not double. This is why a receiver's bandwidth is kept no wider than the signal requires, and why the matched filter is optimal.

Answer: B

The order is AM at a third, SSB and DSB-SC at unity, and wideband FM at hundreds, which is the exact reverse of the bandwidth ordering. That inversion is the central trade of analogue communication: every improvement in noise performance is bought with spectrum, and every saving in spectrum is paid for in noise. The reversed and jumbled orders each misplace at least one scheme.

Answer: C

It is the ratio of transmitted to message bandwidth, so it is two for AM and DSB, one for SSB, and about twelve for FM broadcasting. The output-over-channel ratio is the figure of merit, its natural partner in any comparison. The noise-to-3 dB bandwidth ratio is a filter detail. And deviation over message frequency is the modulation index. Plotting figure of merit against expansion factor is how the schemes are properly compared. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: D

The first two are standard results. The third is the one to watch: SSB and DSB-SC both achieve unity, because SSB's halved sideband power is exactly offset by its halved receiver bandwidth. SSB's real advantage is spectral rather than in noise, and remembering that keeps the comparison honest. Expecting a noise advantage from every efficiency is the intuition this result corrects.

Answer: A

The first three are right and the fourth is not, so three match. Atmospheric noise arrives from outside and is entirely independent of the receiver's design, which is why improving the front end buys nothing where it dominates. Sorting the mechanisms by what each depends on is what lets a measurement identify which one is present, since varying temperature, current or frequency changes each differently.

Answer: B

Because the figure of merit grows with the cube of the index, more deviation buys a large improvement, provided the receiver bandwidth is widened to match and the link stays above threshold. Narrowing the audio raises the index too but at the cost of information. Intermediate frequency choice affects images and selectivity rather than noise. And audio gain amplifies signal and noise together. The qualification about threshold is essential, since below it the trade reverses.

Answer: C

Boltzmann's constant appears, in \( kTB \), linking temperature to energy. Planck's constant governs the quantum correction that matters only at very high frequencies. The electronic charge appears in shot noise, through \( 2qIB \). And permittivity belongs to electromagnetics rather than to noise. Recognising which constant belongs to which mechanism is a quick way to identify an unfamiliar noise formula and to check that it is being applied to the right phenomenon.

Answer: D

The electronic charge appears, since shot noise arises from the granularity of charge itself and its density is \( 2qI \) per hertz. Boltzmann's constant belongs to thermal noise. Permeability and the gas constant have no part in either. The presence of \( q \) rather than \( k \) is the immediate signature of a current-dependent mechanism rather than a temperature-dependent one.

Answer: A

The standard reference is 290 kelvin, which is why 3 dB noise figure corresponds to an equivalent noise temperature of 290 K and why the noise floor is quoted as -174 dBm per hertz. 273 K is the ice point and 300 K a common round figure in physics, but neither is the convention here. Absolute zero would make noise figure meaningless. Conventional reference values are worth knowing exactly, since a small difference shifts every derived number.