Noise in Analog Systems Questions and Answers

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.

Answer: B

Equivalent noise temperature is in kelvin, and it is the preferred measure in satellite and radio astronomy work because it discriminates better among very low noise figures. Noise figure is in decibels, noise factor is the same quantity as a plain ratio, and signal-to-noise ratio is a ratio or its decibel form. Four ways of expressing closely related ideas, and knowing which units each takes prevents them being mixed in one calculation. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: C

It describes how far a calibrated noise source exceeds the thermal noise of a resistor at the reference temperature, and it is the known quantity in a Y-factor measurement. Receiver degradation is the noise figure. The shot-to-thermal ratio is a device characteristic without a standard name. And the signal margin is the signal-to-noise ratio. Instrument specifications and system specifications use different vocabularies for related ideas, which is worth keeping straight.

Answer: D

The audio stage sits after all the gain, so the Friis formula divides its contribution by an enormous factor and it is effectively irrelevant. The first stage dominates, the feeder before it adds its loss directly, and the mixer matters because its noise is divided only by the modest preselector gain. Ordering the stages by their weight in the cascade tells a designer exactly where to spend effort and money.

Answer: A

A passive loss ahead of the gain adds its decibels straight to the system noise figure, which is why front-end filters are made as low-loss as their selectivity allows and why an amplifier is sometimes placed before them. Being passive does not exempt a component from the cascade formula; on the contrary, a lossy passive stage is the worst place to put attenuation. And the effect is the full loss rather than half of it. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: B

Placing a filter first improves selectivity and out-of-band rejection but adds its loss to the noise figure; placing the amplifier first protects the noise figure but exposes it to strong out-of-band signals. That is the genuine dilemma of front-end architecture, and it is resolved differently in a broadcast receiver and in a crowded military band. The other three pairs are real trades elsewhere but not the one this ordering decision forces. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.