Noise in Analog Systems Questions and Answers

Answer: D

Since the receiver is quiet on a matched load, its own noise is normal, so the noise is external and arrives through the antenna - man-made interference, atmospheric noise, or a nearby source. An excess-noise first stage would hiss on the termination too, which is precisely why that substitution is the standard test. Misalignment would change sensitivity and selectivity rather than produce this asymmetry. And a detector below threshold needs a signal to demonstrate it. Substituting a known quiet source for the antenna separates internal from external noise in one step, and it should be the first measurement taken.

Answer: A

This is the threshold effect: below a critical carrier-to-noise ratio the limiter can no longer suppress the noise impulses and the output collapses abruptly, which is why FM coverage has a sharp edge rather than a gradual fade. Capture would replace the wanted programme with another rather than produce crackle. A failed de-emphasis network would make the audio permanently bright, not signal-dependent. And over-deviation distorts at all signal levels. The signal-dependence of a fault is itself diagnostic, and here it points unambiguously at threshold.

Answer: B

If de-emphasis is missing, the transmitter's high-frequency boost is never undone and both the programme's treble and the triangular noise remain lifted, which is exactly bright and hissy on every station. Every transmitter losing pre-emphasis simultaneously is not a credible fault. An early limiter would affect the noise performance but not the tonal balance. And an off-centre discriminator distorts and reduces output rather than brightening it. The fact that all stations are affected localises the fault to the receiver, and the tonal signature names the stage.

Answer: C

The narrower receiver has the lower noise power and therefore the better sensitivity, so long as its bandwidth still accommodates the signal. The proviso is the whole point: narrow past the signal's own width and the signal is mutilated, which is why the matched filter and not the narrowest filter is optimal. Extra bandwidth collects noise, not signal, since the signal occupies only its own spectrum. And gain affects neither the ratio nor the sensitivity once the noise figure is fixed. Sensitivity is always a statement about noise figure and bandwidth together.

Answer: D

Narrowing the bandwidth to match the signal reduces the noise power admitted and so improves sensitivity, and it costs nothing in devices - only in filter design. Extra gain after the front end raises signal and noise together and changes nothing about the ratio, a point that surprises people who expect gain to help. A second detector adds nothing the first did not do. And audio power concerns loudness rather than the weakest usable signal. When the devices are fixed, bandwidth is the remaining lever, and it is often the cheapest one.

Answer: A

Because an amplifier raises signal and noise by the same factor, the ratio between them is unchanged, and this is the single most useful thing to know about receiver noise: the ratio is decided at the point where the noise enters, and nothing downstream can recover it. Later gain does add a little noise but that is a second-order effect divided down by the gain ahead of it. Friis quantifies the contribution rather than forbidding gain. And gain does not change bandwidth. Once a ratio is lost it stays lost, which is why front-end design receives so much attention.

Answer: B

The figure of merit is defined at equal transmitted power and equal message bandwidth, with the channel bandwidth left free to differ - which is what makes the comparison meaningful, since the extra channel bandwidth is precisely what FM spends to buy its advantage. Holding channel bandwidth equal would forbid FM the resource it trades in. Equalising the output ratio asks a different question, namely what power each scheme needs. And receiver noise figure is common to both and irrelevant to the comparison. Stating the conditions of a comparison is as important as the numbers it yields.

Answer: C

FM spends channel bandwidth, roughly twice the deviation plus twice the highest message frequency, and receives an output ratio improvement growing with the cube of the index. Transmitter power is held constant in the comparison, which is what makes the trade worth having. Dynamic range is relaxed rather than spent, since a limiter tolerates amplitude variation. And FM does not need a linear modulator, another of its practical advantages. Naming the resource spent and the benefit obtained in the same sentence is the compact way to hold any scheme comparison in mind.

Answer: D

Because an FM signal's amplitude is constant by design, clipping it discards noise without discarding information, whereas an AM signal's amplitude is the message and a limiter would destroy it. That single asymmetry is the origin of most of FM's noise advantage. Peak-to-average ratio, bandwidth and detector linearity are all real differences between the schemes but none of them decides whether limiting is permissible. Every noise-reduction technique exploits an asymmetry between signal and noise, and here the asymmetry is that one is constant in amplitude and the other is not.

Answer: A

The allocated channel bandwidth caps the index, since bandwidth grows with deviation and a station may not spill into its neighbours. That is why broadcast FM settled on an index of about five rather than the far larger value the noise arithmetic would favour. Audio linearity affects distortion rather than the index. Noise figure sets sensitivity. And oscillator stability concerns frequency accuracy. Regulatory allocation, rather than physics, is what stops the bandwidth-for-noise trade being pushed further, and that is worth remembering when the arithmetic suggests otherwise.