FM Transmitters and Receivers Questions and Answers

Answer: D

Selectivity is about rejecting energy at a different frequency, so the adjacent channel is its business; capture ratio governs two signals inside the same channel. Internal noise is a sensitivity matter. And amplitude modulation on the carrier is dealt with by the limiter and reported as amplitude modulation suppression. Four different complaints, four different specifications, is the structure worth carrying out of this topic. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Interference from a different channel is a selectivity failure, and better intermediate frequency filtering is the remedy. Capture ratio would matter if the interferer were on the same channel. Sensitivity is evidently adequate, since the distant station was usable before. And amplitude suppression concerns noise on the wanted carrier. Reading the geometry of the complaint - same channel or next channel - points straight at the specification at fault.

Answer: B

The indicator responds to the 19 kHz pilot, so noise or a spurious component being read as a pilot is the explanation, and the decoder will be adding a noisy difference channel to no purpose. De-emphasis failure gives bright, hissy audio. Excessive deviation distorts. And an image would appear as a second programme. Faults in indication and faults in reproduction are worth separating, since the first can be misleading about the second.

Answer: C

Listeners' aerials point every which way, from a car whip to a set-top dipole, so radiating both polarisations gives a usable signal whatever the receiving orientation. It does not double the power, and indeed splitting between two planes costs something in each. Harmonics are a filtering matter. And bandwidth is set by the modulation. Antenna polarisation is chosen to suit the population of receivers rather than the transmitter.

Answer: D

It delivers the whole composite baseband, audio sum, difference sidebands, pilot and radio data together, which the following stages then separate. The envelope is constant and carries nothing. The pilot is one component of the composite rather than the output. And a strength-proportional voltage comes from the limiter or a separate detector for the meter. Understanding that the discriminator hands over a multiplexed baseband explains the whole decoder that follows.

Answer: A

Everything up to the composite's top, around 60 kHz with radio data, must reach the decoder, which is why the path between discriminator and decoder is wider than any audio stage. Limiting it to 15 kHz would discard the difference signal entirely and leave a mono receiver. The intermediate frequency is translated long before. And passing the pilot alone would give a reference with no data to decode. Bandwidth requirements differ at every point in the chain, which is worth tracing once.

Answer: B

The order is mixer, intermediate filter, limiter, discriminator, then stereo decoder: translate, select the channel, strip amplitude, convert frequency to baseband, then demultiplex. Every alternative breaks a dependency, most obviously discriminating before limiting, which would admit all the amplitude noise the limiter exists to remove. Checking a block diagram against dependencies rather than memory is the reliable method.

Answer: C

Clipping the outer Bessel sidebands distorts the frequency variation the discriminator reads, so the audio distorts, worst on loud passages where deviation is greatest. The carrier sits in the middle of the passband and survives. The pilot is a baseband component recovered after detection, unaffected by intermediate filtering except through the general distortion. And image rejection depends on circuits before the mixer. Filter width faults always show as level-dependent distortion.

Answer: D

The limiter does, and it is the stage that has no counterpart in an AM receiver because in AM the amplitude is the message. Its presence on a block diagram is a reliable way to identify an angle-modulation receiver. The mixer translates frequency, de-emphasis shapes the recovered audio, and the output transformer matches the loudspeaker. Each stage does exactly one thing, and the limiter's one thing is what buys FM its noise advantage over the whole amplitude family. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: A

The 19 kHz pilot is doubled to 38 kHz, giving a subcarrier locked in both frequency and phase to the one the transmitter suppressed, which is exactly what coherent detection of the difference signal requires. The audio limit is not a reference. The radio data subcarrier is itself derived from the pilot, at three times its frequency. And the intermediate frequency belongs to a different part of the receiver entirely. Deriving every subcarrier from one pilot is what makes the multiplex coherent.