FM Transmitters and Receivers Questions and Answers

Answer: C

Its output follows frequency linearly by construction, so distortion is low and stays low without any tuned-circuit alignment to drift. Image rejection depends on the preselector. Passband width is set by the intermediate filters. And co-channel behaviour is the capture effect, common to any limiting FM receiver. Replacing an adjustment with a topology that needs none is the same argument that favours synthesisers over pulled oscillators in transmitters.

Answer: D

It replaces the tuned intermediate frequency transformers of older designs, giving steeper skirts, a flatter passband and no alignment. The limiter, de-emphasis network and preselector all serve different functions and remain. Substituting a fixed component for an adjustable one, wherever the frequency is fixed, is the pattern that made mass-produced receivers reliable, and it is worth recognising in any modernised block diagram.

Answer: A

A 200 kHz channel is a negligible fraction of the very high frequency spectrum and an impossible one at medium wave, so wideband FM could only be accommodated up there. Propagation is in fact more limited, being essentially line of sight, which suits local coverage. Antennas are indeed smaller and cheaper, which is a benefit rather than the reason. And receiver noise is not markedly lower. Spectrum availability decides where a wideband service can live.

Answer: B

Very high frequency signals travel little beyond the horizon, so an FM station covers a local area and the same frequency can be reused a modest distance away. Night-time extension and ionospheric dependence belong to medium and short wave. And terrain matters greatly, which is why transmitter sites are chosen on high ground. Frequency choice determines propagation, which in turn determines the whole structure of a broadcasting service.

Answer: C

Because coverage is limited by line of sight, two distant transmitters do not reach one another's audiences and the frequency can be reused with adequate geographic spacing. Receivers do not reject co-channel signals; the capture effect merely favours the stronger, which is why spacing is still needed. Phase synchronisation is used in some single-frequency networks but is not the general mechanism. Frequency reuse is a planning consequence of propagation.

Answer: D

The composite drives the modulator, so every component of it appears as a corresponding deviation of the carrier frequency, and the receiver's discriminator recovers the whole composite again. The envelope stays constant throughout, which is what makes class C amplification possible. Subcarriers exist in the baseband rather than around the intermediate frequency. And the phase reference is the pilot, one component of the composite. Following one signal through both ends of the link is the clearest way to see the system.

Answer: A

The decoder needs the composite recovered with its amplitudes and phases intact, since the difference signal is detected coherently against the regenerated subcarrier. Deviation and carrier frequency describe the transmitted signal rather than what the decoder consumes. Pilot level and intermediate frequency are conditions rather than the signal. And audio bandwidth and de-emphasis act after decoding. Coherent detection always imposes requirements on phase as well as amplitude, which is why the pilot exists.

Answer: B

Coherent detection depends on phase, so an error leaks part of each channel into the other and separation degrades, which is exactly why the pilot is transmitted and doubled rather than a free-running oscillator being used. Pitch shifts follow frequency errors in single sideband rather than here. The sum signal is recovered independently of the subcarrier. And muting responds to level. Every quadrature system fails first in crosstalk when its phase reference is imperfect.

Answer: C

Channel separation is the figure, measured by driving one channel and reading the level appearing in the other, and it exercises the matrix, the subcarrier regeneration and the phase accuracy together. Capture ratio concerns two competing signals, image rejection one interfering frequency, and amplitude suppression the limiter. Each specification tests one mechanism, and separation is the one that tests the stereo system end to end.

Answer: D

Because the amplitude never varies, no information is lost when the amplifier saturates, so class C or switching stages can be used and efficiency is high. Filtering removes harmonics but cannot restore a distorted envelope, so it is not the reason. The carrier is present throughout. And deviation limiting controls bandwidth rather than enabling nonlinear amplification. Constant envelope is the single property that decides the amplifier class available, in this scheme as in every other.