FM Transmitters and Receivers Questions and Answers

Answer: A

The pilot's job is to be detected, so too low a level leaves receivers unable to recognise a stereo transmission and they stay in mono. The difference signal is still transmitted, and indeed still occupies its deviation, so the power is spent for no benefit. Bandwidth is unaffected by the pilot's level. And the sum signal is independent. Faults that waste transmitted capacity without producing an audible error are the ones most easily overlooked in monitoring.

Answer: B

Channel separation, measured by feeding one channel only and checking how little appears in the other, tests the whole chain from matrix through subcarrier to decoder. Antenna current does not change with FM modulation at all. Standing wave ratio tests the antenna. And modulator input level tests only the audio front end. An end-to-end measurement must exercise every stage in between, which separation does and none of the alternatives do.

Answer: C

Because the information is in the angle, everything the noise does to the amplitude can be thrown away by a limiter, which is the mechanism behind both the noise advantage and the capture effect. FM occupies far more spectrum, not less. Its detector is more complex than a diode. And tuning tolerance is comparable rather than better. Stating an advantage together with the mechanism that produces it is what distinguishes understanding from recitation.

Answer: D

Capture is a contest between two signals, so the measurement needs two generators on the same frequency, one level adjusted until the other is suppressed. A single generator cannot create the contest. A spectrum analyser observes rather than tests. And a distortion meter measures the result without providing the stimulus. Any specification describing behaviour between two signals requires two sources, which is a useful check on whether a proposed test can measure what it claims.

Answer: A

The exciter delivers a finished modulated signal at a few watts, which the power amplifier then raises to the transmitted level. Separating the two makes maintenance and redundancy practical, since a spare exciter can be switched in without touching the power stage. An unmodulated carrier is what the oscillator alone gives. The composite is the exciter's input. And bias is a supply matter. Equipment vocabulary repays learning because it maps onto how systems are actually maintained.

Answer: B

Redundancy at the low-power end is cheap and covers the most complex part of the chain, so a standby exciter restores service in seconds. Running two in parallel would not add power, since they feed one amplifier. The frequency reference is internal to each. And two programmes would need two transmitters and two channels. Placing redundancy where complexity is high and cost is low is a general principle of broadcast engineering.

Answer: C

On a strong signal the noise is dominated by the demodulation gain, which the deviation sets, together with the improvement de-emphasis contributes. The audio amplifier's noise is negligible by comparison. Antenna aperture affects the signal delivered rather than the ultimate ratio once the signal is ample. And extra filters improve selectivity without changing the noise floor. Which quantity binds depends on the signal level, which is why FM specifications quote sensitivity and ultimate ratio separately.

Answer: D

Limiting flattens amplitude and filtering removes the harmonics that flattening created, leaving a clean constant-envelope signal carrying the same modulation. That is exactly why nonlinear amplification is harmless to angle modulation and why class C stages can be cascaded. Conversion to amplitude modulation is what a slope detector does deliberately. Deviation is unchanged. And the signal survives precisely because the information is not in the amplitude.

Answer: A

Loud passages deviate furthest, so a filter that clips the outer sidebands distorts them while quiet passages pass cleanly. That level dependence is the signature of a bandwidth problem, at the receiver or in the transmitter's deviation. A pilot fault gives mono. A muting fault gives silence or noise. And excessive audio gain would clip at the output, which is also level dependent but would not respond to retuning the station. Noting whether a symptom follows level, frequency or tuning is what separates the possibilities.

Answer: B

Distortion follows any curvature in the frequency-to-voltage characteristic, so its straightness across the deviation range is what matters. Filter Q sets selectivity. Audio bandwidth sets the reproduced range. And limiter gain determines only whether limiting is complete. This is why the phase-locked loop displaced the discriminator family: its linearity comes from the loop rather than from the shape of a tuned circuit's response.