FM Transmitters and Receivers Questions and Answers

Answer: A

Ceramic filters at 10.7 MHz give the 200 kHz passband with steep skirts, and several are usually cascaded. The antenna provides broad preselection at best. De-emphasis and audio roll-off act on the recovered audio, long after the channels have been separated or confused. Selectivity is always a filtering job at a fixed frequency, which is the superheterodyne's central economy in FM exactly as in AM.

Answer: B

It strips amplitude variation, which in FM carries only noise and fading, and that single action is the source of most of FM's noise advantage and of the capture effect. Deviation is set at the transmitter and no receiver stage restores it. The stereo reference comes from the pilot. And converting frequency to amplitude is the discriminator's function, which follows the limiter. The limiter is the stage with no counterpart in an AM receiver.

Answer: C

A phase-locked loop needs no tuned transformer, holds its alignment and integrates onto a chip, which is why it displaced the discriminator family in consumer equipment. All three alternatives depend on carefully tuned inductive components that drift and resist integration. The advantage is one of manufacturability rather than theory, and it is a good reminder that circuit choices are often decided by production rather than performance.

Answer: D

It undoes the transmitter's treble boost and in doing so attenuates the noise that rose with frequency, which is the net gain in signal-to-noise ratio the pair delivers. Discriminator nonlinearity is a design matter. Filter phase error is corrected, if at all, by filter selection. And residual amplitude variation is the limiter's problem. The pair must be matched end to end, which is why the European and North American time constants cannot be mixed.

Answer: A

Capture ratio is the difference in decibels by which one signal must exceed another for the weaker to be suppressed, and a good receiver achieves one or two decibels. It is a figure peculiar to FM, arising from limiting and discrimination together, and it has no counterpart in an AM specification. The other three describe gain distribution, spectral occupancy and modulation, none of which is what the term means.

Answer: B

It measures the limiter's effectiveness at rejecting amplitude modulation on the incoming signal, which is a direct test of the mechanism behind FM's noise immunity. Transmitter amplitude modulation is not a parameter of an FM system. Subcarrier leakage is a separate stereo specification. And image attenuation is quoted as image rejection. Each figure of merit tests one mechanism, and knowing which is which makes a specification sheet readable.

Answer: C

It is the input level at which the output noise falls by a stated amount, commonly quoted as a signal-to-noise-and-distortion figure, and it is the FM equivalent of the AM sensitivity specification. The muting threshold is a separate adjustable level. The stereo indicator responds to the pilot rather than to the noise. And residual noise with no signal is a different measurement altogether. FM sensitivity is defined by the noise improvement because of the threshold effect.

Answer: D

Because of the threshold effect, the output collapses into noise below a certain input level, so muting spares the listener. It is the same reasoning as squelch in a two-way radio. The limiter draws no more current on weak signals. Nothing is damaged. And while a stereo decoder does need a usable pilot, mono muting operates regardless. Muting is a response to a sharp performance cliff rather than a protective measure. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: A

The decoder needs a detectable 19 kHz pilot, and on a weak or noisy signal it reverts to mono rather than reproduce a noisy difference channel - a sensible degradation, since the difference signal sits highest in the baseband where discriminator noise is worst. De-emphasis is a fixed network. Discriminator amplitude is held constant by limiting. And frequency control has no bearing on the decoder. Blending to mono is a deliberate design response to where the noise lies.

Answer: B

The discriminator's noise rises with baseband frequency, so the difference signal at 23 to 53 kHz suffers far more than the sum signal at 0 to 15 kHz, which is why stereo reception needs a stronger signal than mono and why receivers blend to mono when it weakens. The deviation allocation is comparable. Regeneration is a phase matter rather than a noise one. And the matrix is passive. The triangular noise spectrum explains the whole behaviour.