FM Transmitters and Receivers Questions and Answers

Answer: A

A reactance modulator pulls an oscillator's tuned circuit with the message and so varies the frequency directly, which is the simplest route to large deviation. A balanced modulator with a filter produces single sideband. Collector modulation is high-level amplitude modulation. And a detector cannot be run backwards as a modulator in any useful sense. Matching each modulator to the quantity it varies is the quickest way through a transmitter diagram.

Answer: B

It holds the centre frequency against drift by comparing the oscillator with a crystal reference through a time constant long enough not to cancel the wanted modulation. That last detail is the subtle part of the design, since too fast a loop would flatten the audio. Deviation is set by the modulator's sensitivity and checked separately. Audio distortion is a linearity matter. And antenna matching belongs to the output network. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: C

Varying an oscillator's tuned circuit means it cannot also be crystal controlled, so its centre frequency drifts with temperature and supply and must be steered back. There is no multiplier chain in a direct transmitter, which is the Armstrong method's feature. The deviation limiter acts on audio. And a well designed audio chain has no drifting offset. The choice between direct and indirect generation is essentially a choice about where the stability comes from.

Answer: D

Multiplication scales the small phase deviation a linear phase modulator can produce up to the seventy-five kilohertz broadcasting needs, taking the carrier frequency with it. A bigger varactor belongs to the direct method. More audio would simply drive the phase modulator into its nonlinear region. And filters do not create deviation. The chain exists because the modulator's linear range and the required deviation differ by a large factor. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

The deviation is scaled by the same factor as the carrier, so the modulation index rises too, which is the whole purpose. The audio rate is a property of the modulating signal and is untouched by any radio-frequency operation. Amplitude is held constant by limiting. And the audio filter's bandwidth is fixed in the audio chain. Being clear about what scales and what does not is the key to understanding indirect generation. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: B

Multiplication scales carrier and deviation together, so a mixer is used to shift the carrier without touching the deviation, letting the designer reach the required output frequency and the required deviation independently. Carrier suppression plays no part in FM. Harmonics are an unwanted by-product rather than a goal. And the modulator needs no external phase reference. Having two operations with different effects is what makes the chain flexible.

Answer: C

A deviation limiter caps the audio peaks so the occupied bandwidth stays inside the channel however loud the programme. It sits after pre-emphasis, because pre-emphasis itself raises deviation on bright material. The output tuned circuit and the antenna network handle radio frequency rather than modulation depth. And the control loop holds the centre frequency. Each regulatory requirement maps to one identifiable stage. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: D

Pre-emphasis comes first and limiting second, because boosting the treble increases deviation and the limiter must see the signal that will actually be transmitted. Reversing them would let pre-emphasis push the peaks back over the limit. De-emphasis belongs in the receiver, not the transmitter. And limiting at the microphone would control level rather than deviation. The order follows from what each stage assumes about its input. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

Class C is ideal, because FM's envelope is constant and amplitude distortion therefore costs nothing, so efficiency can be pushed high. The three linear classes are needed only where an envelope must be preserved, as in single sideband. That freedom is one of FM's practical advantages over the amplitude family, alongside its noise performance, and it is why an FM transmitter of given output runs cooler than an SSB one. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

The band is 88 to 108 MHz in most of the world, with 200 kHz channels accommodating Carson's 180 kHz and a guard band. 540 to 1600 kHz is the medium-wave AM broadcast band. The 10.7 MHz figure is the FM receiver's intermediate frequency rather than a broadcast band. And 174 to 230 MHz is a television and digital radio band. These conventional numbers anchor every bandwidth calculation in the topic to real allocations. Framing the question as which quantity the change acts upon makes the answer follow without calculation.