AM Transmitters and Receivers Questions and Answers

Answer: A

A signal at that particular offset is the image, so preselection ahead of the mixer is what must improve - or the intermediate frequency must be raised so the image falls further out. Adjacent-channel selectivity acts on the wrong offset. Sensitivity is unrelated. And gain control range governs how strong a signal can be handled rather than which frequencies get through. The offset named in the complaint identifies the mechanism uniquely.

Answer: B

The image mixes to the same intermediate frequency as the wanted signal, so no filter after the mixer can separate them and the rejection must happen before. Impedance matching is a secondary benefit of the input circuit. Oscillator isolation matters but is not the preselector's purpose. And audio overload is prevented by gain control. This is the same reasoning as anti-alias filtering, and it recurs wherever an operation is irreversible.

Answer: C

If the oscillator drifts, the signal slides off the intermediate frequency passband and the station fades and distorts, which is why oscillator design and thermal compensation matter so much in a receiver. Distortion at all levels points to the detector or an overloaded stage. Image rejection depends on the preselector. And gain control hunting is a loop stability problem. Each fault has a signature, and drift shows itself as a station that will not stay put.

Answer: D

Sensitivity is set by the noise added before the gain, so improving the first stage's noise figure is the only change that helps materially. Additional audio gain amplifies noise and signal alike. A narrower audio filter helps a little by reducing the noise bandwidth, at the cost of fidelity. And the loudspeaker affects loudness rather than sensitivity. The cascade formula's weighting of the first stage is the reason low-noise preamplifiers are placed at the antenna.

Answer: A

It is the gain, or more often the loss, a signal experiences in being translated by the mixer, and a passive diode mixer has a conversion loss of several decibels while an active mixer can show gain. Audio gain is a separate figure. The frequency ratio is not a gain. And the benefit of double conversion is in image rejection and selectivity rather than gain. Knowing where a chain loses signal is what makes a noise budget possible.

Answer: B

The situation needs both sensitivity for the weak wanted signal and selectivity against the strong neighbour, and R alone has both. P is sensitive but would be swamped by the adjacent channel. Q rejects the neighbour but cannot hear the wanted station. The requirements do conflict in design, which is why R is the more expensive receiver, but they are not mutually exclusive. Reading a specification table against the actual operating problem is the skill being tested.

Answer: C

A class C stage conducts for only part of the cycle, so its output is rich in harmonics which the tuned circuit and output filter must remove. The oscillator and buffer are operated to produce a clean sinusoid. And the audio pre-amplifier works at frequencies far below the carrier. The harmonics are the price of the efficiency, which is why filtering is not optional in a transmitter that uses class C.

Answer: D

It measures the mismatch between the feeder and the antenna, and hence how much power is reflected rather than radiated. A high ratio wastes power and can damage the output stage. Modulation depth needs a monitor. Harmonic content needs a spectrum analyser or a filter measurement. And frequency accuracy needs a counter. Antenna matching is the one transmitter parameter that changes with weather, icing and nearby objects, which is why it is monitored continuously.

Answer: A

Since the transmitted bandwidth is twice the highest audio frequency, limiting the audio to four or five kilohertz is what keeps the signal inside a nine or ten kilohertz channel. Audio power depends on the modulation depth rather than the bandwidth. Linearity is a circuit property. And over-modulation is prevented by a peak limiter, which is a separate stage. The audio chain's filter, limiter and level are three distinct controls serving three distinct regulatory requirements.

Answer: B

The chain runs level control, band limiting, peak limiting, then the modulator: set the working level, remove content the channel cannot carry, hold the peaks below full modulation, and only then modulate. Peak limiting must come after band limiting, because filtering can create new peaks. And nothing can follow the modulator, which is the end of the audio path. Ordering a signal chain by what each stage assumes about its input is the general method here.