AM Transmitters and Receivers Questions and Answers

Answer: A

The dial reads the signal frequency, which is the oscillator frequency less the intermediate frequency, so correct tracking is what makes the calibration true across the band. Padder and trimmer adjustments exist precisely to align it at three points. Audio gain, detector characteristics and loudspeaker impedance have no bearing on which frequency the dial indicates. Alignment is the process of making a mechanical scale agree with an electrical relationship.

Answer: B

Sensitivity is quoted as the input level needed for a stated output signal-to-noise ratio, so a generator of known output is essential. Audio power needs a load and a voltmeter. Loudspeaker impedance needs a bridge. And idle current needs only an ammeter. Measurements that reference an input level always require a calibrated source, which is why the signal generator is the central instrument on a receiver bench.

Answer: C

Fidelity is the term, and it is limited chiefly by the intermediate filter width and by the detector's linearity. Selectivity is the rejection of other channels, sensitivity the ability to use weak signals, and image rejection the suppression of one particular interfering frequency. The three classical figures of merit pull against one another, which is why a receiver specification is a set of compromises rather than a list of maxima.

Answer: C

To raise the level of very weak signals from an antenna you would use an RF amplifier.

The signal at the antenna is at radio frequency and may be only microvolts. An RF amplifier, usually a low-noise amplifier at the front of the receiver, boosts it at its own frequency before the mixer. Because it comes first, its gain and noise figure largely set the sensitivity of the whole receiver, and it also improves image rejection.

Oscillators, RF or audio, generate signals rather than amplifying them; an RF oscillator is the receiver's local oscillator. An audio amplifier works only after detection, at audio frequencies, so it cannot boost the RF signal from the antenna.