Superheterodyne Receiver Questions and Answers

Answer: A

Phase noise on the oscillator mixes strong adjacent signals into the intermediate frequency passband, so a noisy oscillator undoes the filter's selectivity. That is why a synthesiser's phase noise is specified as carefully as its stability. Conversion gain and injection level are drive matters. And a mean frequency shift is drift rather than phase noise. A receiver's selectivity is only as good as the cleanliness of the signal it converts with.

Answer: B

A filter narrower than the signal removes part of the signal along with the noise, so the useful limit is the signal's own occupied bandwidth - a few hundred hertz for Morse, a few kilohertz for speech. Audio stability is unrelated. Crystal quality factors permit far narrower filters than are wanted. And gain is not the constraint. The matched filter rather than the narrowest filter is optimal, and that holds for every kind of receiver.

Answer: C

A radiating oscillator appears as an unwanted transmission that other receivers hear, and in some services it also reveals that a receiver is present. It does not raise the set's own noise figure, detune the preselector or upset the gain control. Shielding and careful antenna coupling are the remedies. A receiver is not a purely passive device, and the obligations of a transmitter apply in miniature to the oscillator inside it.

Answer: D

The assertion is true but the reason is wrong: selectivity is uniform because the intermediate frequency filter works at a fixed frequency, not because the preselector is broad. The preselector's breadth is a limitation it happens to have, and its actual job is image rejection. A true conclusion supported by the wrong mechanism is the most instructive kind of error, because it survives every check except tracing the cause.

Answer: A

The reason is true but the assertion is false: the image arrives at the intermediate frequency itself, so narrowing that filter rejects the wanted signal and the image equally. Image rejection is set before the mixer, by the preselector and by how far out the intermediate frequency places the image. Two true statements about the same component can still fail to explain one another, and this pair is a clean example.

Answer: B

All three are correct, and together they describe the architecture completely: the high first stage handles the image, the low second stage handles the neighbours, and because the second conversion is between two fixed frequencies its oscillator need not tune at all and can therefore be crystal controlled. Each choice follows from the job that stage has to do, which is why the architecture is easier to reconstruct than to memorise.

Answer: C

The first three are right and the fourth is not, so three match. Output level constancy is the work of the automatic gain control loop; the detector merely supplies the voltage the loop measures. Attributing each specification to the block that actually sets it is what makes a specification sheet diagnostic, because a disappointing number then points straight at the stage to examine.

Answer: D

On a crowded band interference dominates, so Receiver B wins on the two parameters that matter, a narrower passband and much steeper skirts, despite its poorer noise figure. Receiver A would be the better choice on a quiet band. The comparison is decidable precisely because the environment names which parameter is limiting, and without that context neither receiver is simply better.

Answer: A

The pairing is A-P, B-Q, C-R. The image must go before conversion because afterwards it is indistinguishable; an adjacent channel remains distinguishable and so is the intermediate frequency filter's business; and hum enters through the supply and is cured there. Every unwanted signal has exactly one place in the chain where it can still be told apart from the wanted one, and that place is where the remedy belongs.

Answer: B

The order is 1, 2, 3, 4: fix the intermediate frequency first, then the low end with the padder, then the high end with the trimmer, then verify in the middle. Aligning the front end against an unaligned intermediate frequency strip would build the error in permanently. Establish the fixed reference, adjust the extremes, verify between - a sequence that recurs whenever two adjustments interact at opposite ends of a range.