Superheterodyne Receiver Questions and Answers

Answer: C

Freedom from spurious responses is not among its advantages - the architecture actually creates them, the image being the most important, because a mixer will combine any pair of frequencies whose harmonics differ by the intermediate frequency. The other three are its genuine benefits. An architecture chosen for good reasons still brings its own characteristic problems, and honest comparison requires listing both.

Answer: D

A wave trap is a tuned circuit that notches out one particular offending frequency, typically a strong local transmitter, before it can reach the mixer. It narrows rather than broadens the response, matching is the antenna coupling's job, and injection comes from the oscillator. When one interferer dominates, a notch at that frequency is far cheaper than improving selectivity generally, which is a useful principle wherever a single dominant problem can be identified.

Answer: A

If a transmitter operated at the intermediate frequency, the strip could pick it up directly and it would be heard at every dial setting. That is why 455 kHz sits below the medium wave band and 10.7 MHz between the short wave allocations. Component cost, oscillator capacitance and detector time constants do not influence the choice. Standard values usually encode a constraint that is invisible until it is violated, and this is a clear example.

Answer: B

Misaligned transformers reduce the gain at the true intermediate frequency and skew the passband, so sensitivity and selectivity suffer together. Image rejection is decided before the mixer and is untouched. The oscillator is a separate circuit. And distortion on loud peaks points at the detector or the audio chain. Because a misalignment degrades two specifications at once, it is worth suspecting whenever a set is uniformly poor rather than poor in one respect.

Answer: C

Raising the intermediate frequency moves the image further out, where the existing preselector attenuates it more, and it requires no change to the front end at all. Narrowing the filter cannot help, since the image arrives at the intermediate frequency itself. Extra gain and slower gain control are irrelevant. Where a problem depends on a distance, moving the target is as effective as sharpening the instrument, and often much cheaper.

Answer: D

The cost of the improvement is that a narrow, steep filter is much harder to build at a higher frequency, so adjacent channel selectivity suffers. The oscillator's ratio actually narrows slightly with a larger offset. Conversion loss does not scale that way. And the detector's time constant becomes easier rather than harder to satisfy. Naming the price alongside the benefit is what turns a fact about a parameter into a design judgement.

Answer: D

The beat (intermediate) frequency is 10.7 MHz.

In a superheterodyne receiver the local oscillator beats with the incoming signal, and the difference frequency is the IF. Broadcast FM receivers (88 to 108 MHz) standardise on 10.7 MHz, high enough to keep the image frequency, 21.4 MHz away, outside the FM band, and wide enough to pass the 200 kHz FM channel.

455 kHz is the standard IF of AM broadcast receivers, and 910 kHz is twice it. 21.14 MHz is near 21.4 MHz, the image separation (twice the IF), not the IF itself.

Answer: B

The signal may overload the later IF and AF stages.

Automatic gain control reduces the gain of the RF and IF amplifiers as the received signal grows stronger, keeping the signal level at the detector roughly constant. Without AGC, a strong station is amplified by the full gain, so the later IF and audio stages are driven beyond their linear range: they saturate and clip, causing distortion and blasting.

Underloading is the problem of weak stations, not strong ones. 'Cut-off' and 'cut-in' are not the terms for this effect.

Answer: D

The correct matching is a-ii, b-iv, c-iii, d-i.

• A mixer multiplies two signals to shift a signal to a new frequency, which is frequency conversion (ii).
• A voltage-controlled oscillator changes its frequency with the input voltage, which generates FM (iv).
• A ring (balanced) modulator multiplies the message by the carrier and suppresses the carrier, generating DSB-SC (iii).
• Codes here refers to convolutional codes, whose encoder convolves the input bit stream with its generator sequences (i).

The other codes give the mixer or the ring modulator the wrong function.

Answer: D

The characteristic is fidelity.

Fidelity is a receiver's ability to reproduce faithfully all the modulating frequencies of the original audio signal, without distortion. It depends on the frequency response of the IF and audio stages; good fidelity needs a flat response across the audio band, for example 50 Hz to 10 kHz for AM broadcast.

Sensitivity is the ability to pick up weak signals and give a usable output. Selectivity is the ability to separate the wanted station from adjacent ones. Image rejection is a superheterodyne receiver's ability to reject the image frequency. None of these is about faithful reproduction of the audio.