Superheterodyne Receiver Questions and Answers

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.

Answer: A

The key marks both statements as true, with the reason explaining the assertion. In a superheterodyne receiver the incoming signal is mixed with the local oscillator voltage, and the difference frequency is converted to a fixed lower frequency, the intermediate frequency, where amplification and selectivity are easier. The choice of that frequency is a compromise: it should not be too low, because image rejection gets poorer, and it should not be too high, because selectivity gets poorer, so it lies in a range between the two, which is why the reason explains why a fixed intermediate frequency of that kind is used. The answer is that both are true and (R) is the correct explanation of (A).

Answer: B

Both statements are true but the reason is not the correct explanation, so option (B) is right. Radio and television receivers are generally of the superheterodyne type, because it gives good selectivity and sensitivity by converting every station to one fixed intermediate frequency, which makes the assertion true. The reason, that wireless communication is possible by receiving signals through superheterodyne receivers, is a true remark in itself, but it does not explain why these receivers are in common use, since wireless communication is also possible with other types of receiver, such as the tuned radio frequency receiver. Option (A) calls it the explanation.

Answer: A

The sequence in a superheterodyne receiver is the RF amplifier, then the mixer, then the AM detector, then the AF amplifier, which is (iii), (ii), (i), (iv), the first option. The mixer converts the incoming signal to the intermediate frequency, and the detector then recovers the audio, which the AF amplifier boosts.

The other options place the detector before the mixer or put the AF amplifier ahead of the detector, which would amplify a signal not yet demodulated.

Answer: D

The keyed order is 1, 2, 4, 5, 3, option (D): antenna and RF stage, mixer with local oscillator, IF amplifier, detector, and then the amplifier. This is the order of a superheterodyne receiver, where the detector recovers the audio and the following amplifier raises it.

Options A and C put the amplifier before the IF stage or the detector, and option B starts with the detector, which comes after the IF stage. The block is printed as class C, which is not usual for audio amplification, but the key places it last, and a reviewer may wish to note this.

Answer: A

The architecture's whole point is that whatever the station's frequency, it is translated to a single fixed intermediate frequency where all the gain and selectivity live. Amplifying at the signal frequency is the tuned radio frequency receiver the superhet replaced, precisely because its filters had to be retuned and its bandwidth changed across the band. Two detectors in succession describes nothing in this chain. And an oscillator on the signal frequency would produce zero difference, which is the direct conversion receiver rather than a superhet. Fixing the frequency at which the hard work is done is the single idea from which every other property of the architecture follows.

Answer: B

In a tuned radio frequency set every filter tunes with the signal, so its bandwidth and gain vary across the band and the selectivity is worst where the frequency is highest. Converting to a fixed frequency makes the selectivity identical for every station, which is the decisive advantage. An audio stage, a detector and an antenna match are all still needed and are common to both architectures. Identifying what an architecture actually fixes, rather than listing everything it contains, is how its merits are judged.