Superheterodyne Receiver Questions and Answers

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 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.