Superheterodyne Receiver Questions and Answers

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.

Answer: C

The local oscillator supplies the second input to the mixer, and it is ganged to the preselector so that their difference stays constant as the dial turns. The preselector is the tuned circuit ahead of the mixer that selects the wanted region of the band. The intermediate frequency amplifier provides most of the gain after conversion. And the detector recovers the modulation at the end of the chain. Naming each block by the one job it does is the foundation for reasoning about any receiver fault.

Answer: D

A mixer is a non-linear stage that yields the sum and the difference of its two inputs, and the intermediate frequency strip selects the difference. It does not simply amplify, which is what an amplifier does. Recovering the audio directly is the detector's role much later in the chain. And multiplication of a carrier belongs to a transmitter's frequency multiplier. Every property of the superhet, image response included, traces back to the fact that a mixer produces two outputs where the designer wants one.

Answer: A

The difference is held constant, because that difference is the intermediate frequency and the whole architecture depends on it never moving. The oscillator frequency plainly changes, since it must track the signal. Holding the ratio constant would make the difference vary with frequency, which is exactly the tracking error the padder and trimmer exist to correct. And the preselector's bandwidth drifts with tuning as an unavoidable side effect rather than being held fixed. Asking which quantity a mechanism holds constant is usually the quickest route to understanding it.

Answer: B

The intermediate frequency amplifier supplies most of the gain, because it works at one fixed frequency where high, stable gain with well-defined selectivity is easy to obtain. The preselector provides modest gain, chiefly to keep the mixer's noise from dominating. A passive mixer has loss rather than gain. And the audio stage delivers power rather than voltage gain. Concentrating gain where the frequency never changes is the engineering economy that makes the architecture worth its extra blocks.

Answer: C

The preselector's two jobs are attenuating the image frequency before it can reach the mixer, and keeping strong out-of-band signals from overloading it. Main selectivity belongs to the intermediate frequency strip, which is the point of converting in the first place. The intermediate frequency is set by the oscillator's offset, not by the preselector. And injection level is the oscillator's own affair. A stage placed before a mixer can only help with problems the mixer would otherwise create, and the image is the chief one.

Answer: D

Once the mixer has converted it, the image lands on exactly the intermediate frequency and is indistinguishable from the wanted signal, so no later filter can separate them. Adjacent channels differ from the wanted signal after conversion and are removed by the intermediate frequency filter. Hum and audio intermodulation arise later in the chain and are addressed there. The rule is general: any two inputs a stage maps onto the same output can only be separated ahead of that stage.

Answer: A

The image sits two intermediate frequencies from the wanted signal, because each lies one intermediate frequency on either side of the local oscillator. That spacing is the single most useful fact about the image: it explains why a higher intermediate frequency pushes the image further out where the preselector attenuates it more. One intermediate frequency away is the oscillator's own offset. Half is a common confusion. And the oscillator frequency is midway between the two responses rather than being one of them.

Answer: B

The oscillator sits above the signal, and the image sits above the oscillator by the same offset:

\[ \begin{aligned} f_{o} &= 1000 + 455 = 1455 \\ f_{i} &= f_{o} + 455 = 1910 \ \mathrm{kHz} \end{aligned} \]

so 1910 kHz, which is also the signal plus twice the intermediate frequency. 1455 kHz is the oscillator itself. 545 kHz is the signal minus twice the intermediate frequency, the image for low-side injection. And 2000 kHz is simply the second harmonic. Working from the oscillator outwards, rather than memorising a formula, keeps the two injection cases straight.