Superheterodyne Receiver Questions and Answers

Answer: A

The first amplifier dominates, because the Friis cascade divides every later stage's contribution by the gain accumulated ahead of it. The intermediate frequency amplifier matters more than one might expect when the mixer has loss, but it is still divided down. The detector and audio stages are effectively irrelevant. That weighting is why a low-noise front end is worth its cost and why any loss placed before it is so damaging.

Answer: B

Loss in the mixer reduces the accumulated gain, so the intermediate frequency amplifier's own noise is divided by less and contributes more. Oscillator power is a drive requirement rather than a noise consequence. The intermediate frequency is set by the oscillator's offset and does not shift. And noise bandwidth is a filter property. Wherever a chain loses gain, everything after that point becomes more important to the noise figure, which is why a preamplifier usually precedes a passive mixer.

Answer: C

This is the image frequency, sitting twice the intermediate frequency from the wanted signal and one intermediate frequency on the far side of the oscillator. The intermediate frequency is the output of the conversion rather than an input to it. The oscillator is what performs the conversion. And an adjacent channel remains distinguishable after mixing, which is why the intermediate frequency filter can deal with it. The image is singled out precisely because it is the one unwanted response the architecture cannot fix downstream.

Answer: D

The series element is the padder, and it has its greatest influence where the variable tuning capacitance is largest, at the bottom of the band. The trimmer is the parallel element governing the top end. A bypass capacitor provides a low-impedance path to earth for radio frequencies. And a coupling capacitor passes signal between stages while blocking direct current. Whether a component sits in series or in parallel is what decides where in a tuning range it acts.

Answer: A

This is the beat frequency oscillator, injected at the detector to supply the carrier those modes deliberately omit. The local oscillator performs the frequency conversion much earlier in the chain. A crystal reference stabilises a frequency. And a voltage controlled oscillator is a building block of a phase-locked loop. Two oscillators in one receiver do quite different jobs, and keeping their roles separate is essential to reading the block diagram correctly.

Answer: B

A frequency multiplier belongs to a transmitter's exciter chain, where a stable low-frequency oscillator is stepped up to the operating frequency. The other three are essential receiver blocks. The distinction is worth keeping because transmitter and receiver chains share vocabulary - both have oscillators, amplifiers and tuned circuits - while the direction of the signal flow reverses the purpose of nearly every stage.

Answer: C

Image rejection is not among them, because the image arrives at the intermediate frequency itself and the strip cannot distinguish it from the wanted signal. The other three are exactly what the strip provides, and they are what makes concentrating the receiver's work at a fixed frequency worthwhile. Knowing what a stage cannot do is as useful as knowing what it can, and here that limitation is the reason the preselector exists.

Answer: D

Correct alignment is a remedy rather than a cause; the other three are genuine mixing mechanisms that place unwanted inputs inside the passband. Spurious responses arise from the mixer's willingness to combine any pair of frequencies whose harmonics happen to differ by the intermediate frequency, which is a large set. Their cure is selectivity before the mixer and a clean oscillator, since nothing after conversion can help.

Answer: A

With a first intermediate frequency above the highest signal frequency, the image lies beyond the band the receiver covers and a simple low-pass filter rejects it completely. That is why general coverage receivers convert 0 to 30 MHz up to 45 MHz or higher. Ganging is still needed or replaced by synthesis. The detector is unaffected. And a high intermediate frequency makes narrow filtering harder, which is why a second conversion usually follows. Moving a problem outside the range of interest is often better than solving it.

Answer: B

Because the second conversion is from one fixed frequency to another, the oscillator needs no tuning at all and can therefore be crystal controlled for stability. Drift there would move the signal off the narrow second filter. Injection level and conversion loss are unrelated to the oscillator's type. And smooth tuning is exactly what a crystal does not offer, nor does it need to. Where a frequency need not vary, fixing it with the most stable available reference costs nothing.