Superheterodyne Receiver Questions and Answers

Answer: C

With high-side injection the oscillator always sits one intermediate frequency above the signal:

\[ f_{o} = 600 + 455 = 1055 \ \mathrm{kHz} \]

so 1055 kHz. 145 kHz is the low-side alternative, and it is not used on the medium wave band because the oscillator would then have to cover a far wider frequency ratio. 600 kHz is the signal, and 455 kHz the intermediate frequency. High-side injection is standard precisely because the narrower tuning ratio is much easier to achieve in one oscillator.

Answer: D

With the oscillator above the band, its tuning ratio is much smaller than the signal band's, which a single variable capacitor can cover: on medium wave the oscillator spans about 1055 to 2055 kHz, a ratio under two, while low-side injection would demand a ratio of nearly ten. The image distance is twice the intermediate frequency either way. Injection level and the choice of intermediate frequency are unaffected. Practical constraints on component ranges, rather than theory, often decide an architecture's details.

Answer: A

Raising the intermediate frequency pushes the image further from the wanted signal, where the preselector's skirts attenuate it more, so rejection improves. That is the central argument for a high first intermediate frequency. The mixer's difficulty does not enter into it. Rejection depends on both the preselector's selectivity and the image's distance, so it is not the preselector alone. And a higher intermediate frequency makes narrow filtering harder rather than easier, which is exactly the tension the next question addresses.

Answer: B

A high intermediate frequency improves image rejection, while a low one makes narrow, steep-skirted adjacent channel filtering far easier to build, and no single frequency does both well. That conflict is why 455 kHz became the compromise for broadcast AM and why double conversion exists at all. The other three pairs name real requirements that do not compete over this particular choice. Recognising which two requirements a design parameter is caught between is what makes its value comprehensible.

Answer: C

Double conversion takes both benefits in turn: a high first intermediate frequency for image rejection, followed by a low second one where sharp adjacent channel filtering is practical. The two mixers need different oscillators, the second usually crystal controlled. Two filters at one frequency would improve selectivity but do nothing for the image. And a second detector is not involved. Taking a conflict in stages, rather than compromising once, is a recurring move in receiver design.

Answer: D

A spurious response is any input frequency other than the wanted one that the mixing process lands inside the intermediate frequency passband, the image being the most important but harmonic and half-intermediate-frequency responses also occurring. Audio distortion is a separate defect. Drift is an oscillator problem. And gain falling off at a band edge is a tracking or alignment matter. Grouping every unwanted response under one heading is useful because they all share one cure, namely selectivity ahead of the mixer.

Answer: A

Tracking is the oscillator's ability to stay exactly one intermediate frequency away from the preselector's tuned frequency right across the band, and it is what ganged tuning with padder and trimmer capacitors is arranged to achieve. The detector following an envelope is demodulation. Gain following signal strength is automatic gain control. And polarisation is an antenna matter. Each of these is a following behaviour, so the term must be attached to the right pair of quantities to be useful.

Answer: B

The padder, a capacitor in series with the oscillator coil, has most effect where the tuning capacitance is large, which is the low-frequency end. The trimmer, in parallel, dominates at the high-frequency end where the tuning capacitance is small. The antenna coil affects coupling and sensitivity. And the intermediate frequency transformer is aligned separately at a fixed frequency. Knowing which adjustment bites at which end of the band is what makes an alignment procedure follow a sensible order rather than a random one.

Answer: C

The trimmer is in parallel with the tuning capacitor, so its fixed value matters most when the variable part is at its smallest, at the top of the band. The padder in series governs the bottom end. Adjusting the coil core changes inductance and shifts the whole range rather than one end. And the detector load has nothing to do with tuning. Series and parallel adjustments act at opposite ends of a tuning range, which is the general principle behind every three-point alignment.

Answer: D

Because the oscillator and signal circuits have different frequency ranges, a practical arrangement can be made exact at only three frequencies, with small errors between them. The residual error is small enough that the intermediate frequency filter's bandwidth absorbs it. The manual reflects the physics rather than causing it. The intermediate frequency does drift a little as a consequence, but that is the symptom rather than the cause. And the count of tuned circuits is unrelated. Accepting a bounded error where exactness is impossible is ordinary engineering practice.