Superheterodyne Receiver Questions and Answers

Answer: C

Injection level is the oscillator drive applied to the mixer, which must be adequate to switch it properly but not so large as to worsen its noise or linearity. Signal strength, detector bias and audio level all belong to other stages. Too little injection gives poor conversion gain, too much gives spurious products, so the specification has a window rather than a maximum - the same shape as the detector time constant's requirement.

Answer: D

The antenna trimmer compensates for the particular aerial in use, peaking the transfer of signal into the input circuit. Oscillator tracking at the low end is the padder's job, intermediate frequency alignment is done at the transformers themselves, and the gain control threshold is set by biasing. Each adjustment exists to absorb a different unknown, and the aerial is the one unknown the manufacturer cannot predict.

Answer: A

Over-coupling produces two peaks with a dip between them, which broadens the passband at the cost of a flat top. It is deliberately used where a wider response is wanted, as in FM sets. Critical coupling gives the flattest single peak, and under-coupling a narrower and lower one. There is no loss of gain at centre and no level-dependent shift. Coupling is a design parameter with a continuum of useful settings rather than a single correct value.

Answer: B

Critical coupling is the point at which the two peaks just merge, giving the flattest single-peaked response. Above it the response splits into two humps; below it the response narrows and the gain falls. Maximum centre gain occurs at critical coupling too, but flatness rather than gain is what the question asks about. A parameter tuned to a boundary condition often optimises two things at once, which is why critical coupling is the default choice.

Answer: C

A broadcast FM signal occupies about 200 kHz against an AM channel's 10, so the filter must be wide enough to pass it undistorted. Detector selectivity is not a meaningful property here, oscillator drift is comparable, and gain control is unaffected by bandwidth. The passband always follows the signal's occupied width, which is why the FM standard also puts the intermediate frequency far higher: a 200 kHz filter at 455 kHz would be impossible.

Answer: D

The limiter clips the signal to a constant amplitude, discarding amplitude noise before the discriminator sees it, and most of FM's noise advantage depends on it. Carrier reinsertion is the beat frequency oscillator's job in suppressed-carrier modes. Narrowing is the filter's role. And FM sets often need no gain control at all, precisely because the limiter makes level irrelevant. A stage that discards information the signal does not carry is the cleanest form of noise reduction available.

Answer: A

Because the limiter already holds the amplitude constant, the output is independent of input level over a wide range and a gain control loop would add nothing. The strip has ample gain. The detector's frequency-sensitivity is real but it is the limiter that removes the amplitude dependence. And received FM levels vary as much as any other. A stage that solves a problem incidentally can make a whole subsystem unnecessary, which is worth noticing when comparing block diagrams.

Answer: B

It reads the gain control voltage, which is proportional to carrier strength, and that is why an FM set with no gain control needs a different arrangement entirely. Audio power varies with modulation rather than carrier. Stage current is affected by the gain control but is not what is metered. And injection level is fixed. An instrument on a panel is always measuring some internal quantity that correlates with what it claims to show, and knowing which explains its quirks.

Answer: C

A ten kilohertz error against a six kilohertz passband puts the carrier outside the flat region:

\[ \begin{aligned} \text{error} &= 10 \ \mathrm{kHz} \\ \text{half-passband} &= 3 \ \mathrm{kHz} \end{aligned} \]

so one sideband is attenuated more than the other and the result is reduced output with distortion rather than silence, since the skirts still pass something. Asymmetric sideband attenuation is the mechanism, and it is why accurate tuning matters more as a filter narrows.

Answer: D

Gains and losses add in decibels once the sign is respected:

\[ -6 + 60 = 54 \ \mathrm{dB} \]

giving 54 dB. Adding without regard to sign gives 66 dB, and the remaining options multiply or divide quantities that should be added. Working in decibels reduces a cascade to arithmetic, but only if a loss is entered as a negative gain, which is the single convention the notation depends on.