AM Transmitters and Receivers Questions and Answers

Answer: C

Band changing switches different inductors into the preselector and oscillator circuits so the same ganged capacitor tunes each band in turn, while the intermediate frequency stages remain untouched. That reuse of the fixed-frequency stages is precisely the superheterodyne's advantage. A wider filter would spoil selectivity. Audio gain and antenna size affect level rather than coverage. One architecture serving many bands is why the design displaced the tuned radio-frequency receiver entirely.

Answer: D

Low-side injection works, and the image then sits two intermediate frequencies below the wanted signal rather than above it. The intermediate frequency itself is unchanged, being the difference either way. The receiver works perfectly well, though the oscillator's tuning ratio across a band is wider, which is why high-side injection is usual in broadcast sets. And audio inversion is a single-sideband phenomenon. Both choices are valid and the trade is in the oscillator's tuning range.

Answer: A

Working it through:

\[ f_{image} = 800 - 2 \times 455 = -110 \ \mathrm{kHz} \]

which is negative, so no real image exists for this combination - an incidental benefit of low-side injection at the bottom of a band. 1710 kHz is the high-side image. 345 kHz is the oscillator. And 1255 kHz is the high-side oscillator. The result is worth noticing because it shows that image behaviour depends on the tuning position as well as the injection side.

Answer: B

Because its selectivity comes from circuits tuned to the signal frequency, a tuned radio-frequency receiver's bandwidth widens as it is tuned higher, so selectivity is inconsistent across the band. The superheterodyne fixes it by doing the filtering at one frequency. It receives AM perfectly well, needs no local oscillator, and certainly has an audio stage. Understanding what problem the superheterodyne solved is the best way to remember why its architecture looks as it does.

Answer: C

Ganging lets one knob tune both circuits so they stay an intermediate frequency apart, which is what makes a single-dial receiver possible. The tuning range depends on the capacitance ratio rather than on ganging. Intermediate frequency selectivity is fixed and unaffected. And phase noise is a circuit property. The mechanical elegance of ganging is also the source of the tracking problem, which padder and trimmer capacitors then correct.

Answer: D

The master oscillator does, and everything downstream amplifies or modulates what it produces. The modulator impresses the message onto that carrier. The driver raises the level for the output stage. And the output filter removes harmonics. Assigning one function to each block, and one block to each function, is what makes a transmitter diagram something to reason from rather than memorise.

Answer: A

The intermediate frequency amplifier supplies the bulk of it, since fixed tuned stages can be cascaded freely at one frequency. The mixer usually has a small conversion loss. The detector is passive in a diode design. And the preselector contributes modest gain and image rejection. Concentrating gain where the frequency is fixed is the same economy that concentrates selectivity there.

Answer: B

The loop holds the detector's input roughly constant, preventing overload and distortion when a strong station is tuned in while allowing full gain on weak ones. Drift is a frequency matter. Image interference is a filtering matter. And harmonic radiation is the transmitter's problem entirely. Each protective mechanism in a receiver addresses one specific failure, and naming that failure is the quickest route to the answer.

Answer: C

455 kHz is the long-standing convention for AM broadcast receivers, chosen as a compromise between image rejection and achievable selectivity. 10.7 MHz is the FM broadcast standard, where the wider channel makes a higher intermediate frequency practical. 38 MHz appears in television. And 9 kHz is the AM channel spacing rather than an intermediate frequency. These conventional numbers are worth knowing because they anchor the arithmetic to real equipment.

Answer: D

The channel is nine or ten kilohertz wide, so both sidebands together permit only four or five kilohertz of audio, and the receiver's filter is set accordingly. Loudspeakers and amplifiers are easily capable of more. The detector's time constant must follow the envelope but is not the binding limit. The restriction is regulatory and physical rather than a matter of component quality, which is why better components cannot make AM broadcasting sound like FM.