Superheterodyne Receiver Questions and Answers

Answer: A

Cross-modulation transfers one station's modulation onto another, and that requires non-linearity acting while both signals are still present, which is before the intermediate frequency filter has separated them. Once the filter has removed the interferer there is nothing left to cross-modulate. It is not specific to amplitude modulation. Gain is not the issue; linearity is. And the audio stage is far too late. Any interaction between two signals must be located at a point where both still exist.

Answer: B

The envelope detector - a diode, a capacitor and a load resistor - recovers amplitude modulation, and its cheapness is the reason full-carrier AM persisted in broadcasting. A phase-locked loop demodulator is used for frequency modulation. The ratio detector is likewise an FM circuit. And a product detector is needed for suppressed-carrier modes, where no envelope exists to follow. Each modulation scheme has the detector its own waveform admits, and the pairing is worth holding in mind.

Answer: C

The time constant must be long enough to smooth the carrier ripple yet short enough to follow the fastest modulation, which brackets it from both sides. Too long and the detector clips the modulation's negative peaks, a defect called diagonal clipping. Gain figures, gain control timing and loudspeaker impedance all belong elsewhere in the chain. A component bracketed by two opposing requirements always has a design window rather than a single correct value.

Answer: D

It is the detector's inability to follow the envelope down when its load time constant is too long, so the recovered waveform slopes away rather than tracking the fall. The distortion is worst at high modulation depths and high modulation frequencies. Audio overload clips symmetrically. Mixer limiting is a different defect. And losing one half cycle is what the rectifier is supposed to do. Naming a distortion by the mechanism that causes it is what makes the cure obvious, and here the cure is a smaller capacitor.

Answer: A

It reduces the gain on strong signals and restores it on weak ones, so the output stays roughly constant across a wide range of input levels. It cannot improve a signal-to-noise ratio, since it scales signal and noise together, which is the point most often misunderstood about it. Selectivity is fixed by the filters. And tracking is a tuning matter. A control loop can only regulate the quantity it measures, and here that quantity is level.

Answer: B

It is taken from the detector, whose direct component is proportional to carrier strength, and fed back to bias the earlier stages. The oscillator's level says nothing about the received signal. Audio supply current varies with modulation rather than carrier. And the preselector's voltage is too small and too far forward to be useful. A regulator needs a measurement of the quantity it regulates, and the detector is where carrier strength first appears as a usable direct voltage.

Answer: C

The delay is in level rather than in time: the loop is biased so that it does nothing until the signal exceeds a threshold, leaving full gain available for weak stations. Slowing the response in time is a separate matter of attack and decay constants, and the similarity of the names is exactly the trap here. It is applied to the radio and intermediate frequency stages. And the detector it comes from is incidental. When a term could describe two different quantities, the definition rather than the name has to settle it.

Answer: D

Squelch mutes the audio while no carrier is received, so an operator is not subjected to full-gain noise between transmissions. Limiting audio power is a different protective function. The image is a front-end matter. And oscillator stability is unrelated. The circuit is a good example of a feature that improves nothing measurable about the receiver and a great deal about using it, which is why every practical set has one.

Answer: A

It reinserts a carrier so that continuous-wave Morse becomes an audible tone and single sideband becomes intelligible speech, both being modes with no carrier of their own to beat against. Frequency modulation needs a discriminator instead. Amplitude modulation already carries its own carrier and needs no help. And making internal harmonics audible is a defect rather than a purpose. A missing carrier has to be supplied somewhere, and doing it at the receiver is what these modes rely on.

Answer: B

The image is \( 910 \ \mathrm{kHz} \) away, so the detuning ratio is large:

\[ \begin{aligned} \rho &= \tfrac{1910}{1000} - \tfrac{1000}{1910} \approx 1.39 \\ \text{ratio} &\approx \sqrt{1 + (50 \times 1.39)^{2}} \approx 70 \end{aligned} \]

which is about 37 dB, so about 40 dB. The rejection depends on the quality factor and on how far out the image lies, which is why both the preselector and the choice of intermediate frequency appear in the answer.