Superheterodyne Receiver Questions and Answers

Answer: A

Poor tracking detunes the preselector relative to the oscillator, so sensitivity falls at one end of the band while the other end is normal. That frequency-dependence is the diagnostic signature. Distortion affecting every station equally points at the detector or the audio chain. A dead oscillator kills reception entirely. And hum on strong signals suggests a supply or modulation problem. Whether a fault varies with the dial position is the first question worth asking about it.

Answer: B

The intermediate frequency strip is aligned first, at its fixed frequency, and only then are the oscillator and preselector adjusted to feed it. Aligning the front end against a misaligned intermediate frequency would simply build the error in. Detector and audio stages have no tuned adjustments in this sense. The rule generalises: calibrate the fixed reference before adjusting anything that must be set relative to it.

Answer: C

A birdie is a tone appearing at particular dial settings because harmonics of the receiver's own oscillators mix down into the intermediate frequency, so it is present with the antenna disconnected. External static is unrelated to dial position. An alignment tone comes from a signal generator. And a beat between two transmitters is heterodyne interference arriving from outside. Whether a spurious tone survives with the antenna removed settles at once whether it is internal or external.

Answer: D

The intermediate frequency filter does this work, since it operates at a fixed frequency where a steep, narrow response can be built and left alone. The preselector is far too broad to separate adjacent channels; it deals with the image instead. A tone control shapes audio and cannot recover a channel already mixed in. And gain control adjusts level rather than discriminating between frequencies. Each kind of unwanted signal is removed by the one stage able to distinguish it, and adjacent channels are distinguishable only after conversion.

Answer: A

Shape factor is the ratio of the width at 60 dB down to the width at 6 dB down, so a value near one describes almost vertical skirts and a large value a sloppy filter. Gain over bandwidth is a different figure entirely. The section count influences the shape factor without defining it. And insertion loss is a separate specification. A single number that captures skirt steepness is what lets two filters of the same nominal bandwidth be compared honestly.

Answer: B

Mechanical and crystal filters achieve very high quality factors and so very steep skirts, which is why communications receivers use them where broadcast sets make do with tuned transformers. A single tuned transformer is far too gentle for close channel spacing. An RC stage has no useful selectivity at radio frequencies. And an attenuator has no frequency discrimination at all. Matching the filter technology to the channel spacing the service demands is the practical decision behind these choices.

Answer: C

Different modes occupy different widths, so a switchable filter lets the operator admit no more bandwidth than the signal needs and so keep the noise down - narrow for Morse, wider for single sideband, wider still for AM. Drift is handled by oscillator design. Image rejection is a front-end matter. And detector switching is a separate control that usually accompanies it. Choosing bandwidth to match the signal is the same matched-filter principle that governs noise performance generally.

Answer: D

Blocking is the reduction in gain a wanted weak signal suffers when a strong signal is present nearby in frequency, and it is a large-signal property of the front end and mixer. Image rejection is a separate specification. Squelch threshold concerns muting. And intermediate frequency leakage at the antenna is a radiation matter. Weak-signal and strong-signal specifications describe different failure modes, and a receiver can be excellent at one and poor at the other.

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.