Superheterodyne Receiver Questions and Answers

Q: What happens to image rejection as the intermediate frequency is raised?

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.

Q: How does double conversion resolve that conflict?

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.

Q: What is a spurious response in a superheterodyne receiver?

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.

Q: What is meant by tracking in a superheterodyne receiver?

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.

Q: Which component corrects tracking error at the low-frequency end of the band?

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.

Q: Which component corrects tracking at the high-frequency end?

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.

Q: What symptom does poor tracking produce?

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.

Q: In what order is a superhet normally aligned?

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.

21.

What happens to image rejection as the intermediate frequency is raised?

A.It improves, because the image moves further out
B.It worsens, because the mixer must work harder
C.It is unchanged, since rejection depends on the preselector alone
D.It improves, because the intermediate filter becomes narrower

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.

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22.

Which pair of requirements pulls the choice of intermediate frequency in opposite directions?

A.Antenna matching and squelch threshold
B.Image rejection and adjacent channel selectivity
C.Sensitivity and audio output power
D.Oscillator stability and detector linearity

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.

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23.

How does double conversion resolve that conflict?

A.A single IF with two filters in cascade
B.Two detectors sharing one IF strip
C.A high first IF then a low second IF
D.Two mixers driven by the same oscillator

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.

Discuss in Forum

24.

What is a spurious response in a superheterodyne receiver?

A.A distortion product created in the audio stage
B.Oscillator drift away from its nominal frequency
C.Loss of gain at one end of the tuning range
D.An unwanted frequency that reaches the IF strip

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.

Discuss in Forum

25.

What is meant by tracking in a superheterodyne receiver?

A.The oscillator keeping a constant offset from the signal
B.The detector following the modulation envelope
C.The gain control following the signal strength
D.The antenna following the transmitter's polarisation

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.

Discuss in Forum

26.

Which component corrects tracking error at the low-frequency end of the band?

A.The intermediate frequency transformer
B.The padder capacitor
C.The trimmer capacitor
D.The antenna coupling coil

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.

Discuss in Forum

27.

Which component corrects tracking at the high-frequency end?

A.The oscillator coil's core
B.The detector's load resistor
C.The trimmer capacitor
D.The padder capacitor

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.

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28.

Why is three-point tracking accepted rather than perfect tracking?

A.Three points are all a service manual specifies
B.The intermediate frequency varies slightly with tuning
C.The preselector has only three tuned circuits
D.Exact tracking across a whole band is not achievable

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.

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29.

What symptom does poor tracking produce?

A.Reduced sensitivity at one end of the dial
B.Distorted audio on every station equally
C.Complete loss of the local oscillator
D.Excessive hum on strong signals only

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.

Discuss in Forum

30.

In what order is a superhet normally aligned?

A.Audio stages first, then everything ahead of them
B.Intermediate frequency stages first, then the front end
C.Front end first, then the intermediate frequency stages
D.Detector first, then the mixer, then the oscillator

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.

Discuss in Forum

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