Superheterodyne Receiver Questions and Answers

Answer: C

High-side injection puts the oscillator one intermediate frequency above each end:

\[ \begin{aligned} 550 + 455 &= 1005 \\ 1650 + 455 &= 2105 \end{aligned} \]

so 1005 to 2105 kHz, a ratio of about 2.1 against the signal band's 3.0. The low-side alternative would demand a ratio of nearly thirteen. The compression of the tuning ratio is a real and often overlooked benefit of putting the oscillator above the band.

Answer: D

Taking the oscillator above the signal and the image above the oscillator:

\[ \begin{aligned} f_{o} &= 7 + 9 = 16 \\ f_{i} &= 16 + 9 = 25 \ \mathrm{MHz} \end{aligned} \]

so 16 MHz and 25 MHz. A high first intermediate frequency has pushed the image eighteen megahertz away from the wanted signal, where even a modest preselector attenuates it heavily. That is the up-conversion argument in a single calculation.

Answer: A

Shape factor is the ratio of the wider skirt to the narrower passband:

\[ \frac{12}{6} = 2.0 \]

so 2.0, respectable for a mechanical filter though bettered by a good crystal ladder. The reciprocal inverts the definition, and the remaining options combine the wrong quantities altogether. A figure defined as a ratio of two widths must always come out greater than one, which is a useful check on the arithmetic.

Answer: B

A signal half an intermediate frequency from the oscillator produces the half-IF spurious response, because its second harmonic beats with the oscillator's second harmonic to land in the passband:

\[ 2 \times 227.5 = 455 \ \mathrm{kHz} \]

The image lies a full two intermediate frequencies from the signal instead. Recognising a spurious response by its offset is how the mixing order responsible for it is identified.

Answer: C

A live audio stage with nothing coming through it points ahead of the detector, and a dead oscillator stops all conversion at once while leaving everything after the mixer perfectly healthy. An open loudspeaker would silence the hiss too. A shorted detector would also kill the hiss, since the audio path would be loaded. And lost gain control bias makes a set overload rather than go silent. Working out how far along the chain a fault must lie, before suspecting any component, is what makes a diagnosis quick.

Answer: D

Sensitivity failing at one end of the band and not the other is a tracking symptom, and the trimmer is the adjustment that governs the high-frequency end. A transformer fault or a leaky audio capacitor would affect every station equally, since they sit after conversion. And a long detector time constant distorts loud modulation rather than reducing sensitivity. Whether a symptom varies with dial position localises the fault to before or after the mixer in one observation.

Answer: A

Repeats at several dial settings mean unwanted frequencies are reaching the mixer, so the preselector is not rejecting them - a misaligned or overloaded front end. A detector fault would distort rather than duplicate. Slow gain control affects level, not frequency. And a misaligned intermediate frequency filter reduces sensitivity and selectivity everywhere at once. Duplicate reception is always a front-end story, because only the front end decides what the mixer sees.

Answer: B

This is the capture effect: the limiter and discriminator together suppress the weaker of two signals on the same channel rather than letting them mix as an AM set would. Blocking involves a signal on a different frequency. A de-emphasis failure changes tone rather than which station is heard. And over-deviation distorts. The effect is a genuine advantage of the architecture with FM, and recognising it as normal behaviour saves chasing a fault that does not exist.

Answer: C

Surviving with no antenna proves the source is inside the set, so it is an internal spurious response, typically oscillator harmonics mixing to land in the passband. An external transmitter would disappear with the antenna. Audio instability would whistle at every dial setting. And a microphonic component responds to vibration rather than to tuning. Disconnecting the antenna is the cheapest diagnostic available and settles the internal or external question outright.

Answer: D

Distortion that appears only with a strong signal points at a stage driven beyond its linear range, usually because the gain control is not reducing gain enough. Insufficient gain would show as poor sensitivity on weak signals, the opposite dependence. An off-frequency oscillator mistunes every station equally. And an open antenna coupling reduces signal rather than distorting it. Noting whether a symptom grows or shrinks with signal strength usually names the stage responsible.