Superheterodyne Receiver Questions and Answers

Answer: C

The front end is everything from the antenna up to and including the first mixer, and it is the part that determines noise figure, image rejection and overload behaviour. The audio stages are the back end. The intermediate frequency strip sits between the two. And the dial is mechanical rather than electrical. The term is worth being precise about because a surprising number of a receiver's specifications are decided entirely within it.

Answer: D

The image is only \( 910 \ \mathrm{kHz} \) away out of ten megahertz, so the detuning ratio is small:

\[ \begin{aligned} \rho &= \tfrac{10.91}{10} - \tfrac{10}{10.91} \approx 0.18 \\ \text{ratio} &\approx 100 \times 0.18 = 18 \end{aligned} \]

which is about 25 dB, so about 20 dB - poor. The same intermediate frequency that serves the medium wave band well is quite inadequate at ten megahertz, which is the calculation that forces up-conversion on any wide coverage receiver.

Answer: A

The second oscillator must differ from the first intermediate frequency by the second:

\[ 45 + 0.455 = 45.455 \ \mathrm{MHz} \]

so 45.455 MHz for high-side injection, with 44.545 MHz an equally valid low-side alternative rather than a forbidden one. The remaining options confuse the oscillator with one of the intermediate frequencies. Since both inputs to this mixer are fixed, the oscillator can be crystal controlled, which is the practical point of the arrangement.

Answer: B

Building up from the reference density:

\[ \begin{aligned} P &= -174 + 10\log_{10}(2400) + 10 \\ &= -174 + 34 + 10 = -130 \ \mathrm{dBm} \end{aligned} \]

so about -130 dBm. Subtracting rather than adding the noise figure gives -140 dBm, and -174 dBm is the density before any bandwidth is applied. Narrow filtering is what puts a single sideband receiver's floor this low, and it is why bandwidth is chosen to match the mode.

Answer: C

Two carriers reaching the same detector beat together, producing a tone at their difference frequency, here 5 kHz. Suppression of the weaker signal is the FM capture effect and needs a limiter and discriminator. Distortion occurs but the beat note is what dominates. And apparent tuning does not shift. The cure is a narrower filter, which is exactly why adjacent channel selectivity is specified in the first place.

Answer: D

On a crowded band the limiting problem is neighbours rather than noise, so selectivity is what needs tightening - a narrower filter with steeper skirts. Audio power, dial accuracy and gain control timing are all conveniences that do nothing about an interfering station. Deciding which specification is limiting before improving anything is what stops effort going into the parameter that happens to be easiest to change.

Answer: A

With no interference to reject, the limit is the receiver's own noise, so the noise figure is what matters - a quieter first stage, or less loss ahead of it. Extra selectivity, blocking performance and more filter options all address problems the band does not present. The same receiver is limited by quite different specifications in different environments, which is why a single figure of merit for receivers has never been agreed.

Answer: B

With the oscillator on the signal frequency, the intermediate frequency is zero and the image coincides with the wanted signal, so there is no separate response to reject. The price is that both sidebands fold together, which suits single sideband and Morse but not amplitude modulation. It has no intermediate frequency filter in the usual sense, no unusual preselector, and it converts down rather than up. Setting a troublesome parameter to zero sometimes removes a problem instead of shrinking it.

Answer: C

Because the intermediate frequency is zero, the two sidebands land on top of one another and cannot be separated, which distorts a double sideband signal. Oscillator stability is achievable, noise figure is not inherently worse, and adequate gain can be had at audio frequencies. The architecture is excellent for single sideband and Morse and poor for AM, which illustrates that an architecture is suited to modes rather than being good or bad in the abstract.

Answer: D

A synthesiser locked to a crystal reference generates the oscillator frequency digitally, which removes tracking error, gives exact dial readout and allows memories and scanning. Padders and trimmers are the mechanical arrangement being replaced. An extra filter does not address tuning. And widening the passband to absorb error would sacrifice selectivity. When a problem stems from mechanical ganging, replacing the mechanism outright beats refining its adjustments.