Amplitude Modulation Questions and Answers

Answer: A

The channel must hold both sidebands of an audio signal limited to four or five kilohertz, which is twice that width plus a guard band. That is the whole arithmetic of the allocation, and it explains why AM broadcasting sounds bandwidth-limited compared with FM. Carrier drift is tiny by comparison and is controlled by crystal or synthesiser references. The intermediate frequency is a receiver matter with no bearing on the transmitted channel. And oscillator phase noise is orders of magnitude narrower than the channel.

Answer: B

Audio compression raises the average modulation depth while a limiter holds the peaks below unity index, so more of the radiated power carries information and the station sounds louder for the same licensed carrier. It is universal practice in broadcasting for exactly that reason. Raising the carrier power alone lowers the efficiency, since the sideband fraction falls. Widening the bandwidth breaks the channel allocation. And the receiver's intermediate frequency has nothing to do with transmitted efficiency.

Answer: C

The carrier conveys no information at all, yet its presence is what makes the envelope follow the message and so allows a diode detector to work. That is the bargain full-carrier AM strikes: two thirds of the power spent on receiver simplicity. Information lives entirely in the sidebands. Removing the carrier gives DSB-SC, which demands a coherent receiver, so the change is anything but transparent. And the carrier's amplitude is constant; it is the sum of carrier and sidebands whose envelope varies.

Answer: D

Each tone produces its own pair of sidebands at plus and minus its own frequency, so a two-tone message gives four sidebands and the bandwidth is set by the higher tone. Continuous bands appear when the message itself is continuous, as speech is, rather than from adding discrete tones. The carrier is unaffected in amplitude. And the bandwidth certainly grows if the second tone is the higher of the two, which is why the highest message frequency is what the channel must accommodate.

Answer: A

Most of the gain is taken at the intermediate frequency, where fixed tuned amplifiers can be cascaded without retuning and where the selectivity is also provided. That concentration of gain and selectivity at one fixed frequency is the essence of the superheterodyne. The preselector gives modest gain and image rejection. The audio stage supplies power rather than sensitivity. And the mixer usually has a small conversion loss rather than gain.

Answer: B

The loop rectifies the detected level and feeds it back to reduce the gain of the radio and intermediate frequency stages when the signal is strong, holding the detector's input roughly constant across a huge range of received strengths. It cannot touch the transmitter. The listener's volume control sits after the detector and is deliberately left under manual control. And the oscillator frequency is set by the tuning, not by signal strength.

Answer: C

The intermediate frequency stage carries the modulated signal, so it must pass both sidebands:

\[ B = 2 \times 4.5 = 9 \ \mathrm{kHz} \]

giving 9 kHz, which is exactly why broadcast receivers use filters of about that width and why the audio sounds restricted. 4.5 kHz would pass one sideband and half the other, distorting the envelope. 2.25 kHz is narrower still. 18 kHz would admit the adjacent channel along with the wanted one, so selectivity is lost for no benefit.

Answer: D

Selectivity is the rejection of neighbouring channels, provided chiefly by the intermediate frequency filter, and it is quite separate from sensitivity, which is the ability to amplify weak signals. A receiver can be very sensitive and hopelessly unselective, in which case a strong local station will be heard across the dial. Envelope tracking is the detector's job. And frequency tracking is what the tuning and, in some designs, a control loop provide.

Answer: A

A limiter removes amplitude variation indiscriminately, and in AM the amplitude variation is the information, so limiting destroys the message. That is why limiters belong to FM receivers, where amplitude carries nothing and removing it strips the noise. The three impairments listed as alternatives are all amplitude effects a limiter would indeed suppress, along with the signal itself, which is precisely why it cannot be used here.

Answer: B

SSB gives the message bandwidth only and puts every watt into information, which together are worth many decibels of effective improvement over full-carrier AM on a crowded band. The receiver is more complex, needing a reinserted carrier of accurate frequency. It is less troubled by selective fading than double sideband but not immune. And its envelope varies, so linear amplification is required, which is one of its practical costs.