AM Transmitters and Receivers Questions and Answers

Answer: C

The intermediate frequency stages are aligned first, because everything downstream of the mixer must be correct before the tracking of the front end can be judged. Trimmer and padder adjustments follow, at the high and low ends of the band respectively. Audio power is checked last, being independent of tuning. Alignment procedures are ordered by dependency rather than by convenience, and doing them out of order means repeating them.

Answer: D

Hearing one station everywhere means the receiver is no longer separating channels, so the intermediate frequency filtering has failed or been misaligned. A failed audio stage would give silence. An open antenna would give weak reception of everything rather than strong reception of one thing. And excessive gain control would flatten levels without spreading a station across the dial. Reasoning from the symptom's pattern to the stage responsible is the practical skill.

Answer: A

A tuneable whistle is the signature of two carriers beating together, which happens when an image or an adjacent-channel signal reaches the mixer and produces an audible difference frequency. Better preselection or a different intermediate frequency is the cure. De-emphasis belongs to FM. Diagonal clipping distorts the envelope without producing a tone. And over-modulation splatters rather than whistles. Interference diagnostics turn on whether the artefact tunes with the dial.

Answer: B

Total power rises as \( 1 + \mu^{2}/2 \), so the antenna current rising when modulation is applied is exactly what a healthy AM transmitter does, and the increase measures the modulation index. It is not a fault, and it does not by itself indicate over-modulation, which needs the envelope to be examined. Loss of the carrier would reduce the current sharply. Knowing which meter movements are normal is as important as knowing which are not.

Answer: C

A modulation monitor displays or measures the depth and flags peaks beyond the permitted index, which is what keeps a station legal and out of its neighbours' channels. A standing wave bridge checks the antenna match. A counter checks frequency. And a dummy load absorbs power for testing. Each instrument answers one question, and choosing the right one is half of any measurement task.

Answer: D

A class C stage is rich in harmonics, and radiating them would interfere with services on those frequencies, so a low-pass filter attenuates them before the antenna. Modulation linearity is a modulator matter. Matching is the coupling network's job, though the two functions are often combined in one network. And device protection is a circuit design concern. Efficiency bought with a nonlinear stage always has to be paid for with filtering.

Answer: A

The sideband power is what the modulator provides:

\[ P_{sb} = 500 \times \frac{0.36}{2} = 90 \ \mathrm{W} \]

so 90 watts of audio, which for a high-level modulated transmitter means a substantial audio amplifier. 180 W omits the factor of two. 300 W uses the index linearly. And 45 W halves the correct answer. This calculation is what sizes the modulator, and it explains why a hundred-kilowatt broadcast transmitter needed an audio stage of tens of kilowatts before switching supplies arrived.

Answer: B

Taking the voltage ratio in decibels:

\[ 20\log_{10}\left(\frac{40}{10}\right) = 12 \ \mathrm{dB} \]

giving 12 dB of margin. 4 dB is the ratio itself misread as decibels. 30 dB corresponds to a ratio of about thirty. And 6 dB would be a ratio of two. Remembering that voltage ratios take twenty times the logarithm, and power ratios ten, is the single most useful habit in receiver work.

Answer: C

Forty decibels is a factor of a hundred in amplitude:

\[ 10^{40/20} = 100 \]

so the interferer arrives at one hundredth of the wanted amplitude, or a ten-thousandth of the power. A fortieth confuses the decibel figure with a ratio. A tenth corresponds to 20 dB. And a four-hundredth follows from no correct conversion. Rejection figures are always worth converting into a ratio before judging whether they are adequate.

Answer: D

Adjacent-channel selectivity is the specification concerned, provided by the intermediate frequency filter. Sensitivity concerns weak signals and would if anything make the problem worse by amplifying everything. Image rejection deals with a signal two intermediate frequencies away rather than the next channel. And audio bandwidth is downstream of the problem. Matching the complaint to the specification is the first step in any receiver investigation.