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Signal Modulation
31 questions By Tony R. Kuphaldt
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Question 10 of 31
This is a schematic for a simple VCO:
The oscillator is of the RC “phase shift” design. Explain how this circuit works. Why does the output frequency vary as the control voltage varies? Does the output frequency increase or decrease as the control voltage input receives a more positive voltage?
Hint: the JFETs in this circuit are not functioning as amplifiers!
Reveal answerTo understand how the JFETs are functioning in this VCO design, closely examine the “saturation” regions of a JFET’s characteristic curves. Note that these regions appear as nearly straight-line sections. This indicates something about the behavior of a saturated JFET that is exploited in this VCO circuit.
The output frequency decreases as the control voltage becomes more positive.
Notes:Not only does this question allow students to examine the workings of a VCO, but it also provides a good review of JFET theory, as well as a practical example of a special application of junction field-effect transistors.
Note: the schematic diagram for this circuit was derived from one found on page 997 of John Markus’
Guidebook of Electronic Circuits, first edition. Apparently, the design originated from a Motorola publication on using field effect transistors (“Low Frequency Applications of Field-Effect Transistors,” AN-511, 1971).
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Question 11 of 31
FM tends to be a far more noise-resistant means of signal modulation than AM. For instance, the “crackling” form of radio interference caused by natural lightning or the “buzzing” noise produced by high-voltage power lines are both easy to hear on an AM radio, but absent on an FM radio. Explain why.
Reveal answerRadio interference manifests itself as additional peaks on the “envelope” of a modulated carrier wave. AM reception is based on the extraction of that envelope from the modulated carrier, and so AM receivers will “pick up” unwanted noise. FM reception is based on the extraction of information from changes in frequency, which is largely unaffected by noise.
Notes:Ask students to explain this principle in their own words, and not just repeat the given answer.
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Question 12 of 31
When transmitting audio information (such as music and speech) in the form of radio waves, why bother modulating a high-frequency carrier signal? Why not just connect a powerful audio amplifier straight to an antenna and broadcast the audio frequencies directly?
Reveal answerThere are several reasons you would not want to try to broadcast electromagnetic (radio) waves at audio frequencies. A few of the most important are listed here:
- The necessary size of the antenna.
- Low transmission efficiency from inability to match antenna length to (changing) audio frequency.
- Interference from other (similar) radio transmitters.
Be prepared to explain why each of these factors effectively prohibits radio broadcasts at audio frequencies.
Notes:The purpose of this question is to have students relate their understanding of basic RF and antenna theory to a very practical problem of broadcasting low-frequency (in this case, audio) information. A fun exercise to do along with this question is to calculate the necessary physical dimensions of a quarter-wave ([(λ)/4]) antenna at a frequency of 2 kHz, keeping in mind that λ = v/f and v ≈ 3 ×108 meters per second.
