Oscillators Questions and Answers

Answer: D

Self-heating and the enclosure's warming change the reactive components' values until temperatures stabilise, which for a crystal source can take several minutes. The supply settles in milliseconds, the amplitude loop in a fraction of a second, and the feedback fraction is set by components whose values barely matter to frequency. Specifying a warm-up time alongside a stability figure is standard practice for exactly this reason, and quoting the figure without it would be misleading.

Answer: A

Ageing is the gradual, largely one-directional change in frequency over long periods, caused by stress relief and contamination in a crystal, and it is quoted in parts per million per year. Temperature drift, warm-up and unit-to-unit spread are three other specifications describing quite different behaviours. Four figures are needed to characterise a frequency source honestly, and conflating any two of them makes a comparison between sources meaningless.

Answer: B

A varactor's capacitance varies with the reverse bias applied, so a control voltage tunes the frequency - which is exactly what a phase-locked loop needs to steer its oscillator. It does nothing for amplitude or harmonics, and it lowers the quality factor rather than improving it, since it is a lossier component than a good fixed capacitor. Every voltage-controlled oscillator trades some quality factor for tunability, and how much is the central compromise in its design.

Answer: C

Reaching a wide range means letting the varactor dominate the tank, and since it is the lossiest element the quality factor falls and the phase noise rises. Amplitude and supply are secondary, and the control draws essentially no current. A design that must cover a wide band will always be noisier than one covering a narrow one, which is why systems needing both use several oscillators rather than one.

Answer: D

The loop disciplines a wide-range but drifty oscillator against a narrow but accurate crystal reference, so the output has the crystal's long-term accuracy and the oscillator's tunability. It does not inherently double anything, amplitude control is still needed, and integration is a manufacturing convenience. Combining two components so that each supplies what the other lacks is the essence of the technique, and it is why nearly every modern radio contains one.

Answer: A

The Hartley takes its feedback from a tap on the coil, so the two inductive sections form the divider. The Colpitts uses a capacitive divider, the Clapp is a Colpitts with an extra series capacitor, and the Pierce is a Colpitts with a crystal in place of the inductor. Three of the four are variations on one arrangement, which is worth noticing because it means learning the Colpitts thoroughly gives most of the family.

Answer: B

The Colpitts places two capacitors in series across the tank and takes the feedback from their junction. The Hartley taps the inductor, the Armstrong uses transformer coupling, and the Wien bridge is an RC audio topology entirely. Since capacitors are easier to make accurately than tapped inductors, the Colpitts became the dominant arrangement and its descendants account for most radio-frequency oscillators built today.

Answer: C

The Pierce uses the crystal in its inductive region as the tank inductor, with the two load capacitors forming the usual divider. That is the arrangement inside almost every microcontroller's clock circuit, which is why the two load capacitors appear on every such schematic. Recognising a crystal circuit as a familiar topology with one substitution makes its component values immediately intelligible.

Answer: D

The three-section phase shift network attenuates by twenty-nine at the frequency where it gives a hundred and eighty degrees, so the amplifier must make that up. The Wien bridge needs only three, and the LC types' requirements depend on the divider ratio chosen. A gain requirement is always the reciprocal of the network's transmission at the oscillation frequency, which is why it can be computed rather than looked up.

Answer: A

The Wien bridge network passes a third of the signal at its centre frequency, so a gain of three closes the loop. The phase shift type needs twenty-nine, the Pierce's requirement depends on its capacitor ratio, and a multivibrator is a switching circuit with no such condition. That the Wien bridge's requirement is both small and exactly settable with two resistors is a large part of why it became the standard audio generator.