Oscillators Questions and Answers

Q: Why does an oscillator's frequency drift during the first minutes after switch-on?

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.

Q: What does an oscillator's ageing specification describe?

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.

Q: Why is a varactor sometimes included in an oscillator's tank?

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.

Q: What is the trade-off in a voltage-controlled oscillator's tuning range?

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.

Q: Why does a phase-locked loop combine a crystal with a tunable oscillator?

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.

Q: Which topology uses a tapped inductor for feedback?

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.

Q: Which topology uses a capacitive divider for feedback?

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.

Q: Which topology places a crystal where a Colpitts has its inductor?

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.

Q: Which oscillator needs a gain of twenty-nine?

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.

Q: Which oscillator needs a gain of exactly three?

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.

61.

Why does an oscillator's frequency drift during the first minutes after switch-on?

A.Its supply voltage is still rising towards nominal
B.Its amplitude control loop has not yet settled
C.Its feedback fraction changes as the devices warm
D.Its components are still reaching thermal equilibrium

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.

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62.

What does an oscillator's ageing specification describe?

A.Slow frequency change over months or years
B.The drift measured over the operating temperature range
C.The time taken to reach a stable frequency at switch-on
D.The variation between individual units of the same design

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.

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63.

Why is a varactor sometimes included in an oscillator's tank?

A.To suppress harmonics in the output waveform
B.To allow the frequency to be tuned by a voltage
C.To stabilise the amplitude of the oscillation
D.To improve the tank circuit's quality factor

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.

Discuss in Forum

64.

What is the trade-off in a voltage-controlled oscillator's tuning range?

A.A wider range requires a higher supply voltage
B.A wider range needs a larger control current
C.A wider range costs quality factor and so phase noise
D.A wider range costs output amplitude at the extremes

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.

Discuss in Forum

65.

Why does a phase-locked loop combine a crystal with a tunable oscillator?

A.It doubles the output frequency available from either
B.It removes the need for amplitude stabilisation
C.It allows both to be built on the same silicon
D.It takes the crystal's accuracy and the oscillator's range

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.

Discuss in Forum

66.

Which topology uses a tapped inductor for feedback?

A.Hartley
B.Colpitts, which taps a pair of capacitors instead
C.Clapp, which adds a third capacitor in series with the coil
D.Pierce

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.

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67.

Which topology uses a capacitive divider for feedback?

A.Wien bridge, which uses series and parallel RC arms
B.Colpitts
C.Hartley, which taps the inductor instead
D.Armstrong

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.

Discuss in Forum

68.

Which topology places a crystal where a Colpitts has its inductor?

A.Hartley, which taps the coil for its feedback
B.Armstrong, which couples through a separate winding
C.Pierce
D.Clapp

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.

Discuss in Forum

69.

Which oscillator needs a gain of twenty-nine?

A.The Wien bridge, whose network attenuates by three
B.The Colpitts, whose fraction is set by two capacitors
C.The Hartley
D.The three-section phase shift type

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.

Discuss in Forum

70.

Which oscillator needs a gain of exactly three?

A.The Wien bridge
B.The phase shift type with three RC sections
C.The Pierce, using a crystal as its inductor
D.The astable multivibrator with cross-coupled devices

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.

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