Oscillators Questions and Answers

Q: An oscillator refuses to start although its frequency-setting network is correct. What should be checked first?

A circuit that never starts almost always has a loop gain below one, whether from a wrong feedback fraction, a device with less gain than assumed, or a tank whose losses are higher than designed. Decoupling, buffering and temperature coefficient affect stability and purity rather than starting. Since starting requires the gain condition to be exceeded and running requires only that it be met, a non-starting oscillator is a gain problem until proved otherwise.

Q: An oscillator starts but its output is heavily distorted. What is the likely cause?

Too much loop gain drives the amplitude up until the device clips against the supply rails, and the flattened waveform carries strong harmonics. A loop gain below unity would prevent starting altogether, and a high quality factor would clean the output rather than dirty it. The remedy is either less gain or a gentler amplitude control, and which is appropriate depends on whether a clean sine is actually needed.

Q: An oscillator drifts steadily upward in frequency for ten minutes after switch-on. What is happening?

A drift over minutes in one direction is thermal: self-heating and the enclosure warming change the reactive components until temperatures stabilise. Amplitude loops settle in a fraction of a second and regulators in milliseconds, and ageing acts over months rather than minutes. The time scale of a symptom identifies its mechanism more reliably than anything else, and ten minutes is unambiguously thermal.

Q: Which measurement would distinguish clipping from a genuine sine output?

A spectrum analyser shows the harmonics directly, and their level relative to the fundamental quantifies the distortion. A counter reads frequency regardless of waveform, a voltmeter gives amplitude, and an oscilloscope will show gross clipping but not the fraction of a per cent that matters in a good generator. Choosing the instrument that responds to the quantity in question, rather than the one nearest to hand, is what makes a measurement decisive.

Q: Which oscillator would you choose for a microcontroller's clock?

A Pierce gives crystal accuracy from two capacitors and an inverter, which is why it is built into essentially every microcontroller. A Wien bridge is an audio sine generator, a varactor-tuned Colpitts is for tunable radio work, and a unijunction is a low-accuracy trigger circuit. Matching a topology to an application means naming the specification that matters, and for a digital clock it is accuracy at negligible cost.

Q: Which oscillator would you choose for a tunable radio-frequency source?

A varactor-tuned Colpitts disciplined by a phase-locked loop gives both the range and, through the loop's crystal reference, the accuracy. A Pierce alone cannot be tuned usefully far, and the RC and multivibrator types are audio-frequency circuits with poor stability. Where two specifications conflict, the usual answer is two components each supplying what the other lacks, disciplined by a loop that combines them.

Q: Which oscillator would you choose for a low-cost flashing indicator?

A 555 astable costs pennies, needs two resistors and a capacitor, and its per cent accuracy is irrelevant to a flashing light. The other three are precision circuits whose cost and complexity would be entirely wasted. Choosing the cheapest arrangement that meets the actual requirement, rather than the best available, is as much a design skill as reaching for precision when it is genuinely needed.

Q: Why is an oscillator's output usually buffered before leaving the board?

A buffer isolates the tank from whatever is connected, so a change in load impedance cannot reflect a reactance back and shift the frequency. Amplitude and filtering may also be wanted but are separate functions, and stabilisation belongs inside the loop. Isolating a frequency-determining circuit from the outside world is the single most effective step towards a stable source, and it costs one stage.

91.

An LC tank of quality factor \( 150 \), resonant at \( 5 \ \mathrm{MHz} \), passes a band of roughly

A.330 kHz
B.33 kHz
C.3.3 kHz
D.750 kHz

Answer: B

Bandwidth is the resonant frequency over the quality factor:

\[ B = \frac{5 \times 10^{6}}{150} \approx 33 \ \mathrm{kHz} \]

giving about 33 kHz, against 250 Hz for a crystal of the same frequency with a quality factor of twenty thousand. Setting the two side by side is the quickest way to feel why crystal control displaced LC control wherever accuracy mattered.

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92.

An oscillator refuses to start although its frequency-setting network is correct. What should be checked first?

A.The output buffer's ability to drive the connected load
B.The temperature coefficient of the tank's capacitor
C.The loop gain at switch-on
D.The supply decoupling capacitors near the amplifier stage

Answer: C

A circuit that never starts almost always has a loop gain below one, whether from a wrong feedback fraction, a device with less gain than assumed, or a tank whose losses are higher than designed. Decoupling, buffering and temperature coefficient affect stability and purity rather than starting. Since starting requires the gain condition to be exceeded and running requires only that it be met, a non-starting oscillator is a gain problem until proved otherwise.

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93.

An oscillator starts but its output is heavily distorted. What is the likely cause?

A.A loop gain marginally below unity at switch-on
B.A tank quality factor higher than the design intended
C.A load impedance higher than the buffer expects
D.Excessive loop gain driving hard clipping

Answer: D

Too much loop gain drives the amplitude up until the device clips against the supply rails, and the flattened waveform carries strong harmonics. A loop gain below unity would prevent starting altogether, and a high quality factor would clean the output rather than dirty it. The remedy is either less gain or a gentler amplitude control, and which is appropriate depends on whether a clean sine is actually needed.

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94.

An oscillator's frequency changes when a nearby circuit is switched on. What does this suggest?

A.Stray coupling is pulling the loop
B.The supply voltage has fallen below the design minimum
C.The crystal's ageing has accelerated with temperature
D.The amplitude control loop has become unstable

Answer: A

A frequency that responds to another circuit's activity points at coupling - either an injected signal pulling the oscillator, or a shared supply impedance modulating it. Ageing acts over months and would not switch on and off, and an amplitude loop problem would show as amplitude variation. Shielding, separate decoupling and a buffer between oscillator and load are the standard remedies, and which one applies depends on whether the coupling is through the air or through the supply.

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95.

An oscillator drifts steadily upward in frequency for ten minutes after switch-on. What is happening?

A.The crystal is ageing at its specified rate
B.The components are warming to equilibrium
C.The amplitude control loop is still settling
D.The supply regulator has not yet reached its set point

Answer: B

A drift over minutes in one direction is thermal: self-heating and the enclosure warming change the reactive components until temperatures stabilise. Amplitude loops settle in a fraction of a second and regulators in milliseconds, and ageing acts over months rather than minutes. The time scale of a symptom identifies its mechanism more reliably than anything else, and ten minutes is unambiguously thermal.

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96.

Which measurement would distinguish clipping from a genuine sine output?

A.A voltmeter reading the output's mean amplitude
B.An oscilloscope triggered on the rising edge
C.A spectrum analyser showing harmonic content
D.A frequency counter reading the fundamental accurately

Answer: C

A spectrum analyser shows the harmonics directly, and their level relative to the fundamental quantifies the distortion. A counter reads frequency regardless of waveform, a voltmeter gives amplitude, and an oscilloscope will show gross clipping but not the fraction of a per cent that matters in a good generator. Choosing the instrument that responds to the quantity in question, rather than the one nearest to hand, is what makes a measurement decisive.

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97.

Which oscillator would you choose for a microcontroller's clock?

A.A Wien bridge with lamp amplitude stabilisation
B.A Colpitts tuned by a varactor diode
C.A unijunction relaxation circuit with an RC network
D.A Pierce crystal circuit

Answer: D

A Pierce gives crystal accuracy from two capacitors and an inverter, which is why it is built into essentially every microcontroller. A Wien bridge is an audio sine generator, a varactor-tuned Colpitts is for tunable radio work, and a unijunction is a low-accuracy trigger circuit. Matching a topology to an application means naming the specification that matters, and for a digital clock it is accuracy at negligible cost.

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98.

Which oscillator would you choose for a tunable radio-frequency source?

A.A varactor-tuned Colpitts in a loop
B.A Pierce crystal circuit at a fixed frequency
C.A three-section RC phase shift oscillator
D.An astable multivibrator with adjustable resistors

Answer: A

A varactor-tuned Colpitts disciplined by a phase-locked loop gives both the range and, through the loop's crystal reference, the accuracy. A Pierce alone cannot be tuned usefully far, and the RC and multivibrator types are audio-frequency circuits with poor stability. Where two specifications conflict, the usual answer is two components each supplying what the other lacks, disciplined by a loop that combines them.

Discuss in Forum

99.

Which oscillator would you choose for a low-cost flashing indicator?

A.A Clapp oscillator with a temperature-compensated tank
B.A 555 astable
C.An oven-controlled crystal oscillator
D.A Wien bridge with amplitude stabilisation

Answer: B

A 555 astable costs pennies, needs two resistors and a capacitor, and its per cent accuracy is irrelevant to a flashing light. The other three are precision circuits whose cost and complexity would be entirely wasted. Choosing the cheapest arrangement that meets the actual requirement, rather than the best available, is as much a design skill as reaching for precision when it is genuinely needed.

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100.

Why is an oscillator's output usually buffered before leaving the board?

A.To filter the harmonics from the output waveform
B.To provide the amplitude stabilisation the loop needs
C.To keep load changes from pulling the frequency
D.To raise the output amplitude for the following stage

Answer: C

A buffer isolates the tank from whatever is connected, so a change in load impedance cannot reflect a reactance back and shift the frequency. Amplitude and filtering may also be wanted but are separate functions, and stabilisation belongs inside the loop. Isolating a frequency-determining circuit from the outside world is the single most effective step towards a stable source, and it costs one stage.

Discuss in Forum

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