Oscillators Questions and Answers

Q: What does a unijunction transistor provide in an oscillator?

A unijunction has a peak point beyond which its emitter current rises as its voltage falls, so once the timing capacitor reaches that point the device conducts heavily and discharges it. That negative resistance is what makes it a self-triggering switch rather than an amplifier. It has no useful current gain and provides no reference. A device whose characteristic has a negative-slope region can do things no ordinary three-terminal device can, and generating a sawtooth with two components is the clearest example.

Q: What waveform does a unijunction relaxation oscillator naturally produce?

The capacitor charges slowly through its resistor and discharges almost instantly through the device, giving a sawtooth, while the discharge current through the base resistor appears as a narrow pulse. That pulse is what made the circuit the standard trigger for thyristors. Neither a sine nor a symmetrical square wave arises, and a triangle would need equal charge and discharge paths. Reading a waveform off the charge and discharge paths is quicker than recalling it.

Q: Why is a Schmitt trigger used in a relaxation oscillator?

The two thresholds of a Schmitt trigger are exactly the upper and lower limits between which the capacitor charges and discharges, so the circuit needs nothing more than the trigger, a resistor and a capacitor. Its gain is not the point, the capacitor is essential, and amplitude follows the supply. A component whose defining property is a pair of thresholds is a natural fit for a circuit that needs exactly two, which is why the arrangement is so economical.

Q: What is the chief drawback of a relaxation oscillator as a frequency source?

The period is a resistor times a capacitor, so the frequency inherits both components' tolerance and drift, and stabilities better than a per cent are hard to reach. The square output is often what is wanted, the component count is low, and these circuits work to megahertz. A frequency set by absolute component values can never compete with one set by a mechanical resonance, which is why every accurate clock in a system comes from a crystal and every cheap one from an RC.

Q: What is meant by an oscillator's phase noise?

Phase noise is the random fluctuation of the output's phase, seen as skirts either side of the carrier in the spectrum and as jitter in the time domain. Harmonic content is distortion, slow drift is a stability specification measured over minutes or hours, and amplitude variation is a separate impairment. The three describe different imperfections on different time scales, and a source can be excellent on one and poor on another - which is why a data sheet quotes all three.

Q: Which oscillator property most reduces phase noise close to the carrier?

A sharp resonance corrects a phase disturbance with a very small frequency excursion, so the noise skirts are narrow and low. Amplitude helps by raising the signal against a fixed noise floor but matters less close in, and neither supply voltage nor device bandwidth is decisive. The same steep phase characteristic that gives long-term stability also gives low phase noise, which is why one component choice improves two apparently unrelated specifications.

Q: Why does an oscillator's frequency shift when its load changes?

Any load that is not purely resistive presents a reactance which appears across the frequency-determining network and shifts it. Gain changes affect amplitude, and heating is a slower and separate mechanism. The remedy is a buffer stage between oscillator and load, so that the oscillator sees a constant impedance whatever the world outside does. Isolating a sensitive circuit from a variable one with a buffer is the standard answer wherever a specification must not depend on the load.

Q: What is the usual remedy for load pulling?

A buffer presents a constant impedance to the oscillator and drives whatever the load happens to be, so changes outside never reach the tank. A larger tank capacitance reduces sensitivity somewhat but at the cost of the tuning range, a higher supply does nothing, and a series resistor wastes signal without providing isolation. Adding a stage whose only purpose is isolation is worthwhile precisely when the interface, rather than the gain, is the problem.

51.

Why can a plain 555 astable not produce an exactly symmetrical square wave?

A.Its timing capacitor never fully discharges
B.The charge path includes a resistor the discharge path does not
C.Its internal thresholds are not symmetrical about mid-supply
D.Its output stage is faster on one edge than the other

Answer: B

Charging goes through both resistors while discharging goes through only the second, so the high time always exceeds the low time and the duty cycle cannot reach a half. The thresholds at one third and two thirds of the supply are symmetrical, and the output stage is not the issue. The standard fix is a diode across the second resistor so that charging bypasses the first, which makes the two paths equal - a good example of solving an asymmetry by giving each direction its own path.

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52.

What does a unijunction transistor provide in an oscillator?

A.A stable voltage reference for the timing threshold
B.A crystal-controlled frequency reference
C.A negative resistance region that triggers discharge
D.A high current gain for the amplifying stage

Answer: C

A unijunction has a peak point beyond which its emitter current rises as its voltage falls, so once the timing capacitor reaches that point the device conducts heavily and discharges it. That negative resistance is what makes it a self-triggering switch rather than an amplifier. It has no useful current gain and provides no reference. A device whose characteristic has a negative-slope region can do things no ordinary three-terminal device can, and generating a sawtooth with two components is the clearest example.

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53.

What waveform does a unijunction relaxation oscillator naturally produce?

A.A sine wave across the capacitor
B.A symmetrical square wave at both terminals
C.A triangular wave with equal rise and fall times
D.A sawtooth across the capacitor and pulses at the base

Answer: D

The capacitor charges slowly through its resistor and discharges almost instantly through the device, giving a sawtooth, while the discharge current through the base resistor appears as a narrow pulse. That pulse is what made the circuit the standard trigger for thyristors. Neither a sine nor a symmetrical square wave arises, and a triangle would need equal charge and discharge paths. Reading a waveform off the charge and discharge paths is quicker than recalling it.

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54.

Why is a Schmitt trigger used in a relaxation oscillator?

A.Its hysteresis defines the two switching thresholds
B.Its gain is higher than an ordinary comparator's
C.It removes the need for a timing capacitor
D.It makes the output amplitude independent of supply

Answer: A

The two thresholds of a Schmitt trigger are exactly the upper and lower limits between which the capacitor charges and discharges, so the circuit needs nothing more than the trigger, a resistor and a capacitor. Its gain is not the point, the capacitor is essential, and amplitude follows the supply. A component whose defining property is a pair of thresholds is a natural fit for a circuit that needs exactly two, which is why the arrangement is so economical.

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55.

What is the chief drawback of a relaxation oscillator as a frequency source?

A.It cannot be made to work above a few kilohertz
B.Its frequency depends directly on component values
C.Its output is a square wave rather than a sine
D.It requires more components than an LC oscillator

Answer: B

The period is a resistor times a capacitor, so the frequency inherits both components' tolerance and drift, and stabilities better than a per cent are hard to reach. The square output is often what is wanted, the component count is low, and these circuits work to megahertz. A frequency set by absolute component values can never compete with one set by a mechanical resonance, which is why every accurate clock in a system comes from a crystal and every cheap one from an RC.

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56.

What is meant by an oscillator's phase noise?

A.The slow drift of its frequency with ambient temperature
B.The amplitude variation caused by supply ripple
C.Random jitter of its zero crossings in time
D.The harmonic content present in its output waveform

Answer: C

Phase noise is the random fluctuation of the output's phase, seen as skirts either side of the carrier in the spectrum and as jitter in the time domain. Harmonic content is distortion, slow drift is a stability specification measured over minutes or hours, and amplitude variation is a separate impairment. The three describe different imperfections on different time scales, and a source can be excellent on one and poor on another - which is why a data sheet quotes all three.

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57.

Which oscillator property most reduces phase noise close to the carrier?

A.A large output amplitude at the load
B.A low supply voltage for the amplifier
C.A wide bandwidth in the amplifying device
D.A high resonator quality factor

Answer: D

A sharp resonance corrects a phase disturbance with a very small frequency excursion, so the noise skirts are narrow and low. Amplitude helps by raising the signal against a fixed noise floor but matters less close in, and neither supply voltage nor device bandwidth is decisive. The same steep phase characteristic that gives long-term stability also gives low phase noise, which is why one component choice improves two apparently unrelated specifications.

Discuss in Forum

58.

Why does an oscillator's frequency shift when its load changes?

A.The load reflects a reactance into the tank
B.The amplifier's gain falls under a heavier load
C.The supply current rises and heats the components
D.The feedback fraction changes with output amplitude

Answer: A

Any load that is not purely resistive presents a reactance which appears across the frequency-determining network and shifts it. Gain changes affect amplitude, and heating is a slower and separate mechanism. The remedy is a buffer stage between oscillator and load, so that the oscillator sees a constant impedance whatever the world outside does. Isolating a sensitive circuit from a variable one with a buffer is the standard answer wherever a specification must not depend on the load.

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59.

What is the usual remedy for load pulling?

A.A resistor in series with the output
B.A buffer stage after the oscillator
C.A larger tank capacitance in the oscillator
D.A higher supply voltage for the amplifier

Answer: B

A buffer presents a constant impedance to the oscillator and drives whatever the load happens to be, so changes outside never reach the tank. A larger tank capacitance reduces sensitivity somewhat but at the cost of the tuning range, a higher supply does nothing, and a series resistor wastes signal without providing isolation. Adding a stage whose only purpose is isolation is worthwhile precisely when the interface, rather than the gain, is the problem.

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60.

What is injection locking?

A.The output amplitude collapsing under a heavy load
B.The frequency doubling when the drive is increased
C.An external signal capturing the oscillator's frequency
D.The oscillator failing to start at low temperatures

Answer: C

A small signal at a nearby frequency, coupled into the loop, can pull the oscillator into step with it and hold it there, which is a nuisance when it happens by stray coupling and a useful technique when done deliberately. Starting failures, amplitude collapse and frequency doubling are unrelated phenomena. That a weak external signal can capture a much stronger oscillator is worth knowing, because it explains a whole class of interference problems in equipment containing several sources.

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