Oscillators Questions and Answers

Q: What does pulling a crystal mean?

A load capacitance in series or parallel shifts the operating frequency slightly within the narrow window between the two resonances, which is how a crystal oscillator is trimmed to exactness or modulated in a phase-locked loop. Overdriving, overtone operation and selection are all separate matters. The pulling range is small precisely because the quality factor is high, so a crystal that must be tuned over any real range is the wrong component for the job.

Q: Why is a crystal often specified with a stated load capacitance?

A parallel-resonant crystal reaches its marked frequency only when the circuit presents the specified capacitance across it, commonly twelve or twenty picofarads, so the load capacitors are part of the frequency-determining network rather than incidental. It will oscillate with other loads but at a different frequency, drive level is set elsewhere, and overtone selection needs a tuned circuit. A component specified together with its circuit conditions has to be used with those conditions, and this is the case where it matters most.

Q: Why is a crystal's drive level specified?

The crystal is a mechanical resonator, and too much power drives it beyond its elastic limit, ageing its frequency and eventually cracking it. Too little drive is a starting problem rather than a rating, and drive does not set frequency directly. That a purely electrical component has a mechanical failure mode is easy to overlook, and it is why a series resistor limiting drive is a standard part of a crystal oscillator rather than an optional refinement.

Q: A crystal specified at \( 50 \) parts per million over its temperature range drifts by at most

Parts per million multiply the nominal frequency directly:\[ \begin{aligned} 10 \times 10^{6} \times 50 \times 10^{-6} \\ &= 500 \ \mathrm{Hz} \end{aligned} \]giving 500 Hz. Expressing stability in parts per million rather than in hertz lets one figure describe a crystal at any frequency, which is why data sheets use it - but converting to hertz is what tells you whether it is good enough for the application.

Q: What distinguishes a relaxation oscillator from a sinusoidal one?

A relaxation oscillator charges a capacitor until a threshold is crossed, then discharges it, so its waveform is a ramp or an exponential rather than a sine and its output is naturally a square wave. A resonant circuit is what the sinusoidal family uses, an active device is essential, and the frequency depends directly on the timing components. The two families differ in whether energy oscillates between two storage elements or is repeatedly dumped, and every other difference follows from that.

Q: What determines an astable multivibrator's frequency?

Each half is held off while a capacitor discharges through a resistor towards the supply, and the time constant of that pair sets each half period. Supply voltage cancels out of the timing to first order, which is a real advantage, and the collector resistors set the amplitude and the edge speed rather than the period. A timing that depends on a resistor and a capacitor and not on the supply is what makes the circuit usable without regulation.

61.

Why does a crystal oscillator hold its frequency so much better than an LC type?

A.Its feedback fraction is closer to unity
B.Its quality factor is orders of magnitude higher
C.Its amplitude is more tightly controlled
D.Its amplifier operates at a lower gain

Answer: B

A crystal's quality factor runs from tens of thousands to over a million, against a few hundred for a good LC tank, so the phase characteristic is enormously steeper and any phase disturbance in the loop moves the frequency by a correspondingly tiny amount. Amplitude control, gain and feedback fraction affect the output rather than the frequency. Every question about frequency stability comes back to the steepness of the phase characteristic, and quartz provides the steepest available at reasonable cost.

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62.

A crystal has \( Q = 20000 \) at \( 10 \ \mathrm{MHz} \). Its bandwidth is about

A.50 Hz
B.200 Hz
C.500 Hz
D.5 kHz

Answer: C

Bandwidth is the resonant frequency divided by the quality factor:

\[ B = \frac{10 \times 10^{6}}{20000} = 500 \ \mathrm{Hz} \]

giving 500 Hz, or fifty parts per million of the centre frequency. That such a narrow window exists at all is what makes the crystal a frequency standard, and it explains why the oscillator's frequency cannot be pulled far even deliberately.

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63.

What does pulling a crystal mean?

A.Driving it beyond its rated dissipation
B.Operating it at an overtone rather than its fundamental
C.Selecting a crystal from a batch by measurement
D.Shifting its frequency with an external reactance

Answer: D

A load capacitance in series or parallel shifts the operating frequency slightly within the narrow window between the two resonances, which is how a crystal oscillator is trimmed to exactness or modulated in a phase-locked loop. Overdriving, overtone operation and selection are all separate matters. The pulling range is small precisely because the quality factor is high, so a crystal that must be tuned over any real range is the wrong component for the job.

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64.

Why is a crystal often specified with a stated load capacitance?

A.The frequency depends on what the circuit presents to it
B.The crystal will not oscillate without a minimum load
C.The load capacitance sets the drive level applied
D.The load determines which overtone is selected

Answer: A

A parallel-resonant crystal reaches its marked frequency only when the circuit presents the specified capacitance across it, commonly twelve or twenty picofarads, so the load capacitors are part of the frequency-determining network rather than incidental. It will oscillate with other loads but at a different frequency, drive level is set elsewhere, and overtone selection needs a tuned circuit. A component specified together with its circuit conditions has to be used with those conditions, and this is the case where it matters most.

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65.

Why is a crystal's drive level specified?

A.The amplifier's gain depends on the drive applied
B.Excessive drive ages or fractures the crystal
C.Low drive prevents the oscillator from starting
D.Drive level sets the frequency within the window

Answer: B

The crystal is a mechanical resonator, and too much power drives it beyond its elastic limit, ageing its frequency and eventually cracking it. Too little drive is a starting problem rather than a rating, and drive does not set frequency directly. That a purely electrical component has a mechanical failure mode is easy to overlook, and it is why a series resistor limiting drive is a standard part of a crystal oscillator rather than an optional refinement.

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66.

A crystal specified at \( 50 \) parts per million over its temperature range drifts by at most

A.5 kHz at 10 MHz
B.50 kHz at 10 MHz
C.500 Hz at 10 MHz
D.50 Hz at 10 MHz

Answer: C

Parts per million multiply the nominal frequency directly:

\[ \begin{aligned} 10 \times 10^{6} \times 50 \times 10^{-6} \\ &= 500 \ \mathrm{Hz} \end{aligned} \]

giving 500 Hz. Expressing stability in parts per million rather than in hertz lets one figure describe a crystal at any frequency, which is why data sheets use it - but converting to hertz is what tells you whether it is good enough for the application.

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67.

What does an oven-controlled crystal oscillator improve?

A.The starting reliability at switch-on
B.The purity of the output waveform
C.The pulling range available for tuning
D.The frequency drift caused by ambient temperature

Answer: D

Holding the crystal at a constant temperature above the highest ambient removes the principal source of drift, taking a good crystal from tens of parts per million to a few parts in a hundred million. Starting, waveform purity and pulling range are unaffected. It costs power, warm-up time and money, which is why the technique appears in frequency standards and base stations rather than in consumer equipment - a clear case of a specification bought at a price that only some applications will pay.

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68.

What distinguishes a relaxation oscillator from a sinusoidal one?

A.It charges and discharges a timing element between thresholds
B.It uses a resonant circuit rather than an RC network
C.It requires no amplifying device of any kind
D.Its frequency is independent of the components used

Answer: A

A relaxation oscillator charges a capacitor until a threshold is crossed, then discharges it, so its waveform is a ramp or an exponential rather than a sine and its output is naturally a square wave. A resonant circuit is what the sinusoidal family uses, an active device is essential, and the frequency depends directly on the timing components. The two families differ in whether energy oscillates between two storage elements or is repeatedly dumped, and every other difference follows from that.

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69.

What determines an astable multivibrator's frequency?

A.The transistors' saturation voltages
B.The cross-coupling resistors and capacitors
C.The supply voltage and the transistors' current gain
D.The collector load resistors alone

Answer: B

Each half is held off while a capacitor discharges through a resistor towards the supply, and the time constant of that pair sets each half period. Supply voltage cancels out of the timing to first order, which is a real advantage, and the collector resistors set the amplitude and the edge speed rather than the period. A timing that depends on a resistor and a capacitor and not on the supply is what makes the circuit usable without regulation.

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70.

Why does an astable multivibrator's frequency barely depend on supply voltage?

A.The transistors saturate at the same voltage regardless
B.The cross-coupling removes any supply dependence entirely
C.The threshold scales with the supply along with the charging
D.The capacitors are charged from a constant current source

Answer: C

The capacitor charges towards the supply and the switching threshold is a junction drop, so a higher supply charges faster but has further to go, and the two effects largely cancel. No current source is involved, and saturation voltage is a small correction. That a timing circuit can be made insensitive to its supply by arranging the threshold to scale with it is a recurring idea, and it is why this simple circuit outlasted more elaborate alternatives.

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