Oscillators Questions and Answers

Answer: B

Piezoelectricity couples the electrical and mechanical domains in both directions, so a circuit can drive and sense the crystal's mechanical resonance. The other three effects connect electricity to magnetism, heat and light respectively and have no bearing here. It is the mechanical resonance, far sharper than any electrical circuit can achieve, that gives the crystal its quality factor and hence its stability.

Answer: C

The crystal is inductive only in the narrow window between series and parallel resonance, and it is that window every Pierce oscillator works in. Below and above it the crystal is capacitive, and the equivalent circuit's inductance is cancelled outside the window by the series capacitance or the holder capacitance. A window a few kilohertz wide at ten megahertz is what makes the frequency so tightly defined.

Answer: D

Ageing is the slow, largely one-way change over months and years, quoted in parts per million per year. Temperature drift, warm-up and phase noise describe behaviour over a temperature range, over minutes and over microseconds respectively. Four specifications covering four time scales, and a source that is excellent on one may be unremarkable on another - which is why all four are quoted rather than a single figure of merit.

Answer: A

Phase noise describes the short-term random phase fluctuation, seen as jitter in time and as skirts around the carrier in the spectrum. Ageing and drift are slow effects, and harmonic distortion concerns the waveform's shape rather than its timing. Where a specification sits on the time scale is what distinguishes these four, and matching the specification to what the application actually cares about is the practical use of the distinction.

Answer: B

The period is a resistor times a capacitor, so both tolerances appear directly in the frequency and a per cent is hard to better. Supply dependence largely cancels in a well-designed circuit, current gain does not enter the timing, and the load should be buffered. A frequency set by absolute component values can never rival one set by a mechanical resonance, which is the whole reason crystals exist.

Answer: C

Oscillation grows only where the phase condition and the gain condition are satisfied at the same frequency, and the frequency-determining network arranges for that coincidence at one point. The amplifier is far broader, the non-linearity acts on amplitude, and the transient is broadband. It is the coincidence rather than either condition alone that selects, which is why a sharp phase characteristic gives a precisely defined frequency.

Answer: D

A Wien bridge whose amplitude is regulated by a lamp or a slow control loop never clips, so the output stays a clean sine with distortion in the hundredths of a per cent. A multivibrator and a unijunction produce square and sawtooth waveforms by construction, and a clipping Colpitts carries strong harmonics. The limiting mechanism decides the purity, so a topology plus its amplitude control has to be named together before any distortion figure means anything.

Answer: A

Both are true and the second is the reason: a steeper phase characteristic means any phase disturbance in the loop is corrected by a far smaller frequency excursion. That is the whole of the stability argument, and it explains why raising the quality factor is the only reliable route to a better frequency source. Every other improvement, from oven control to careful buffering, addresses a particular disturbance rather than the loop's sensitivity to disturbance in general.

Answer: B

The reason states the criterion correctly, but the assertion misapplies it: a loop gain of exactly one would sustain an existing amplitude and could never build one from noise, so a practical oscillator starts with a loop gain above one and is pulled back to unity by a non-linearity. The criterion describes the steady state rather than the starting condition. Distinguishing the condition for sustaining from the condition for growing is the central practical point in oscillator design.

Answer: C

Both hold and the second is the mechanism: the transistor's junction capacitances drift with temperature and bias, but a small series capacitor with a good coefficient decides the frequency and their contribution is negligible beside it. Making the frequency depend on the most stable available component, rather than on a sum including unstable ones, is a general technique and this is its clearest application in oscillator design.