Oscillators Questions and Answers

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.

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.

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.

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.

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.

Answer: D

An astable multivibrator is a relaxation circuit producing a square wave by charging and discharging between thresholds, with no resonant element and no sinusoidal behaviour. The other three sustain a sine by satisfying the Barkhausen conditions around a frequency-selective network. The two families are distinguished by whether energy oscillates between two storage elements or is repeatedly dumped, and every other difference between them follows from that.

Answer: A

Extra gain margin helps the circuit start reliably but does nothing for the frequency, and if it leads to hard clipping it can make matters worse. The other three attack the three principal causes of instability: loop phase disturbances, load pulling and thermal drift. Sorting proposed remedies by which disturbance each addresses is what stops effort going into the one that happens to be easiest to change.

Answer: B

This is the Barkhausen criterion. Nyquist's test examines encirclement in the complex plane, Bode's is the straight-line magnitude and phase construction, and Hartley is an oscillator topology rather than a criterion at all. The same condition appears in the feedback amplifier topic as the thing to avoid and here as the thing to achieve, which is worth noticing since the mathematics is identical and only the intent differs.

Answer: C

This is the Pierce arrangement, and it is what sits inside almost every microcontroller clock. The Clapp is a Colpitts with an extra series capacitor, the Armstrong uses transformer feedback, and the Wien bridge is an RC audio topology. Three of these are variations on one LC arrangement, so learning the Colpitts thoroughly gives most of the family with one substitution each.

Answer: D

A filament lamp's resistance rises as it heats, so more amplitude means more negative feedback and the loop gain settles at exactly one without ever clipping. A falling resistance would produce runaway rather than regulation, and a constant one would provide no control at all. A component whose imperfection is precisely the wanted behaviour is a neat inversion, and its thermal inertia is what keeps the control slow enough not to distort the waveform.