Answers to RF Cafe Quiz #14

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Return to RF Cafe Quiz #14

1. Who from this list is usually credited with being the first to experiment with high frequency oscillator circuits?

b) Nikola Tesla

Nikola Tesla's work with high frequency oscillators predated those of Marconi (whose name is not in the list of options to avoid contention). An extensive archive of Tesla's achievements can be found at the Tesla Society website.

2. In the four basic oscillator circuits shown below, what is the primary function of the transistor?


c) Compensations for tank circuit losses

A parallel tank circuit composed of an ideal (lossless, i.e., purely reactive) inductor and and ideal capacitors will theoretically "ring" (oscillate) in perpetuity at its resonant frequency once stimulated. Since real-world components have losses (resistance), the oscillations eventually dampen out to an unusable amplitude. Although the transistor can and usually does provide signal gain to the load, its primary purpose is to feed enough energy back into the tank circuit to sustain oscillations.

RF Cafe Quiz: Clapp oscillator schematic3. Which type of oscillator circuit is shown to the right?


b) Clapp oscillator

The Clapp oscillator, invented by James K. Clapp (page 5), is an improvement over the basic Colpitts oscillator due to the addition of a capacitor in series with the parallel inductor.

RF Cafe Quiz: Hartley oscillator schematic4. Which type of oscillator circuit is shown to the right?


c) Hartley oscillator

The Hartley oscillator, invented by Ralph V. L. Hartley, uses a tank circuit composed fundamentally of a pair of inductors (or a tapped transformer) in parallel with a capacitor. The tap point between the inductors is a reactive voltage divider to provide feedback to the transistor.

RF Cafe Quiz: Pierce oscillator schematic5. Which type of oscillator circuit is shown to the right?


d) Pierce oscillator

The Pierce oscillator, invented by George W. Pierce, replaces the LC tank circuit with a crystal as the frequency determining element. Capacitors are used to stabilize the circuit.

RF Cafe Quiz: Colpitts oscillator schematic6. Which type of oscillator circuit is shown to the right?


a) Colpitts oscillator

The Colpitts oscillator, invented by Edwin H. Colpitts, uses a tank circuit composed fundamentally of a pair of capacitors in parallel with an inductor. The tap point between the capacitors is a reactive voltage divider to provide feedback to the transistor.

7. Which factor is most important in maintaining spectral purity in an oscillator circuit?


a) Overall circuit Q

If all of the circuit components have extremely high Qs but the layout and/or shielding is poor, then the overall Q will be diminished. Similarly, a clean layout with poor component Qs will result in suboptimal performance.

8. What does the term "frequency pulling" refer to?


b) Change in frequency due to load variations

Although the other two factors, temperature and power supply variations for the load can cause its impedance to change and pull the oscillator frequency, they are lumped into the load variations definition, so b) is the most accurate choice. 1/2 credit goes for an answer of d).

9. In a phase-locked loop (PLL) oscillator circuit, what does the phase detector do?


b) Compares relative phase between the reference input and oscillator output and introduces a shift as necessary to maintain phase coherence

10. Complete the following adage used in the days before computer simulators. "If you want an oscillator, design an amplifier. If you want an amplifier, design a(n) ____."


c) Oscillator

In the days of yore, when dinosaurs roamed the Earth, it was nearly impossible to design high frequency circuits that behaved as intended on the first pass. The relatively large size of components made isolating circuit inputs from circuit outputs very difficult, and often times stray energy from the output of an amplifier circuit would feed back to the input at just the right phase to induce oscillations. Tweaking of component values and physical locations would be needed to tame the beast. Similarly, attempts to lay out oscillator circuits to intentionally get just the right feedback in phase and amplitude for sustaining oscillation would fall short of expectation, again necessitating tweaking. Use of modern circuit and EM simulators has removed a lot of the unknowns, permitting high confidence in getting it right the first time.