How Does a Capacitor Resistor Oscillator Work?

A simple capacitor-resistor network determines an oscillator's frequency.

Electronic designers have created many basic oscillator circuits. One of the simplest uses the time constant produced by a resistor-capacitor (RC) network. This is called a relaxation oscillator. The values of the resistor and capacitor determine the oscillator's frequency. The RC network triggers an electronic switch which discharges the capacitor and creates a repeating charge-discharge cycle. Wave shapers can be added to this circuit, making a variety of waveforms available from the same oscillator.

Network

The RC network forms the heart of the relaxation oscillator. A resistor connects to a current source. Current from the resistor flows into a capacitor, building electric charge in it. The rate of charge depends on both the resistor and capacitor. Large resistances will lower the current and make the capacitor charge more slowly. A larger capacitor takes longer to charge. The time it takes to charge the capacitor to 63 percent full is given by the formula T = R x C, where T is time in seconds, R is resistance in ohms and C is capacitance in farads. The capacitor charges on a logarithmic curve, taking 4 time constants to reach 99 percent full.

Comparator

An electronic switch called a comparator detects a voltage rising across the capacitor as it charges. When it passes a critical threshold, the comparator causes the capacitor to discharge its current. For most oscillators, the discharge is nearly instant compared to the charge time. The resistor still feeds current to the capacitor, so it charges again until the comparator discharges it. This charge-discharge cycle produces a voltage waveform across the capacitor, called a sawtooth wave.

Current Source

Since the oscillator's frequency depends on how fast current flows into it, a good-quality oscillator will have a carefully controlled current source to prevent the frequency from drifting. Typically, the oscillator's power supply will have good voltage regulation. If not, designers may use a zener diode to keep voltage at a fixed level.

Wave Shapers

Designers frequently add other circuits to derive triangle square, and sine waves from the original sawtooth wave. Having more waveforms makes the oscillator more useful. Standard circuits shape a triangle wave from a sawtooth, then a sine wave from the triangle. You can create a square or rectangular pulse wave from sawtooth or triangle by using a separate comparator.

Voltage Control

Since the oscillator's frequency depends on how fast the capacitor charges, current can be fed into it from sources other than the resistor. A second resistor can connect to the capacitor. This resistor takes an optional external voltage and creates another current. The two currents add together to charge the capacitor at a different rate. This technique, called voltage control, allows the oscillator's frequency to be varied with an external voltage.

References

• Absolute Astronomy: Electronic Oscillator
• Electronics Tutorials: RC Time Constant
• Dublin Institute of Technology: Sine Wave Shapers