Relaxation Oscillators

An oscillator is a type of circuit that is designed to transmit an intermittent signal, usually with no input signal. As an example, a non-electronic oscillator is a pendulum in a grandfather clock. The pendulum rocks back and forth on a set path with two primary locations.

Providing reliable and steady electronic communications can be a challenging task without the use of oscillators to reduce the risk of failure. An electronic oscillator is a type of electronic circuit that produces a repeating waveform of specific frequency, duration, and amplitude to create a constant output that results in circuit stability.

Generally, a battery will be connected to a capacitor, which is a device that stores a very small amount of energy. The capacitor is connected to a working device, a light bulb for example. The battery transfers a charge to the capacitor untilthe capacitor is full. When the capacitor is full, it releases all of its charge at once to the light bulb, causing it to burn very bright for a moment, and then die gradually. The battery is meanwhile recharging the capacitor so the process can repeat. As a result, a blinking light is produced.

Oscillators are also effective in system timing applications, which are present in nearly every field of electronics. The timing controls for signal processing and its triggers are commonly regulated with oscillator circuits, and oscillations themselves are the product of the controlled storage and release of electrical energy on a periodic basis.

A regular source of oscillation is required for most cyclical measuring instruments, any device that initiates measuring, and instruments that operate with periodic states or waveforms. Oscillators are found in a vast range of applications, including radiofrequency receivers, multimeters, oscilloscopes, computers and computer peripherals, and the majority of digital instruments. Depending on the given application, an oscillator can serve as a source for regularly spaced pulses, or it may rely more on its stability and accuracy in generating waveforms. Relaxation oscillators are common types of electronic oscillation devices that offer straightforward design characteristics, relatively low expense, and an efficient level of stability.

RC Relaxation Oscillator

By charging a capacitor through a current or a resistor and rapidly discharging it once a predetermined voltage threshold is reached, a simple oscillating cycle can be produced. A device functioning under this principle is known as a relaxation oscillator, which can also be created by having an external circuit configured to reverse the polarity of the current when it crosses the same voltage threshold. Resistor-capacitor (RC) relaxation oscillators are generally produced using operational voltage amplifiers or integrated circuits equipped with timers. When power is applied, the amplifier causes a capacitor to begin charging toward the voltage threshold under a preset time constant. As it reaches half the voltage supply, the amplifier switches operation and the capacitor starts to discharge at the same time constant. This cycle continuously repeats, independent of the supply voltage.

Relying on current sources to charge the capacitor can yield stable triangle waves, but sometimes a low-noise oscillator may be more appropriate for an application, which entails a different design strategy. A simple circuit using an inverter array to form an RC oscillator can produce square waves and reduce sideband noise density to a significant degree. Even lower noise levels can be achieved using an external circuit that modulates output frequency. This arrangement can produce asymmetrical triangle waveforms and have the inverters switch the base drive polarity at each half-cycle interval. While these circuits yield very low sideband noise, they are also more sensitive to voltage supply changes than other types of oscillators.

L/R Relaxation Oscillator

Unlike RC oscillator circuits, which calculate the time constant from the product of resistance multiplied by capacitance, inductor-resistor (L/R) circuits have a time constant determined by the quotient of inductance over resistance. The time constant difference between the two configurations results in corresponding changes in the oscillator circuit response. RC units have a more rapid response when working with low resistance and a slower response with high resistance, while L/R devices offer quicker response with high resistance and slower response with lower resistance. Much like capacitors in RC oscillator systems, the transformers within an L/R oscillator provide cross-coupling and positive feedback. The transformers also have collector windings that are alternately charged from the power supply current and discharged through a series of diodes.

In some types of L/R oscillators, the oscillation frequency is determined by the wire resistance of a collector winding. However, if base resistance drops below a certain level, the oscillation frequency is no longer dependent on the collector winding resistance and instead becomes the function of flux density. This mode of operation benefits from simplicity and can be effective in low-power applications, but the dissipation of base resistance may lead to an overall decrease in efficiency, reducing the accuracy and predictability of the oscillator circuit’s performance.

Variable Parts in Relaxation Oscillators

When choosing a relaxation oscillator, there are two very important aspects common in any electronic power source: the amp and voltage capabilities. Amperes, or amps, represent electric charge in motion. Technically, one amp is equivalent to 6.242 x 10¹⁸ electrons passing by a given point in one second. Voltage represents the amount of driving force between two given electrons in a charge. The higher the voltage, the “higher” the electric current and therefore the more driving power.

To determine if a relaxation oscillator is appropriate for a given application, it is necessary to find the appropriate amp and voltage capabilities of both a power source (battery) as well as the capacitor. If the electric charge is too great for the capacitor or device, a short out can occur when the device is overcharged. Alternatively, an electric charge that is too low, or “flat” will fail to “turn over,” or activate, a device.

Additionally, relaxation oscillators are composed of resistors which can control the charge flow. This is represented by the battery in the example above. Some oscillator functions require a linear, or constant input of charge. This is called a “constant current source,” and reflects the way current will be constantly added to the capacitor in order to affect a constant charge. To differentiate from a constant current source, see a camera’s flash capability. The capacitor is charged only once, until the threshold is met and the flash is discharged.

Finally, relaxation oscillators can be affixed with different kind of “desynchronous” controls. These controls allow the charge to be controlled by a third control connection. In other words, the charge is produced by a separate signal and then converted into use by the capacitor. Oscilloscopes are examples of oscillators that employ synchronous control pulses.