ring oscillator

A ring oscillator is an electronic oscillating circuit; it is a self-oscillating oscillator that generates a square wave voltage like a multivibrator. It consists of a series connection of several inverters, where the output of each inverter is connected to the input of the following one. From the last inverter there is a feedback to the input of the first one.

For ring oscillators the number of inverters must be odd, otherwise such a circuit shows the behavior of a flip-flop. Frequency determining factors are the number of inverters and the delay time of the single inverters, which is caused by the existing or unwanted load capacitances and which is the inherent delay of the inverter circuits. The oscillation period results from the multiplication of both values, multiplied by two, the frequency from the quotient of the delay time.

Construction of a ring oscillator from three inverters (NOT gate)

Ring oscillators can consist of transistors which are connected in stages one after the other and whose collectors are each connected to the base of the following transistor. The phase rotation, resp. inversion is done in the transistor circuit. Likewise, a ring oscillator can be formed from inverting digitallogics, the NOT gates.

The oscillation state of a ring oscillator is statistically triggered by noise, which is amplified and fed back in the amplifier stages. The subsequent switching functions into stable switching states are those of Schmitt triggers. The output signal is a free-running square-wave signal whose frequency depends very much on the supply voltage and can only be determined to a limited extent.

Ring oscillators are used in sensor technology and for switching time measurements. They are not suitable as pure oscillators.