Ring oscillator

A ring oscillator is an electronic oscillator circuit. He swings at independently and requires no external components, such as capacitors or coils. For this you can predict the value of the frequency generated only imprecisely.

Principle

An inverter or transistor in the emitter circuit is the signal of the input level with the opposite (reverse phase 180 °) at its output. The same applies to the series connection of an odd number 2n 1 coupled inverters; thereby terms of DC voltage, the operating point set ( negative feedback ). If the number you get a flip-flop -like behavior with two stable states.

If you couple the output to the input, the circuit oscillates, since the oscillation condition is satisfied:

• Due to a phase rotation of each stage of 120 ° in the vicinity of the cutoff frequency of the three built- inverter ( see picture), the total phase shift is achieved from 0 °. The stages behave here as an RC low-pass filters.
• The loop gain exceeds at all levels clearly the critical value 1 Therefore, this heavily controlled oscillator does not generate sinusoidal but rectangle-like oscillations.

In the upper diagram consists of three inverters of the oscillator, but any other odd number is also useful, the phase shifts of the individual steps can then be smaller than 120 °. The fourth serves as a driver for extracting the signal.

The circuit below uses three NF- transistors whose internal capacity is highly dependent on the operating current. Therefore, the frequency between 0.5 MHz at UB change = 0.5 V and about 15 MHz when UB = 5V. The circuit can be used as voltage controlled oscillator. The reason for this relatively low frequency is the Miller effect.

, The loop gain is much too great with three or more inverters, it suffices in principle, a single amplifier with hysteresis ( Schmitt trigger). The term of both the saved amplifier can be modeled by an RC element with which the frequency can be lowered very far.

Calculation of frequency

Because of the finite processing speed of an inverter, the input signal appears after the term tD at the output. With n identical inverters results for the period of oscillation

And the frequency

If the ring three common logic blocks with tD ≈ 2 ns used, f ≈ 83 MHz.

Applications

From the frequency measurement of a chain of inverters, the cutoff frequency of an element can be determined. This is often used for determining the transit time of signals in newly developed logic blocks or the cutoff frequency determination of newly developed transistors.

Ring oscillators are much more likely used in sensors to convert an analog signal such as pressure, humidity or temperature in a to ( almost) proportional to frequency. Reason for this is that their frequency is highly dependent on the speed of signal processing in the inverters. This can already be influenced by very small variation of the switching capacity between the input and output of any or all inverters. To a capacitive sensor is placed in parallel to Collektor -emitter path of a transistor.

As the frequency generated is strongly dependent on the junction capacitance, and this varies with the temperature, the ring oscillator can also be used as a thermometer to processor chips as ring oscillators VCO are seldom used because they produce too much phase noise.