What is Ring Oscillator : Working and Its Applications

An oscillator is used to generate a signal which has a specific frequency, and these are useful for synchronizing the computation process in digital systems. It is an electronic circuit that produces continuous waveforms without any input signal. The oscillator converts a dc signal into an alternating signal form at the desired frequency. There are various types of oscillators depending on the components which are using in the electronic circuits. The different types of oscillators are Wien bridge oscillator, RC phase shift oscillator, Hartley oscillator, voltage controlled oscillator, Colpitts oscillator, ring oscillator, Gunn oscillator, and crystal oscillator, etc. By the end of this article, we will know, what is ring oscillator, derivation, layout, frequency formula, and applications.

What is a Ring Oscillator?

The definition of the ring oscillator is “an odd number of inverters are connected in a series form with positive feedback & output oscillates between two voltage levels either 1 or zero to measure the speed of the process. In place of inverters, we can define it with NOT gates also. These oscillators have an ‘n’ odd number of inverters. For instance,  if this oscillator has 3 inverters then it is called a three-stage ring oscillator. If the inverter count is seven then it is seven stage ring oscillator. The number of inverter stages in this oscillator mainly depends on the frequency which we want to generate from this oscillator.

The designing of the ring oscillator can be done using three inverters. If the oscillator is employed with a single-stage, then the oscillations & gain are not sufficient. If the oscillator has two inverters, then the oscillation and gain of the system are a little bit more than the single-stage ring oscillator. So this three-stage oscillator has three inverters that are connected in the form of series with a positive feedback system. So the oscillations & the gain of the system are sufficient. This is the reason to choose the three-stage oscillator.

“Ring oscillator uses an odd number of inverters to achieve more gain than a single inverting amplifier. The inverter gives a delay to the input signal and if the numbers of inverters are increases then oscillator frequency will be decreased. So the desired oscillator frequency depends on the number of inverter stages of the oscillator.”

The s frequency of oscillation formula for this oscillator is

Here T = time delay for single inverter

n = number of inverters in the oscillator

Ring Oscillator Layout

The above two diagrams are showing the schematic and output waveforms for 3 stage ring oscillator. Here, the PMOS size is double than of the NMOS. The NMOS size is 1.05 and PMOS is 2.1

From these values, the time period of the three-stage ring oscillator is 1.52ns. By this time period, we can say that this oscillator can produce signals with a frequency of 657.8MHz range. To generate the signal which is less than this frequency means we should add more inverter stages to this oscillator. By this, the delay will increase and operating frequency will decrease. For example to generate 100MHz signals or lesser than frequency signals 20 number of inverter stages need to be added to this oscillator.

The below figure shows the ring oscillator layout. This is a 71 stage oscillator to produce the signal at 27MHz frequencies. The inverters which are used in this oscillator are connected using L1M1 and PYL1 contact. With this contact, the input and outputs of the inverters are connected together. And Vdd pin is for source connection purposes.

Ring Oscillator using Transistor

The ring oscillator is a combination of inverters connected in a series form with a feedback connection. And the output of the final stage is again connected to the initial stage of the oscillator. This can be done through the transistor implementation also. The below figure shows the ring oscillator implantation with a CMOS transistor.

• Input can be given to this oscillator through pin 6 and pin 14 connected to Vdd and pin 7 connected to ground.
• C1, C2, and C3 are the capacitors having a value of 0.1uF.
• Here pin 14 i.e. should get the supply voltage of 3.3V.
• The output of this oscillator can be taken from after the pin 12 port.
• Set the Vdd value to the 3.3V and set the frequency to 250Hz. And C1, C2, and C3 capacitors measure the rise time and falling time at each inverter output stage. Note the frequency of oscillation.
• Then connect Vdd pin to 5V and repeat the above process and note down the propagation delay times and frequency of oscillations.
• Repeat the process with several voltage levels, then we can understand, if the supply voltage increases gate delay (rise time and fall time) decreases. If the supply voltage decreases the delay of the gates increases.

Frequency Formula

Based upon using the number of inverter stages in ring oscillators frequency can be derived by the following formula. Here delay time of each inverter also important. The final stable oscillation frequency of this  oscillator is,

Here, n indicates the number of inverter stages used in this oscillator. T is the delay time of each inverter stage.

This oscillator frequency depends only on the stages of delay time and the number of stages using in this oscillator. So, the delay time is the most important parameter in finding the oscillator frequency.

Applications

A few applications of this oscillator will be discussed here. They are,

• These are used to measure the effect of voltage and temperature on an integrated chip.
• During wafer testing, these oscillators are preferred.
• In frequency synthesizers these oscillators are applicable.
• For data recovery purposes in serial data communications, these oscillators are useful.
• In phase-locked loop (PLL) the VCO’s can be designed by using this oscillator.

A ring oscillator has been designed to generate the desired frequency in any condition. The frequency of oscillation is dependent on the number of stages and delay time of each inverter stage. And the effect of temperature and voltage of this oscillator can be tested in five conditions. In all the different test conditions if the temperature increases the time period of the output can be decreased compared with the least temperature value. We need to analyze the phase noise and jitter value if the temperature varies.