What is a Ring Oscillator? Principle, Features, and Frequency Explained

You can find a ring oscillator in many electronics. It makes a regular signal that goes from high to low voltage. This circuit has an odd number of inverting amplifier stages, like NOT gates. These are linked together in a loop. The last stage sends its output back to the first stage. This makes a closed feedback system. It is important to know this design. It helps check how fast and stable chips and circuits are.

Key Takeaways

• A ring oscillator has an odd number of inverters in a loop. This setup makes a signal that keeps going. The design helps the circuit work well.

• You can change the frequency by adding or removing inverters. Making more inverters will slow down the frequency. The speed of inverters also changes the frequency.

• Temperature and power supply must stay steady for good performance. If they change a lot, the frequency and reliability can be hurt.

• CMOS technology helps ring oscillators use less power. It also lets them be small in size. This is good for new electronic devices.

• Ring oscillators are used to test chips and in communication systems. They give important timing signals. They help devices work well and stay reliable.

Ring Oscillator Principle

Structure

A ring oscillator is made from inverters linked in a circle. Each inverter changes the signal from high to low or low to high. The last inverter sends its output back to the first one. This loop shape makes the ring oscillator different from other oscillators. Other types use things like resistors, capacitors, or crystals. The ring oscillator only uses inverters and a loop.

The loop is very important. It lets the signal travel around and come back. This keeps the circuit changing between states.

You need an odd number of inverters for it to work. Here’s why:

• You need at least three inverters.

• Odd numbers make a feedback loop that keeps the signal moving.

• Even numbers do not make the signal move. They stop the circuit from working.

• The Barkhausen stability rule says you need a 360-degree phase shift. Only odd numbers of inverters can do this.

This setup makes the ring oscillator work by itself. It is small and simple to build.

Operation

When you turn on a ring oscillator, the signal moves through each inverter. Each inverter adds a small delay when it flips the signal. This delay is called propagation delay. The last inverter’s output goes back to the first, making a loop.

A tiny voltage change at one inverter starts changes in the next. This keeps happening because of feedback. The signal keeps switching between high and low voltages. This is called positive feedback. It lets the ring oscillator keep running on its own.

You can change the frequency by using more or fewer inverters. You can also change how fast the inverters work. This design lets you pick from many frequencies. It also saves space on a chip.

Inverter Role

Inverters are the main part of the ring oscillator. Each one flips the signal and adds a delay. All the delays together set how fast the oscillator works.

Here is a table that shows how they are related:

Relationship	Description
Propagation Delay (τ_d)	If delay goes up, frequency goes down
Frequency Formula	f = 1/(2Nτ_d)
Number of Stages (N)	Changes how fast the oscillator works

If you make each inverter slower, the oscillator slows down. If you use faster inverters, it speeds up. You need to know the delays to set the right frequency.

The loop needs the inverters to work. Without them, the ring oscillator cannot switch between high and low. Every inverter is needed to keep the signal moving and the circuit working.

Frequency of Ring Oscillator

Formula

The frequency of a ring oscillator depends on how long a signal takes to go through all the inverters. Each inverter makes the signal wait a little bit. All these small waits add up. This total wait time decides how fast the oscillator can switch between high and low. To find the frequency, you use a formula.

The frequency depends on the total delay for the signal to pass through all the inverters. If 'N' is the number of inverters and 'T' is the delay for each one, you can find the frequency (f) with this formula:

f = 1 / (2_N_T)

Both the number of inverters and the delay time are important. If you add more inverters, the signal takes longer to go around. This makes the oscillator slower.

Influencing Factors

Many things can change how fast the oscillator works. You should think about these things when you design or use a ring oscillator.

Factor	Description
Temperature Change	When the temperature changes, the parts inside the circuit can act differently. This can make the frequency change. TCXO and OCXO oscillators help by keeping the frequency steady even if the temperature changes.
Mechanical Vibrations	Vibrations can shake the oscillator and cause phase noise. This makes the signal less steady. Using the right crystal and keeping the oscillator safe from shaking can help fix this problem.
Power Supply	The oscillator needs a steady power supply to work well. If the power changes, the frequency can drift. Using a regulated power supply helps keep the voltage steady and the frequency stable.

Temperature is a big factor. When it gets hotter, the delay in each inverter changes. This happens because the parts inside, like PMOS and NMOS, work differently when it is hot or cold. If the temperature goes up, the timing gets worse. The frequency can change a lot. You need to keep the temperature steady for a stable oscillator.

If you add more inverters, the frequency goes down. The total delay gets longer, so the oscillator switches slower. You can change the frequency by using more or fewer inverters or by making the inverters faster.

Example

Here is an example. Imagine you make a ring oscillator with three inverters. Each inverter has a delay of 253.7 picoseconds. You can use the formula to find the frequency:

f = 1 / (2 * 3 * 253.7e-12)

If you do the math, you get about 657.8 MHz. This means the oscillator switches between high and low 657.8 million times every second. You can change the frequency by adding more inverters or by making each inverter faster.

You should also know about phase noise and jitter. Phase noise means the signal is not always smooth. Jitter means the timing changes a little bit each time. Both can make the oscillator less steady. If you want a very steady oscillator, you need to fix these problems with good design and a steady power supply.

Features

Design

The ring oscillator is easy to build. It only needs inverters. These inverters use PMOS and NMOS transistors. Each inverter connects to the next one. They form a loop together. The loop must have an odd number of inverters. This odd number keeps the signal moving. It helps the oscillator keep working.

Some important things about the design are:

• The inverter flips the signal using PMOS and NMOS transistors.

• You always use an odd number of inverters for feedback.

• Each inverter’s output goes to the next inverter in the loop.

• The total delay through all inverters sets the frequency.

• You do not need extra parts like resistors or capacitors, so it is simple and cheap.

This simple design lets you build a ring oscillator fast. You can change the frequency by adding or removing inverters. You can also change how fast each inverter works.

Performance

A ring oscillator has good and bad points. One big advantage is its tuning range. You can change the frequency by changing the supply voltage. You can also change the current or the load on the inverters. This gives a wider tuning range than a crystal oscillator.

Here is a table that shows how tuning ranges compare:

Oscillator Type	Tuning Range Description
Ring Oscillator	Usually has a wider tuning range than crystal oscillators.
Crystal Oscillator	Has a small tuning range because of a high Q factor.
LC Oscillator	Can have a wide tuning range, but it depends on the design and parts used.

The ring oscillator is flexible in design. But small voltage changes can make the frequency change a lot. You need to watch the supply voltage to keep it stable.

Limitations

There are some limits to using a ring oscillator. Its frequency is not as accurate as other oscillators. Noise can also be a problem. Many things can affect how steady and reliable the output is.

Limitation	Description	Impact
Thermal Noise	Happens in all electronic circuits and affects how well you can measure.	Causes jitter in the frequency, hiding small changes in capacitance.
Phase Noise	This is a big problem for accurate frequency measurements.	Makes the timing less certain, so the frequency can change.
Environmental Noise	Can make phase noise worse and hurt stability and efficiency.	Adds more noise, making the frequency less reliable.

You might also have problems from changes in how chips are made. Even small changes in voltage can make the frequency shift a lot. If the workload changes, the ring oscillator can also change how it works. Because of these things, you should test and watch your design closely.

CMOS Ring Oscillators

Technology

CMOS ring oscillators are found in lots of new devices. People started using them in communication systems in the late 1980s. CMOS means Complementary Metal-Oxide-Semiconductor. This uses PMOS and NMOS transistors together. CMOS circuits use less power and work fast. Engineers pick CMOS because it is good for making oscillators.

Using CMOS makes the oscillator smaller and better. You can put many oscillators on one chip. This saves space and energy. You can also control the frequency more easily. The CMOS process lets you change how big and fast each inverter is. This helps the oscillator work in many different ways.

Note: CMOS ring oscillators are simple to make and test. You can use them to check if a chip works well before using it in a product.

Applications

CMOS ring oscillators do many important things. They help test and measure chips. They also make signals for communication. Here is a table with some common uses:

Application Type	Description
Integrated Chip Testing	You can see how voltage and temperature change a chip.
Wafer Testing	You use them to test wafers before making chips.
Frequency Synthesizers	They help make circuits with many frequencies.
Data Recovery in Communications	You use them to get data in serial communication systems.
Phase-Locked Loops (PLLs)	They work as Voltage-Controlled Oscillators (VCOs) in PLL circuits.

A CMOS ring oscillator can make signals with different phases. This helps in communication circuits. You can also use it to test how fast a chip works. Engineers use these circuits to check chip speed during manufacturing.

CMOS ring oscillators are a simple way to check chip quality. You find them in frequency synthesizers, data recovery systems, and PLLs. These circuits help make sure devices work well and stay reliable.

You now know a ring oscillator uses inverters in a loop. This makes a signal that repeats over and over. It gives steady clock signals for digital circuits. It also helps engineers test new chip designs. You can find ring oscillators in microprocessors, communication systems, and smartphones.

• They give exact timing for digital systems.

• Their simple design lets people use them in many ways.
  Today, people are making them smaller and using new materials. If you want more information, read The Ultimate Guide to Ring Oscillators.

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

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