Why oscillators are important

Oscillators are something that can be easily overlooked in a project, with the attitude to simply grab any old oscillator that is within the frequency range specified in the datasheet that suits board space and cost requirements. There can be substantially more to the choice; however, depending on power requirements for the board, board real estate and the frequency precision that is required.

Some oscillators operate on microamps or less power, where some need several amps to operate. Oscillators fall into two major categories: Harmonic and relaxation. Harmonic oscillators create a sinusoidal waveform, RC, LC, tank circuits, ceramic resonators, and crystal oscillators all fall into this category. When looking at fixed frequency MEMS oscillators, for example, options that are regularly stocked at DigiKey vary between parts per million to 50 parts per billion in terms of frequency stability.

At less than twice the price of the Connor-Winfield device in single quantity volumes, and still less than ten times cheaper than the atomic option. Insensitive to EMI, Vibration and humidity. Fast startup, small size, no additional components or matching issues. Usually sensitive to EMI and humidity. Poor temperature and supply voltage rejection performan. Now that we have had a general overview of the options let's get right into the most basic of oscillators and the principles behind it - the RC oscillator is one you can easily build on a breadboard with very basic components.

An RC Oscillator resistor-capacitor is a type of feedback oscillator which is built using resistors and capacitors, along with an amplifying device such as a transistor or operational amplifier. The amplifying device feeds back into the RC network, which causes positive feedback and generates repeated oscillations. Most microcontrollers and many other digital ICs that require a clock signal to perform actions contain an RC oscillator network within them to create their internal clock source. The RC network of an RC oscillator shifts the phase of the signal by degrees.

The positive feedback is needed to shift the phase of the signal to another degrees. Therefore, the total phase shift of the circuit needs to be 0, , or another multiple of degrees. We can use the fact that a phase shift occurs between the input to an RC network, and the output from the same network, by using interconnected RC elements in the feedback branch.

In the picture above, we can see that each cascaded RC network provides a 60 degree out phase voltage lag. Three networks together produce a degree phase shift. For ideal RC networks, the maximum phase shift can be 90 degrees.

Therefore, to create a degree phase shift, oscillators require at least two RC networks. However, it is challenging to achieve precisely 90 degrees of phase shift with each RC network stage. We need to use more RC network stages cascaded together to produce the required value and the desired oscillation frequency.

A pure or ideal single-pole RC network would produce a maximum phase shift of precisely 90 degrees. For oscillation, we require degrees of phase shift, therefore, to create an RC oscillator, we must use at least two single-pole networks.

The RC network's actual phase depends on the chosen resistor and capacitor value for the desired frequency. By cascading several RC networks, we can obtain degrees of phase shift at the chosen frequency.

This cascade of networks forms the base for the RC oscillator, otherwise known as Phase Shift Oscillator. Adding an amplifying stage utilizing a bipolar junction transistor or inverting amplifier, we can produce a degree phase shift between its input and output to provide the full degree shift back to 0 degrees that we require, as mentioned above.

The primary RC Oscillator circuit produces a sine wave output signal using regenerative feedback obtained from the RC ladder network. Regenerative feedback occurs due to the ability of the capacitor to store an electric charge. The Resistor Capacitor feedback network can be connected to produce leading phase shift phase advance network or can be connected to create a lagging phase shift phase retard network.

One or more resistors or capacitors from the RC phase shift circuitry can be changed to modify the frequency of the network. This change can be made by keeping resistors the same and using variable capacitors because capacitive reactance varies with frequency.

However, for the new frequency, there could be a requirement to adjust the amplifier's voltage gain. If we choose the resistors and capacitors for RC networks, then the frequency of RC oscillations would be:. However, the combination of the RC Oscillator network works as an attenuator, and it reduces the signal by some amount as it passes through each RC stage. So voltage gain of the amplifier stage should be sufficient to restore lost signal.

The RC network needs to be connected to the inverting input of the Op-Amp, making it the inverting amplifier configuration. The inverting configuration gives degrees of phase shift at the output, leading to a total of degrees combined with the RC networks.

The other configuration of RC oscillator is the operational amplifier phase lag oscillator. LC or Inductor-Capacitor Oscillator is a type of oscillator which utilizes a tank circuit to produce positive feedback for sustaining oscillation. The schematic contains an inductor, capacitor, and also an amplifying component.

The tank circuit is a capacitor and inductor connected in parallel, the diagram above also includes the switch and voltage source for ease of demonstration of the working principle when the switch is connecting the capacitor to the voltage supply, the capacitor charges.

When the switch connects the capacitor and inductor , the capacitor discharges through the inductor. The increasing current through the inductor starts to store energy by inducing an electromagnetic field around the coil.

When the switch connects the capacitor and inductor, the capacitor discharges through the inductor. After discharging the capacitor, the energy from it has transferred into the inductor as an electromagnetic field. As the energy flow from the capacity decreases, current flow through the inductor decreases - this causes the inductor's electromagnetic field to fall as well.

This back EMF then begins to charge the capacitor. Once the capacitor has absorbed the energy from the inductor's magnetic field, the energy is stored once again as an electrostatic field within the capacitor. If we had an ideal inductor and capacitor, this circuit could generate the oscillations forever. However, a capacitor has current leakage, and inductors have resistance.

In real life, however, the oscillations would look as below, as energy is lost. This loss is called damping. If we want to sustain the oscillations, we need to compensate for the loss of energy from the tank circuit through the addition of active components to the circuit, such as bipolar junction transistors, field-effect transistors, or operational amplifiers. The primary function of the active components is to add the necessary gain, help generate positive feedback, and to compensate for the loss of energy.

The tuned collector oscillator is a transformer and a capacitor connected in parallel and switched with a transistor. This circuit is the most basic LC oscillator schematic. The primary coil of the transformer and capacitor forms the tank circuit, with the secondary coil providing positive feedback, which returns some of the energy produced by the tank circuit to the base of the transistor.

This circuit consists of two capacitors in series, forming a voltage divider , which provides feedback to the transistor, with an inductor in parallel.

While this oscillator is relatively stable, it can be hard to tune and is often implemented with an emitter follower circuit so as not to load the resonant network. To overcome the difficulties tuning the Colpitts oscillator to a specific frequency in production, a variable capacitor in series with the inductor is often added, forming a Clapp Oscillator. This modification allows the circuit to be tuned during production and servicing to the specific frequency required.

Unfortunately, this type of LC oscillator is still quite sensitive to temperature fluctuations and parasitic capacitances. SiTime oscillators are available in frequencies as low as 1 Hz for low-power devices and as high as MHz.

The frequency of SiTime oscillators is programmable within this range to 6 decimals of accuracy. The use of custom frequencies can optimize system performance.

Frequency stability is a fundamental performance specification for oscillators. It is typically expressed in parts per million ppm or parts per billion ppb which is referenced to the nominal output frequency.

It represents the deviation of output frequency due to external conditions; therefore, a smaller stability number means better performance. It may also include frequency aging over time, solder down frequency shift, and may include electrical conditions such as supply voltage variation and output load variation.

Chipset vendors may specify the required output signal mode for timing chips, or the system designer may have some leeway. Output types fall into two categories: single-ended or differential. Single-ended oscillators are lower cost and easier to implement, but they have limitations.

They are somewhat sensitive to board noise and are therefore typically better suited for frequencies below MHz. Differential signaling is a more expensive option, but it enables better performance and is preferred for higher frequency applications. Since any noise common to both differential traces will be zeroed out, this mode is less sensitive to external noise and it generates lower levels of jitter and EMI. Supply voltage, specified in volts V , is the input power required to operate the oscillator.

Standard voltages for single-ended oscillators include 1. Voltages for modern differential oscillators typically range between 2. SiTime offers oscillators that operate as low as 1. The supply voltage of most SiTime oscillator families is programmable, which reduces the need for external components such as level translators or voltage regulators.

Supply current is the maximum operating current of an oscillator. Typical supply current is measured without load. There are oscillators in computers , metal detectors and even stun guns. In this article, you'll learn the basic idea behind oscillators and how they're used in electronics. One of the most commonly used oscillators is the pendulum of a clock. If you push on a pendulum to start it swinging, it will oscillate at some frequency -- it will swing back and forth a certain number of times per second.

The length of the pendulum is the main thing that controls the frequency. For something to oscillate, energy needs to move back and forth between two forms. For example, in a pendulum, energy moves between potential energy and kinetic energy.

When the pendulum is at one end of its travel, its energy is all potential energy and it is ready to fall. When the pendulum is in the middle of its cycle, all of its potential energy turns into kinetic energy and the pendulum is moving as fast as it can. As the pendulum moves toward the other end of its swing, all the kinetic energy turns back into potential energy. This movement of energy between the two forms is what causes the oscillation.

Eventually, any physical oscillator stops moving because of friction. To keep it going, you have to add a little bit of energy on each cycle.

In a pendulum clock, the energy that keeps the pendulum moving comes from the spring. The pendulum gets a little push on each stroke to make up for the energy it loses to friction.

See How Pendulum Clocks Work for details. Energy needs to move back and forth from one form to another for an oscillator to work.