In the world of electronics and signal processing, oscillators are the fundamental building blocks of waveforms for many applications. From powering clocks in digital devices to producing sound in musical instruments, oscillators are the components that create the periodic signals. But what is an oscillator, and how does it work?
In this post, we will define oscillators, their types, and their applications so you can understand this technology completely. Whether you’re an experienced engineer or just curious, this guide will give you all the information you need to get into the world of oscillators.
Oscillators: The Components in Electronics
Every printed circuit board (PCB) made in the last few years has an oscillator, and most integrated circuits have oscillators. You’re probably wondering: what is an oscillator? Oscillators are the components that generate a periodic electronic signal, usually in the form of a sine wave or square wave. They convert direct current (DC) signals into periodic alternating current (AC) signals, which can be used to set frequencies, audio applications, or clock signals.
All microcontrollers and microprocessors use oscillators to generate the clock signal for their operation. Some devices have built-in oscillators, some have external oscillators, and some have both, a low-accuracy internal oscillator with the option to connect an external signal.
Clock Signals
Electronic devices use clock signals as a time reference so that actions can be executed consistently. Some devices use the oscillator signal to generate other frequencies, which can be used for audio or radio signals.
Knowing the different types of oscillators and their functions will help you choose the right oscillator for your project. For example, if you want to generate a radio signal, you will need a much more accurate oscillator than other devices. Oscillators are often overlooked in projects, and we tend to choose any oscillator within the frequency range specified in the datasheet, considering board space and cost. However, the choice of oscillator is more complex than that, depending on power requirements, board real estate, and the frequency precision needed. Some oscillators run on microamps or less, and others need several amps to work.
Types of Oscillators
Oscillators can be categorized into two main types: harmonic and relaxation. Harmonic oscillators produce a sinusoidal waveform. Examples are resistor-capacitor (RC) oscillators, inductor-capacitor (LC) oscillators, tank circuits, ceramic resonators, and crystal oscillators.
Oscillator Performance Comparison
When evaluating oscillators, note that each type has a wide range of devices available on the market. For example, when looking at fixed frequency MEMS oscillators at DigiKey, you can find options with a frequency stability of 150 ppm to 50 ppm. A corresponding price range comes with this wide range of frequency stability. So, while one oscillator type may have options for high stability or precision over a wide temperature range, another option may be more cost-effective for your specific precision requirement.
A Pricing Example
To illustrate this, let’s take the Connor-Winfield OX200-SC-010.0M, a 10MHz Voltage-Controlled Crystal Oscillator (VCOCXO) with a frequency stability of +/—1.5 ppb. The IQD Frequency Products LFRBXO059244BULK, a 10MHz atomic oscillator, is more than 10 times more expensive for the same +/—1.5 ppb frequency stability. However, the $2000 atomic oscillator may be the better choice for applications that require extreme precision.
Interestingly, IQD Frequency Products also offers a VCOCXO with +/- 1 ppb frequency stability over a wider temperature range than the atomic oscillator. Priced less than twice the Connor-Winfield device in single quantity volumes, this is still cheaper than the atomic oscillator. It’s amazing that atomic clock sources are available today and even more amazing that we can find crystal oscillators that offer more precision at a fraction of the cost.
Frequency Stability
Frequency stability is a key parameter when choosing an oscillator, as it affects the performance of the devices that use these signals. Temperature variations, power supply fluctuations, and mechanical stress can affect frequency stability. So, understanding your application requirements is crucial when selecting the right oscillator. For example, applications in telecommunications or precision timing may require oscillators with tighter stability specs, while less critical applications may be good enough with more affordable options.
Temperature Range
Temperature range is another important factor. Oscillators that can maintain their frequency stability over a wide temperature range are very valuable in applications where temperature variations are common. For example, automotive or outdoor sensors may experience big temperature changes, so you need to select an oscillator that can perform well under such conditions. The ability to withstand temperature changes without compromising performance is a significant factor in the decision-making process.
Cost vs Performance
When choosing an oscillator, you need to weigh the cost against performance requirements. High-end options like atomic oscillators offer the best precision but may not be needed for every application. In many cases, a well-chosen crystal oscillator or MEMS oscillator can provide enough performance at a fraction of the cost. Knowing your project requirements will help you make informed decisions that balance performance and budget constraints.
RC Oscillators: A Primer
Now that we have covered the general overview of the different oscillators, let’s get into one of the most basic: the RC oscillator. This oscillator is not only fundamental in understanding oscillation principles but is also easy to build on a breadboard with very basic electronic components.
What is an RC Oscillator?
An RC oscillator is a type of feedback oscillator that uses resistors and capacitors along with an amplifier like a transistor or operational amplifier. The principle behind an RC oscillator is the feedback mechanism: the amplifier feeds back into the RC network and creates positive feedback that generates repeated oscillations. This allows the circuit to produce a continuous waveform, usually a sine or square wave, depending on the configuration.
Cascading RC Networks: Phase Shift
By cascading several RC networks, we can get 180 degrees of phase shift at a chosen frequency. This configuration of multiple RC networks is the basis for the RC oscillator called the Phase Shift Oscillator. Creating a specific phase shift is crucial for oscillations as it allows us to meet the conditions for sustained oscillations.
Phase Shift Oscillator
A Phase Shift Oscillator uses the principle of phase shift to produce a continuous waveform. In this design, each RC network contributes to the total phase shift. Usually, three RC stages are used, each providing 60 degrees of phase shift and a total of 180 degrees. We need 360 degrees (or 0 degrees when considering the periodic nature of waveforms) to sustain oscillations.
Adding the Amplifier Stage
To complete the phase shift, we add an amplifier stage, which can be implemented using a bipolar junction transistor (BJT) or an inverting operational amplifier. This amplifier stage provides an additional 180 degrees of phase shift between its input and output. When combined with the 180 degrees from the cascaded RC networks, we get the full 360 degrees of phase shift required for oscillations.
The Oscillation Process
The oscillation process starts when the circuit is powered on. The transistor or operational amplifier amplifies the initial noise or disturbance in the circuit, and the output is fed back into the RC network. The feedback loop ensures the output signal is continuously reinforced, and the oscillator keeps oscillating. The result is a stable sine wave or square wave output, depending on the oscillator’s configuration.
RC Oscillation Frequency
The RC oscillation frequency can be calculated by:
𝑓=12𝜋𝑅𝐶𝑁f=2πRCN1
Where:
f = RC oscillation frequency
R = Resistance of the feedback resistors
C = Capacitance of the feedback capacitors
N = Number of RC networks cascaded
This formula calculates the oscillation frequency by showing the relationship between resistance, capacitance, and a number of cascaded networks.
RC Oscillators Applications
RC oscillators are used in many applications, especially in microcontrollers and digital integrated circuits (ICs). Most microcontrollers have an RC oscillator network inside them to create their internal clock source. This clock signal is required for timing operations so that the microcontroller can perform tasks in sync. RC oscillators are also found in audio applications, signal generators, and timer circuits, showing their versatility in electronic design.
Alternative: Phase-Lag Oscillator
Another configuration of the RC oscillator is the Operational Amplifier Phase-Lag Oscillator. In this design, the RC network is arranged to produce a lagging phase shift, which can also be used to generate oscillations. The choice between phase-lead and phase-lag configuration depends on the application requirements, such as frequency response and stability.
Advantages and Disadvantages
Advantages
Simple: RC oscillators are easy to design and build for beginners in electronics.
Cheap: The components required for an RC oscillator are affordable and easily available.
Low Power: RC oscillators consume less power than other types, making them suitable for battery-operated devices.
Disadvantages
Frequency Stability: RC oscillators are sensitive to temperature and component tolerances, which can affect frequency stability.
Limited Frequency Range: RC oscillators have a limited frequency range compared to other types of oscillators, like crystal oscillators.
LC Oscillator
LC Oscillator, also known as an inductive-capacitor oscillator, is a type of oscillator that uses a tank circuit to produce positive feedback, which is necessary for oscillations. This type of oscillator is used in many applications, including radio frequency (RF) generation, signal processing, and audio applications.
LC Oscillator Components
The basic schematic of an LC oscillator consists of three main components:
Inductor (L): The inductor stores energy in a magnetic field when current flows through it. Its ability to resist change in current makes it a critical component in determining the oscillation frequency.
Capacitor (C): The capacitor stores energy as an electric field. It charges and discharges, allowing the energy to oscillate between the inductor and capacitor, which is the oscillator’s fundamental operation.
Amplifying Component: This can be a transistor, operational amplifier, or any other active device that provides the required gain to sustain oscillations. The amplifying component feeds back a portion of the output signal into the tank circuit to ensure the oscillations continue.
LC Oscillator Working Principle
The fundamental operation of the LC oscillator revolves around the tank circuit, which consists of a capacitor and an inductor connected in parallel. This configuration allows oscillations to be generated by exchanging electrical and magnetic energy. The following sections explain the working principle of the LC oscillator.
Charging the Capacitor
When the switch connects the capacitor to the voltage supply, it starts to charge. During this phase, the capacitor accumulates electrical energy in the form of an electric field. The voltage across the capacitor increases as it stores energy, preparing it for the next phase.
Discharging Through the Inductor
Once the capacitor is fully charged, the switch is flipped to connect the capacitor and inductor. At this point, the capacitor discharges its stored energy through the inductor. As the capacitor discharges, the increasing current through the inductor starts to store energy by inducing a magnetic field around the coil. This is crucial as it allows the energy to oscillate between the capacitor and inductor.
Energy Transfer and Electromagnetic Induction
After the capacitor discharges, the energy it contains is transferred to the inductor, and an electromagnetic field is created. As the capacitor’s energy flow decreases, the inductor’s current also decreases. This decrease in current causes the inductor’s magnetic field to collapse. According to the principle of electromagnetic induction, the inductor generates a back electromotive force (back EMF) that opposes the change in current described by the equation back EMF=Ldi/dt.
Recharging the Capacitor
The back EMF produced by the inductor starts to charge the capacitor again. As the inductor’s magnetic field collapses, the energy is transferred back to the capacitor and stored again as an electric field. This cycle of charging and discharging continues, and oscillations occur.
Ideal vs Real-World
In an ideal scenario with perfect components (ideal inductor and capacitor), this circuit can theoretically oscillate indefinitely. But in reality, capacitors have current leakage, and inductors have resistance. These will cause energy loss in the system, and oscillations will die out over time. This is called damping, and it causes the amplitude of the oscillations to decrease.
Sustaining Oscillations with Active Components
To sustain the oscillations and compensate for the energy loss from the tank circuit, active components are added to the circuit. These can be bipolar junction transistors (BJTs), field-effect transistors (FETs), or operational amplifiers (Op-Amps). The main function of these active components is to provide gain and positive feedback and replenish the energy lost due to damping.
By amplifying the signal and feeding it back into the tank circuit, these active components ensure the oscillations can continue for a longer duration and maintain the desired output waveform.
Tuned Collector Oscillator: Introduction
The Tuned Collector Oscillator is a basic type of LC oscillator that uses a transformer and a capacitor in parallel with a transistor to switch the circuit. This is one of the simplest LC oscillator schematics and is used in many applications, including radio frequency (RF) generation and signal processing.
Components of Tuned Collector Oscillator
The tuned collector oscillator consists of the following:
Transformer: The transformer is the heart of the tuned collector oscillator. It has a primary and a secondary coil. The primary coil and the capacitor form the tank circuit, and the secondary coil provides positive feedback to the transistor.
Capacitor: The capacitor, along with the transformer, stores and releases energy to oscillate. The combination of the capacitor and the primary coil of the transformer forms a resonant circuit that determines the oscillation frequency.
Transistor: The transistor is a switch and an amplifier in the circuit. It controls the current and provides the gain to sustain the oscillations. The feedback from the secondary coil of the transformer is fed back to the base of the transistor to amplify the signal.
Applications of Tuned Collector Oscillator
Tuned collector oscillators are used in
RF Signal Generation: They are used in RF transmitters and receivers to generate carrier signals for communication.
Signal Processing: Tuned collector oscillators like mixers and modulators can be used in signal-processing circuits.
Audio Applications: They are used in audio synthesis and effects processing to create oscillating waveforms.
Advantages and Limitations
Advantages
Simple: A tuned collector oscillator is simple to design and build and is suitable for educational purposes and hobby projects.
Good Feedback: The transformer provides good feedback to sustain oscillations at the desired frequency.
Limitations
Component Sensitivity: The tuned collector oscillator is sensitive to temperature changes and component tolerances, which can affect the frequency stability.
Size and Weight: The use of a transformer makes the circuit bulkier than other oscillator designs, which can be a problem in compact applications.
Colpitts Oscillator: Introduction
The Colpitts Oscillator is an LC tank oscillator widely used in radio frequency (RF) applications. It can operate up to several hundred megahertz and is used in many electronic devices, including transmitters, receivers, and signal generators.
Circuit Diagram
The Colpitts oscillator circuit has this configuration:
Two Capacitors in Series: Two capacitors in series form a voltage divider. This is the key to providing feedback to the transistor to sustain the oscillations.
An inductor in parallel: An inductor is parallel to the capacitors, which form the tank circuit. The combination of the inductor and capacitors determines the resonant frequency of the oscillator, which oscillates at a specific frequency.
Transistor: The transistor is the circuit’s amplifier. It provides the gain to sustain the oscillations and is usually configured as a common emitter to amplify the feedback signal from the voltage divider formed by the capacitors.
Advantages and Limitations
Advantages
Stable: The Colpitts oscillator is more stable than other oscillators and is good for RF applications.
Wide Range: It can operate up to several hundred megahertz and is versatile for many applications.
Limitations
Tuning: Tuning of the Colpitts oscillator is tricky because of the interaction between the capacitors and the inductor. Small changes in component values can affect the oscillation frequency.
Loading: The Colpitts oscillator is often used with an emitter follower circuit to prevent loading the resonant network. This configuration helps to maintain the oscillation without adding extra load.
Applications of Colpitts Oscillator
Colpitts oscillators are used in:
RF Signal Generation: They are used in RF transmitters and receivers to generate carrier signals for communication.
Signal Processing: Colpitts oscillators can be used in signal processing circuits like mixers and modulators.
Frequency Synthesizers: They are used in frequency synthesizers to produce stable frequency output.
Clapp Oscillator: A Better Design
The Clapp oscillator is a variation of the Colpitts oscillator that addresses the problem of tuning to a specific frequency during production. By adding a variable capacitor in series with the inductor, the Clapp oscillator provides more flexibility and precision in frequency tuning and is good for many applications.
Circuit Diagram
Clapp oscillator retains the basic components of the Colpitts oscillator but with one key modification:
Variable Capacitor: A variable capacitor is added in series with the inductor. This allows the frequency to be fine-tuned during production and servicing to meet specific frequency requirements.
Capacitors in Voltage Divider: Like Colpitt’s oscillator, the Clapp oscillator has two capacitors, which form a voltage divider that provides feedback to the transistor. This feedback is necessary to sustain oscillations.
Inductor: The inductor is parallel to the capacitors to form the tank circuit, which determines the resonant frequency of the oscillator.
Advantages and Limitations
Advantages
Better Tuning: The addition of a variable capacitor makes the Clapp oscillator more easily tuned to a specific frequency and more versatile in production and servicing.
Stable Frequency Output: The Clapp oscillator can produce stable frequency output and is good for RF applications.
Limitations
Temperature Sensitivity: Like the Colpitts oscillator, the Clapp oscillator is temperature sensitive, and temperature will affect frequency.
Parasitic Capacitance: The presence of parasitic capacitance in the circuit can introduce unwanted effects, potentially impacting the oscillator’s performance.