The Beginner's Guide (OCXO, TCXO, VCXO, & Clocks)
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We'll be honest, crystal oscillators aren't the easiest topic to understand. That's mostly because there's a wide variety of crystal oscillator types that do different things, in different ways, for different purposes. This is largely due to their almost endless applications. From satellite communications in space, to military & defense, to telecom and more... there are so many different needs for crystal oscillators.
In this post, we'll cover the most common types of crystal oscillators, which include:
- Oven controlled crystal oscillators (OCXO)
- Temperature compensated oscillators (TCXO)
- Voltage controlled oscillators (VCXO)
- Clock oscillators (XO)
- And some other key types within these categories
I know it sounds like a lot to cover, but don't worry! We're about to make things a whole lot easier for you. By the end of this post, you'll learn the basic uses, advantages, and limitations of each crystal oscillator type.
Typical Temperature Stability: ±1 x 10-7 to ±1 x 10-9
Typical aging rate: ±2 x 10-7/year to ±2 x 10-8/year
Typical Power Consumption: 1.5 Watts to 2.0 Watts in a steady state condition (at +25°C ambient temperature)
An oven controlled crystal oscillator (OCXO) is a crystal oscillator that is temperature controlled by a mini internal oven. This type of oscillator has a temperature controlling circuit to maintain a consistent temperature of the crystal and other key components.
OCXOs are typically used when temperature stabilities of ±1 x 10-8 or better are required. While this type of oscillator has a tenfold improvement over a TCXO for temperature vs. frequency stability, the OCXO tends to be higher in price and consumes more power.
Temperature Characteristics of OCXO Circuits
The key to an OCXO is to keep the crystal and some of the other oscillator components at one specific temperature while the outside ambient temperature changes. This can be compared to a house in the winter, where a thermostat located inside the house senses a temperature change and controls the furnace to maintain a desired temperature.
What is the desired temperature of operation? The temperature of operation is one of the crystal's turning points (refer to crystal section). At the turning point, the slope of the frequency versus temperature curve is zero. This means that even if the temperature varies up or down slightly, the frequency change is minimal.
Note that for an OCXO, the turning point temperature of a crystal must be higher than the upper limit of your temperature range. This is because you could not control a house's temperature at +25°C with a furnace if the outside temperature is +35°C. A general rule of thumb is that you will need the turning point of the crystal to be 10°C higher than the upper operating temperature of the OCXO oscillator circuit.
For an OCXO, the thermistor (we&#;ll chat more about this in the TCXO section) is equivalent to the thermostat in the house. It is used to sense the temperature of the crystal and crystal oscillator circuitry. The heat source can be either a power transistor or a power resistor. The last component required is a comparator circuit that is used to control the amount of power generated in the heat source.
The Comparator Circuit
The comparator circuit consists of an op-amp and other components (resistors and capacitors) configured as a high gain amplifier. The temperature of operation is called the &#;set point&#; and is adjusted by a selected value resistor chosen during the normal production process.
During normal operation, the thermistor senses an ambient temperature change by changing to a slightly different resistance value. The comparator circuit then adjusts the power generated to return the thermistor back to the original resistance value and the crystal and circuit temperature to the original set point temperature.
Sticking with the house comparison... OCXOs use insulation in a similar fashion to a house. Insulation is used to lessen the effects of ambient temperature changes and to reduce the amount of power required to maintain the set point temperature. The better the insulation used, the less power is required to stay at the set temperature point. More and more of today&#;s RF applications are requiring lower power input, so insulation plays a key role.
The temperature controller circuit of a typical OCXO will hold the set point temperature within ±1°C or less.
The Double Oven OCXO (DOCXO)
A double oven oscillator (DOCXO) might be required if tighter stabilities (±1 x 10-10 to ±5 x 10-11) are required. A DOCXO is made by putting an OCXO inside another oven package. This outer oven will buffer the OCXO from ambient changes and the combination of two temperature controllers can hold the set point temperature to within ±0.10°C.
Some of the biggest downfalls of using DOCXOs include
- They require a larger package size
- They consume more power
- They are typically more expensive
Typical power intake for a double oven oscillator at +25°C ambient is 3.0 Watts to 4.0 Watts in a steady state condition.
Because OCXOs have aging rates of 0.20 ppm/year to 2.0 x 10-8/year, there is a need to adjust the frequency at +25°C to offset aging effects. Most OCXOs have mechanical frequency adjustment similar to TCXO oscillators. The typical adjustment range is +2 ppm to ±0.20 ppm.
Types of Quartz Crystal Cuts in OCXOs
The type of crystal cut will also add to the stability of the oscillator. Some types of cuts have different slopes of frequency versus temperature at their turning points. The 2 most common types of cuts are AT and SC cuts.
For example, the SC cut crystal might have a slope of 5 x 10--9/°C for a +80°C turning point while an AT-type crystal might have a slope of 1 x 10-8/°C for an 80°C turning point. With the same temperature controller, the AT-type crystal will change frequency two times the amount of the SC-type crystal. Temperature stability and operating temperature range requirements dictate the type of crystal cut used.
Typical Temperature Stability: ±0.20 ppm to ±2.0 ppm
Typical aging rate: ±0.50 ppm/year to ±2 ppm/year
Temperature compensated crystal oscillators (TCXOs) act similarly to OCXOs in that they manage the temperature of the crystal oscillator circuit. But there are also many differences.
The basic building block for a TCXO is a VCXO with approximately ±50 ppm deviation range and a temperature sensitive network. This temperature sensitive network (temperature compensation circuit) applies a voltage to the varactor diode that corrects the frequency of the VCXO at any temperature within the operating temperature range.
Typical temperature stabilities achieved from TCXOs would be from ±0.20 ppm to ±2.0 ppm. From this we can see that a TCXO offers about a tenfold improvement in temperature stability over a clock oscillator.
Related: The TXCO Oscillator: 5 Elements of Temperature Compensated Oscillators
The TCXO Circuit
To create a temperature compensation circuit, you&#;ll need something to sense ambient temperature. A thermistor is the typical sensing device in most TCXOs. Thermistors are resistive devices whose resistance is dependent upon the ambient temperature.
There are two types of thermistors:
- Ones with a positive coefficient (their resistance goes up as temperature goes up)
- Ones with a negative coefficient (their resistance goes down as the temperature goes up)
Typical temperature compensation circuits combine thermistors and resistors into a voltage divider network to produce the required correction voltage at any temperature. This correction voltage is then applied to the varactor.
If the temperature-compensation circuit matched a crystal's temperature curve exactly, the oscillator's frequency would remain constant as the temperature changed. This is not obtainable in the real world because of the variability of crystals available and the thermistor coefficients available. Each crystal's temperature stability varies slightly, and the exact thermistor coefficients and values to produce a perfect network area not always available.
Related: Can a Crystal Oscillator Operate Outside of Its Specified Temperature Range?
Typically, given sets of thermistors are used for all TCXOs in a production lot. This will allow most TCXOs to be corrected to acceptable stability. If a tighter temperature stability is required, the thermistor can be adjusted during the production sequence, but the cost of the TCXO will increase because of longer test times.
The other major problem to overcome is the perturbations (deviations from curve fit data) in the crystal temperature stability. These deviations from the smooth temperature curve are difficult to compensate for, and if they are of a narrow duration, impossible to compensate for. If the temperature stability requirement for a TCXO is too tight, some crystals might have to be replaced and production testing started over. This will increase the cost of the TCXO.
Because TCXOs have aging rates of 0.50 ppm/year to 2.0 ppm/year, there is a need to adjust the frequency at +25°C to offset aging effects. Most TCXOs have mechanical frequency adjust similar to clock oscillators. The typical adjust range is ±5 ppm.
Related: Temperature Compensated Crystal Oscillators (TCXOs): Performance & Common Types
Typical deviation ranges: ±10 ppm to as much as ± ppm.
Typical aging rate: ±1 ppm/year to ±5 ppm/year
A voltage controlled crystal oscillators (VCXO) is a crystal oscillator with a frequency that can be adjusted by an externally applied voltage. VCXOs have a wide variety of applications in frequency modulation (FM) and phase-locked-loop (PLL) systems.
The frequency of voltage controlled oscillators is maintained by a device known as a varactor diode. This device is essentially a voltage variable capacitor. The capacitance of a varactor diode is inversely proportional to the voltage applied.
To understand how a diode can be a voltage variable capacitor, first consider what is a capacitor. It&#;s made of two oppositely charged plates separated by a dielectric. The diode is nothing more than a P-N silicon junction. The facing edges of the two regions act as plates. Reversed-bias forces charges to move away from their normal regions and form a depletion layer. The greater the voltage, the wider the depletion layer. This increases the distance between the plates, which decreases the capacitance.
To get larger tuning ranges, some varactors have a hyper-abrupt junction. The doping in a hyper-abrupt varactor is denser near the junction, which causes the depletion layer to be narrower, and the capacitance to be larger. Therefore, changes in reverse voltage have greater effects on capacitance.
The transfer function (or slope polarity) for a VCXO is the direction of frequency change versus control voltage. This can either be positive (meaning a positive change in voltage will cause the frequency to go higher) or negative (meaning a negative change in voltage will cause the frequency to go higher). This parameter needs to be specified or some slope will be assumed by the manufacturer.
As a general rule of thumb, do not specify more deviation range than is necessary. That&#;s because a VCXO with more deviation will be less stable with temperature and time. As an example:
- The temperature stability of a ±25 ppm deviation VCXO might be ±10 ppm over 0°C to +50°C, with a yearly aging rate of ±1 ppm.
- The temperature stability of a ± ppm deviation VCXO might be ±100 ppm over 0°C to +50°C, with a yearly aging rate of ±5 ppm.
Related: How Does a VCXO Work?
Typical aging rate: ±1 ppm/year to ±5 ppm/year
Typical calibration tolerance: For an AT crystal, it would be ±10 ppm
Typical Frequency Adjustment Range: ±10 ppm to ±20 ppm
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The crystal controlled clock oscillator (XO) is a device that achieves its temperature stability from the quartz crystal's inherent temperature stability. This characteristic is typically specified in tens of parts per million (ppm). The initial accuracy at room temperature (+25°C) is dictated by the calibration of the crystal for the most part.
A frequency adjustment electronic circuit could be incorporated so that the nominal frequency at room temperature could be adjusted for aging. This frequency adjustment would be achieved by use of a trimmer capacitor and the typical adjustment range would be ±10 ppm to ±20 ppm. With this type of adjustment, the frequency at +25°C could be set to ±1 ppm typically.
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Choosing the Right TCXO or OCXO for Your Application
In the rapidly evolving field of electronic components, frequency control products, such as oscillators and quartz crystals, play a pivotal role in defining the efficiency and reliability of electronic circuits.
These elements serve as the "heartbeat" of electronic systems, providing the timing and synchronisation that is essential to their operation.
Understanding the nuances of frequency control products can empower you to optimise your applications, whether you are an electronics enthusiast or a professional in the field. It&#;s important to know which frequency control products match best with your application or device.
The crux of this understanding lies in the domain of oscillators, specifically Crystal Oscillators (XOs), Temperature Compensated Crystal Oscillators (TCXOs) and Oven-Controlled Crystal Oscillators (OCXOs). These are among the most commonly used frequency control products in modern electronic circuits due to their exceptional precision and stability.
In the remainder of this blog, we will delve deeper into the role that oscillators play in electronic devices. We will also share valuable information to help you understand how to choose the perfect oscillator for your specific application.
Understanding oscillators
Oscillators form the cornerstone of electronic circuits, producing a continuous, rhythmic output signal without any external input. By generating a repeating waveform that usually has a constant amplitude and frequency, oscillators effectively set the tempo for the operation of many electronic systems.
The core functioning of an oscillator revolves around the principle of oscillation - a repetitive variation, typically in time, of some measure.
The oscillators harness this principle to create a periodic waveform, which can take many forms such as a sine wave, square wave, or a clipped triangle wave. This output signal is generated by a resonant device, often a quartz crystal , whose frequency of vibration is well-defined and consistent.
The generated frequencies are pivotal to the functioning of a multitude of electronic devices. For instance, in a radio, the oscillator creates an electromagnetic wave that can be modulated with information and broadcast.
In modern computers, it drives the processor's clock, which in turn controls the execution speed of operations. In a GPS receiver, oscillators provide the precision timing needed to receive signals from satellites. This ubiquitous nature of oscillators underscores their profound relevance in our tech-driven world.
Types of oscillators and their applications
To fully appreciate the versatility and importance of oscillators, we must delve into the detailed comparison of different types, specifically:
Crystal Oscillators (XOs)
XOs are the basic building block and can either be a self contained module or a crystal with external oscillator circuit, often part of a microcontroller. This is the cheapest option providing much better accuracy than none crystal controlled options but not as good as TCXOs or OCXOs.
Temperature Compensated Crystal Oscillators (TCXOs)
TCXOs are advanced frequency control products that balance the cost and low power of a standard crystal oscillator with the temperature stability approaching that of an OCXO.
A TCXO incorporates a temperature compensation circuit that corrects the natural tendency of the crystal to change frequency with temperature, ensuring a consistent output even across a wide temperature range.
This accuracy, combined with the TCXO's smaller size, lower power consumption, and cost-effectiveness, makes it ideal for various applications, including:
- GPS Receivers
- Wireless and Cellular Communications Equipment
- Portable Test and Measurement Equipment
- Scientific Instruments
- Aerospace and Defence Systems
Oven-Controlled Crystal Oscillators (OCXOs)
On the other hand, OCXOs represent superior precision in frequency control.
An OCXO maintains a constant temperature for the crystal oscillator within an 'oven,' which allows for minimal frequency variation, even when environmental temperatures fluctuate.
Although OCXOs are typically larger, more power-hungry, and more expensive than TCXOs, their superior frequency stability and accuracy make them invaluable in high-performance applications, including:
- Telecommunications: OCXOs ensure clock synchronisation and RF signal stability across networks, retaining system timing even in the absence of GPS-derived timing signals.
- Instrumentation: Laboratory and test equipment, such as signal generators and frequency counters, utilize OCXOs for accurate and stable frequency references.
- Broadcasting: For accurate timing and synchronisation of audio and video signals, broadcast equipment relies on OCXOs.
- Computing: High-performance computing systems, including supercomputers and servers, use OCXOs for precise timing and synchronisation of system events.
Understanding the differences between TCXO, XO, and OCXO, along with their specific applications, is crucial when choosing the right oscillator for your needs. This knowledge will enable you to optimise your applications for performance, reliability, and cost-effectiveness.
Other type of oscillators
Another type of oscillator is Voltage-Controlled Crystal Oscillators (VCXOs). They are frequency control devices that offer precise frequency tuning capabilities through the application of a varying voltage.
Unlike TCXOs and OCXOs, which rely on temperature compensation or stabilisation techniques for frequency stability, VCXOs provide frequency tuning over a narrow range typically centered around a nominal frequency.
VCXOs find applications in a wide range of precision timing and frequency control applications where precise frequency adjustments are required. Some common applications of VCXOs include telecommunications, data communication systems, frequency synthesizers, phase-locked loops (PLLs), test and measurement equipment, and radar systems.
What is the difference between TCXO and VCXO?
One of the primary differences between VCXO and TCXO lies in their frequency stability and tuning capabilities. While TCXO offers excellent frequency stability over a wide temperature range, VCXO provides frequency tuning capabilities that allow for precise adjustments to the output frequency. VCXO typically have a lower frequency stability compared to TCXOs but offer greater flexibility in frequency tuning, making them suitable for applications where precise frequency adjustments are required.
Another distinguishing factor is their power consumption and size. VCXO generally consume less power and have smaller footprints compared to TCXO, making them suitable for space-constrained applications where power efficiency and size are critical considerations.
Choosing the right TCXO or OCXO for your application
Selecting the ideal oscillator for your specific application requires a thoughtful balance of several factors, including size, power consumption, cost, accuracy, and temperature range.
Understanding these factors will empower you to choose an oscillator that perfectly aligns with your needs.
Let us go into further detail about the importance of each of these factors:
Oscillator Size
Size often matters in electronics design, especially in compact or portable devices.
Here, XOs and TCXOs hold a significant advantage. The design of TCXOs and XOs are generally smaller than OCXOs, allowing for integration into space-constrained applications.
Oscillator Power consumption
Power consumption is another critical factor. In battery-powered devices or energy-conscious applications, it's crucial to opt for an oscillator that maintains a low power profile.
Again, XOs and TCXOs outshine OCXOs in this aspect, consuming less power while still delivering impressive frequency stability.
Cost of the Oscillator
Cost can also be a deciding factor, particularly in budget-sensitive projects.
Crystals offer the cheapest solution, closely followed by XOs and then TCXOs, delivering a balance of performance and affordability.
Accuracy
Accuracy is, of course, a fundamental consideration when choosing an oscillator.
If your application requires uncompromised frequency stability and precision, the OCXO becomes the prime candidate. Despite its larger size, higher power consumption, and cost, an OCXO's superior accuracy makes it the best choice for applications demanding high precision.
Phase Noise
Golledge XO modules can provide an excellent low phase noise and low jitter clock source at low cost. Our OCXOs use crystals with higher Q than normal XOs and TCXOs and therefore provide the best phase noise performance.
Temperature range
Finally, the oscillator's temperature range can be crucial, particularly in applications exposed to significant temperature variations.
Both TCXOs and OCXOs shine in this regard, with in-built temperature compensation mechanisms to maintain frequency stability over a broad temperature range.
Why Golledge Electronics should be your go-to solution for oscillators
When it comes to procuring high-quality frequency control products, Golledge Electronics should be your provider of choice.
Offering a wide-ranging portfolio, including quartz crystals oscillators, crystal filters , and saw filters , Golledge Electronics caters to a diverse array of applications and requirements.
But where Golledge Electronics truly shines is in our offering of high-quality TCXOs and OCXOs.
Our TCXOs stand out for their unique blend of compact size, low power consumption, and impressive level of temperature stability, making them a perfect choice for various applications. On the other hand, our OCXOs are renowned for their exceptional frequency stability, unparalleled in applications that require the highest levels of precision and superior phase noise
With a robust commitment to quality and compliance and the goal of always exceeding our customers&#; requirements, Golledge is proud to have cultivated a reputation for providing top-tier products backed by excellent customer service.
Our technical experts are always on hand to provide guidance and advice, helping you select the perfect frequency control product for your specific application.
For more information on OCXOs, TCXOs, and other frequency control products, do not hesitate to reach out to our team of experts. We will be happy to help your project achieve the success it deserves.
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