10 Questions You Should to Know about tcxo manufacturer

The harsh truth is, selecting the wrong quartz crystal oscillator can quickly kill any design. With the wide variety of options and specs available on the market today, selecting the perfect crystal oscillator for your design can be a difficult and time-consuming task.

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That's why we're about to give you four simple questions you can ask yourself that will make it quick and easy to find the perfect crystal oscillator for your design. 

4 Key Questions That'll Help You Find the Perfect Crystal Oscillator for Your Design

1. Do You Need a Crystal or an Oscillator? 

This is a simple mistake that many engineers make. There is actually a big difference between quartz crystals and quartz crystal oscillators.

A packaged quartz crystal is simply a quartz crystal finely cut and polished to resonate at a very specific frequency when an electrical input is applied. A quartz crystal provides clock synthesis and precise timing when connected to a design with integrated oscillator circuitry. The quartz crystal alone does not provide a clock output.

Most consumer and battery-powered applications use a standard quartz crystal (along with a system-on-chip (SoC) device) because of cost, size, and power restrictions.

A quartz crystal oscillator (XO) is a complete device that contains the quartz crystal, oscillator circuit, output driver, and potentially a phase-locked loop (PLL). Unlike a quartz crystal alone, an oscillator does provide a clock output.

Crystal oscillators are more commonly used in higher-end applications such as data centers, telecom, satcom, defense operations and PNT, and so on. This is because oscillators are better at maintaining their frequency and low phase noise even in extreme temperature, vibration, and g-sensitive environments. Another benefit is that oscillators incorporate integrated power supply noise rejection to minimize the impact of board-level phase noise.

2. What Phase Noise Performance Do You Need?

Speaking of phase noise, it's important to know how critical maintaining low phase noise is for your application.

Phase noise, on a very basic level, is the amount of distortion or added noise in the clock's frequency signal. Since an oscillator is usually the main heartbeat of a timing system, maintaining low phase noise is typically desirable. 

Low phase noise oscillators with <250 fs-RMS are crucial in high-performance applications. This is because high phase noise levels can lead to high bit error rates (BER) or even total loss of system communication. Therefore, it's always safer to start with a low phase noise clock source, especially in high-end applications. As you can see in the graph below, a compensated oscillator can significantly minimize phase noise in a high-vibration environment. 

A hardware designer will not typically have a comprehensive set of phase noise requirements for all key components of the system. Reference designs are helpful in this case because the oscillator for the design has already been vetted.

Choosing a trusted and proven oscillator supplier with many low noise options is also a great way to help you find the best fit. It may be worth incurring a slightly higher cost for better phase noise requirements at first, then relax them down the road if needed.

Related: The Ultimate Guide to Understanding Phase Noise

3. Will Your Frequency Need to Change? 

There are a few scenarios you might come across when deciding what frequency you need for your application: 

1. You will need an oscillator that maintains a single, specific frequency (e.g. a 10 MHz oscillator).
2. You may need to change the frequency provided by the crystal oscillator (e.g. a video framer that needs to toggle between two different video frame rates of 297 MHz and 297/1.001 MHz). 
3. You will need to intentionally add a small frequency deviation to the oscillator as part of margin testing to stress-test the system-level setup and hold times. 

There are many times when design engineers might not know exactly which frequency the final design will require, but they know they will need an oscillator to provide the reference. In this case, there are two main types of oscillators that can provide a solution:

1. Dual and quad oscillators that can provide multiple pre-stored frequencies.
2. I2C-programmable XOs that provide consistent low phase noise capabilities over a wide frequency range. This option offers the most frequency flexibility and can be reprogrammed to a nearly infinite number of frequency possibilities. 

4. How Important Is Frequency Stability to Your Design? 

In a nutshell, frequency stability is how much an oscillator's frequency drifts away from the desired frequency output. Frequency drift can occur based on a number of factors, including temperature, external vibrations, and g-forces. Timing errors or complete loss of communication is likely to occur if the frequency drifts beyond what the application demands.

Frequency stability is expressed in parts per million (ppm) and sometimes even parts per billion (ppb) for critical applications such as precise military and defense technology.

Quartz crystals slowly age over long periods of time, which causes the output frequency to drift slowly over time. A good crystal oscillator supplier will show aging levels at different time frames such as 1 year, 5 years, and 10 years at a given temperature (typically 25℃). When in doubt, it is safer to use a timing device with guaranteed specifications over more stringent conditions to provide more design margin.

Related: How to Prevent Frequency Perturbations in Crystal Oscillators

Where to Find the Perfect Crystal Oscillator

Bliley Technologies has almost a century of experience manufacturing quartz crystals and oscillators. We offer a wide variety of OCXOs, TCXOs, VCXOs, and clocks for many different high-performance and low phase noise solutions.

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.

Oven Controlled Crystal Oscillators (OCXO)

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.

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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.

Temperature Compensated Crystal Oscillators (TCXO)

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:

1. Ones with a positive coefficient (their resistance goes up as temperature goes up)
2. 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

Voltage Controlled Crystal Oscillators (VCXO)

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?

Quartz Crystals & Clock Oscillators (XO)

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

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.

Quartz Crystal Oscillators That Will Take You Further

Not to brag, but Bliley Technologies has been a worldwide leader in the design and manufacturing of high-performance crystal oscillators for over 90 years! (Ok... maybe we're bragging a bit.)

We brag for good reason though. Our precision oscillators have been taking our customers' latest innovations further than ever before. We're always pushing the limits on size, weight, power, and cost (SWaP-C) to allow our customers to reach new heights.

Contact us to discuss your requirements of ocxo manufacturer. Our experienced sales team can help you identify the options that best suit your needs.