Quartz crystals such as these are cut to ensure that temperature stability is optimized for the OCXO&#;s internal operating temperature.
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The Power to Control Temperature
It&#;s still sometimes necessary to ensure an even better degree of stability. This can be achieved by placing the crystal in a thermally insulated container with a thermostatically controlled heater.
By heating the crystal to a temperature above that which would normally be encountered within the electronic equipment, the crystal&#;s temperature can be maintained at a constant temperature. This results in a far greater degree of temperature stability. Additionally, the crystal in the OCXO will be cut to ensure that its temperature stability is optimized for the internal operating temperature (see figure).
The internal temperature for a crystal oven is commonly run at about 75°C. It needs to be above the highest temperature likely to be encountered, or else the temperature control will not work.
The typical specification for an OCXO might be ±5 × 10&#;8 per degree Celsius (0.05 ppm). A non-oven-controlled oscillator, on the other hand, may be between 10 and 100 times poorer.
To ensure that the optimum overall accuracy is maintained, combating elements such as aging of the crystal itself may be required as well as a periodic calibration of the OCXO. Typical calibration periods may be on the order of six months to a year, but the actual period will depend on the OCXO itself and the requirements of the application in which it is being used.
More Physical Abilities of an OCXO
OCXOs are physically much larger than a simple crystal oscillator. This is because they:
- Incorporate the crystal oscillator itself
- Contain a heater
- Contain control circuitry
- House thermal insulation around the crystal oscillator
Typically, the heater will run from a different supply to the oscillator. The supply for the OCXO&#;s heater may be quite current-hungry. Some units may require an amp or so upon warm-up. This figure will reduce as the temperature inside the OCXO rises and less heat is needed. As you might imagine, the temperature is thermostatically controlled.
Temperature-Compensated Crystal Oscillators (TCXOs)
A standard TCXO has quite a few performance qualities and can even completely solve the two major problems with quartz crystals. Here are some of the most common performance characteristics of TCXOs:
TCXO PPM performance: The TCXO temperature performance is better than that of a normal crystal oscillator. Figures of between 10 and 40 times improvement can often be seen. Figures of better than ±1.5 ppm over a 0 to 70°C temperature range are difficult to achieve, though. That&#;s because they fall into a high-precision category, where costs increase significantly.
Power dissipation: The power dissipation of a TCXO will be greater than an ordinary oscillator due to the additional circuitry required. In addition, the cost is greater. One should also remember that it takes a short while after startup for the oscillator to stabilize (and hopefully stay stabilized). This may be on the order of 100 ms, or possibly longer, depending on the design.
TCXO package: TCXOs come in a variety of packages, depending on the way they have been designed and the requirements of the end user. The most common form of construction is to build the circuit on a small printed circuit board (PCB) that can be housed in a plated metal package. This then becomes suitable for mounting onto the main circuit board of the overall equipment. Since the crystal itself is sealed, this means that sealing of the overall TCXO package is not critical, or even required for most applications.
Note: Package sizes such as 5 × 3.2 × 1.5 mm or 5 × 3.5 × 1 mm are widely used for TCXOs; smaller packages are available if required.
Output format and level: With many TCXOs being used to drive digital circuits, most of the small oscillator packages produce what is termed a clipped sine wave. This is suitable for driving a logic circuit, although in many cases it&#;s wise to put it through a logic buffer to ensure it&#;s sufficiently square. Often the output is an open collector circuit. If a sine wave output is required, this must be chosen at the outset; it will limit the choice available.
Power requirements: Actual power requirements will be predicated on the device. Many operate from supplies of 3 V and may draw as little as 2 mA, although this will depend on the general type, the manufacturer, and the particular device chosen.
4 Common Types of TCXOs
Although TCXOs are normally referred to in this manner, occasionally more detailed descriptions are used. Consequently, a variety of techniques can be used to provide the temperature compensation.
ADTCXO: This is an analog digital TCXO, widely used in cell phones. ADTCXOs leverage analog technology to provide temperature correction to the oscillator. It has the advantage that changes take place slowly and no phase jumps are experienced, as is the case with some all-digital types.
DTCXO: As you probably surmised, this is a digital TCXO. A DTCXO begins with a temperature sensor. Logic and mathematical functions use digital circuitry along with a lookup table. The resulting digital correction figure is converted to an analog signal using a digital-to-analog converter (DAC).
DCXO: This is a form of oscillator where any correction is calculated by the host processor within the equipment. In this way, the TCXO is not a separate entity, but the processing is incorporated within that of the overall equipment. This can help save costs in some instances.
MCXO: This form of TCXO uses a microprocessor to provide a considerably increased level of processing to deliver more accurate compensation under a variety of circumstances. While performance is a little better, costs are above those of the other forms.
TCXOs are widely used where accurate frequency sources are needed. They are less expensive and smaller than OCXOs. As such, they offer an ideal solution for many portable units requiring a reasonably accurate source.
To read more on common misconceptions about crystal oscillator stability, click here.
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.
Contact us to discuss your requirements of tcxo tc. Our experienced sales team can help you identify the options that best suit your needs.
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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
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
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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.
Consider browsing our high-performance oscillator offerings to see if Bliley can take you (and your application) further at a better price.
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