Frequency Stability, Here We Come!

It is 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 temperature of the crystal 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.

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The internal temperature for a crystal oven is commonly run at a temperature of 75°C. The temperature needs to be above the highest temperature likely to be encountered&#;otherwise, the temperature control will not work.

The typical specification for an OCXO might be ±5 × 10-8 per degree Celsius (0.05 ppm), whereas a non-oven controlled oscillator may be between 10 and 100 times poorer. As the oscillator assembly will also contain buffering circuitry, as well as supply voltage regulation, the other characteristics of the oscillator should also be good. Typically, it might be expected that frequency stability would be around ±5 × 10-9 (0.005 ppm) per day and ±5 × 10-7 (0.5 ppm) per year, and 1 × 10-7 for a 5% change in supply voltage. These are far better than would be expected from a simple crystal oscillator.

To ensure that the optimum overall accuracy is maintained, combating elements such as aging of the crystal itself is needed. A periodic calibration of the OCXO may be required. Typical calibration periods may be of the order of six months to a year, but the actual period will depend upon the OCXO itself and the requirements of the application in which it is being used.

OCXO Physical Considerations

OCXOs are physically much larger than a simple crystal oscillator. Not only do they need to incorporate the crystal oscillator itself, but also the heater, control circuitry, and the thermal insulation around the crystal oscillator.

Typically, the heater will be run from a different supply to the oscillator. It does not need the same level of regulation, and indeed the oscillator is most likely to have its own regulator to remove any stray noise and RF that may appear on the supply line and thereby degrade the performance of the OCXO.

The supply for the heater in the OCXO may be quite current hungry. Some units may require an amp or so on warm-up. This figure will reduce as the temperature inside the OCXO rises and less heat is needed. As will be imagined, the temperature is thermostatically controlled.

These OCXO units are naturally more expensive than crystals on their own, but the performance of an OCXO is considerably enhanced on that of a simple crystal in an unregulated electrical and physical environment. They also consume much more power than a traditional crystal or non-ovenized oscillator.  This is considered one of the main drawbacks of OCXOs and has driven the rise of a new class of low-power OCXOs that draw significantly less power than traditional ovens.

Frequency stability versus temperature is one of the key parameters of quartz oscillators, with several design approaches used to achieve it (see Figure 1).1,2 With a simple quartz oscillator (XO), the frequency versus temperature stability is provided only by the quartz resonator itself, primarily by choosing the cut of the quartz crystal. Frequency stability (Δf/f0) versus temperature for an XO can reach = ±10 to ±15 x 10-6 from -40°C to +85°C. With a temperature compensated quartz oscillator (TCXO), additional components apply a control voltage to a varactor diode that compensates for temperature effects on the frequency. Frequency stability versus temperature for a TCXO can reach ±1 to ±3 x 10-7 from -40°C to +85°C. An oven controlled quartz oscillator (OCXO) design places the quartz resonator and all basic circuits inside an oven at constant temperature. Frequency stability versus temperature for an OCXO can reach ±1 to ±5 x 10-11 from -40°C to +85°C. Of these three designs, only the OCXO, which has the highest frequency stability versus temperature, is discussed here.

Figure 1 Typical frequency stability vs. temperature for XO (a), TCXO (b) and OCXO (c) oscillators.

OCXO DESIGN

Figure 2 OCXO block diagram.

With the OCXO design, all temperature sensitive elements are located inside an oven and maintained at almost constant temperature (see Figure 2). The temperature inside the oven is set slightly above the upper operating temperature of the OCXO, usually 5°C to 15°C, and it is located near the quartz resonator&#;s lower turnover point or upper turnover point to minimize the frequency variation with temperature (see Figure 3).

The need to maintain a high oven temperature increases the turn-on power consumption; however, as soon as the temperature inside the oven reaches a defined level, the power decreases significantly. Related to this is the warm-up time, which is determined by the time to meet the frequency accuracy specification. Usually, the warm-up time from room temperature ranges from 2 to 5 minutes to achieve an accuracy of ±2 × 10-8.

A basic OCXO provides a frequency stability versus temperature from ±1 x 10-8 to ±5 x 10-10 depending upon the design. This can be improved through several ways:

• Double oven design (DOCXO)

  - This is an efficient approach usually achieving a frequency stability versus temperature of up to ±1 x 10-10. However, it is relatively large and has a limited OCXO upper operating temperature to maintain a difference between the operating temperature and the oven temperature.

• Additional temperature compensation

  - The frequency versus temperature characteristic of an XO is more or less linear, enabling compensation. The disadvantage of this approach is because the frequency versus temperature characteristic has a rather steep slope, which reduces the improvement that can be achieved. The slope can be reduced using an oven and employing temperature compensation with an OCXO enables up to a 5x increase in stability.

• Improvement in the basic design

  - This results in the best performance yet is the most sophisticated method, involving careful calculation and a multi-iterative process designing the specific type of oscillator to obtain better frequency stability, typically by decreasing the temperature gradients. The resulting frequency stability versus temperature can be equivalent to a DOCXO while retaining the size - and especially the height - of the basic design.

Figure 3 Typical XO frequency vs. temperature dependency.

Figure 4 Frequency vs. temperature cycling test showing OCXO aging.

Obtaining exceptionally high frequency stability versus temperature, as high as 1 x 10-11, requires using all the above approaches.

OTHER CONSIDERATIONS

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During both operation and measurement, additional factors can affect stability, which need to be considered. The higher the OCXO&#;s frequency stability versus temperature, the greater influence these factors will have.

Aging

The frequency of an OCXO changes over time, which is known as aging, making the operating time of the oscillator extremely important. An OCXO operating for several weeks will age around 10-11, while an OCXO that operates for only one day will age around 10-10. This contribution will be noticeable when measuring the frequency versus temperature, especially when it is small and comparable to the aging; so aging must be accounted for when frequency versus temperature is measured.

It is straightforward and necessary to fix the OCXO&#;s frequency at constant temperature. A model of the frequency change over time can be calculated for small time intervals, such as hours, using a simple linear model. Usually, when testing OCXOs with very high temperature stability, several heating and cooling cycles are required to ensure that the OCXO meets the stability requirements. Figure 4 shows an example of aging in the test results of a Morion OCXO.

Frequency vs. Temperature

If additional compensation is used to increase the temperature stability, areas with a steep slope may be present in the final frequency versus temperature characteristics. While this is not very pronounced for OCXOs, it is very noticeable with rubidium oscillators. To illustrate, two frequency versus temperature curves are shown in Figure 5. In Figure 5a, the slope of the frequency change with temperature is relatively small; however, Figure 5b shows an oscillator where a small change in temperature results in significantly more frequency swings, equaling the full range over temperature of the oscillator shown in Figure 5a.

Temperature Shock

Due to the design of the temperature compensation or a &#;bad&#; OCXO design, large frequency changes can be observed with rapid temperature changes. This is called temperature shock (see Figure 6). With an OCXO with high temperature stability a change in the shape and magnitude of the frequency versus temperature characteristic can be observed, due to convection inside the OCXO. For a properly designed OCXO, this dependence should be minimized and evaluated during testing.

Figure 5 Frequency vs. temperature for oscillators with linear (a) and highly changing (b) characteristics.

Voltage Control

Figure 6 OCXO designed to minimize frequency changes with temperature shocks.

Voltage control directly affects the stability of the OCXO. When considering small instabilities, the contribution due to the presence of frequency adjustment is especially acute. An OCXO without voltage control has better frequency stability versus temperature and better short-term stability than one with it. The frequency stability versus temperature of an oscillator without voltage control can be up to ±1 × 10-11, while the stability of an oscillator with voltage control may be only ±2 x 10-11. If better frequency stability is needed, an OCXO without frequency adjustment should be chosen for the application, if possible.

Frequency adjustment of the oscillator can be provided either with an analog or digital circuit. Digital voltage control uses a digital-to-analog converter (DAC) with an I2C or serial peripheral interface. With digital control, degradation of the frequency stability versus temperature is minimal; however, when changing the control code, the short-term stability and phase noise may degrade. Another limitation with digital control is the minimum tuning step, which depends on the bit capacity of the DAC. For a 20-bit DAC, the tuning step is from 5 x 10-13 to 1 x 10-13. With analog adjustment to adjust the nominal frequency, the control voltage must be applied to the control input and the location of the ground will affect the frequency stability. If a common ground is used (see Figure 7a), the current through the oven heating transistors will raise the voltage on the ground pin of the OCXO, which will add to the control voltage and degrade both the frequency stability versus temperature and the short-term frequency stability. To reduce this source of instability, the common resistance of the supply and control pins must be reduced, which is commonly done using separate grounds for the supply and control circuits (see Figure 7b).

Materials

Figure 7 OCXO designs with analog frequency adjustment using common (a) and separate (b) grounds.

When different conductors are used, thermoelectric effects at the connections can degrade the frequency stability versus temperature.

CONCLUSION

OCXOs with high frequency stability versus temperature can be successfully employed in many areas where very stable frequency sources are needed. They can even compete with rubidium oscillators in some applications, offering smaller size and lower power consumption. The OCXO frequency stability versus temperature characteristic is more linear with a lower slope, so with small changes in ambient temperature, the frequency stability can be better than that of a rubidium oscillator. The only disadvantage with the OCXO is greater aging; in the case of an extremely small change in frequency with a change in temperature, this effect can be compensated.

References

1. J. R. Vig, &#;Quartz Crystal Resonators and Oscillators: a Tutorial,&#;

   US Army Communications-Electronics Research

   , Development & Engineering Center Fort Monmouth, NJ, USA, March .
2. A. Kotyukov, Y. Ivanov and A. Nikonov, &#;Precise Frequency Sources Meeting the 5G Holdover Time Interval Error Requirement,&#;

   Microwave Journal

   The company is the world’s best ocxo oc supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

   , Vol. 61, No. 5, May .