Selecting Quartz Oscillators with High Frequency Stability vs. Temperature
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.
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.
Obtaining exceptionally high frequency stability versus temperature, as high as 1 x 10-11, requires using all the above approaches.
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.
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.
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.
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).
When different conductors are used, thermoelectric effects at the connections can degrade the frequency stability versus temperature.
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.
- J. R. Vig, “Quartz Crystal Resonators and Oscillators: a Tutorial,” US Army Communications-Electronics Research, Development & Engineering Center Fort Monmouth, NJ, USA, March 2004.
- A. Kotyukov, Y. Ivanov and A. Nikonov, “Precise Frequency Sources Meeting the 5G Holdover Time Interval Error Requirement,” Microwave Journal, Vol. 61, No. 5, May 2018.