Application and Operation of a Double OCXO

Application Note

Application and Operation of a Double OCXO

Timo Reinhardt
FOQ Piezo Technik GmbH
Bad Rappenau, Germany

Central switching centers make up the junction of mobile and stationary communications. In order to handle increasing data flows, this operation is performed at ever-higher frequencies and data speeds, and the demand on frequency stability is steadily increasing. Neither basestations nor the oven-controlled crystal oscillators (OCXO) of the 50 parts per billion (ppb) class offer sufficient precision. Switching junctions require stabilities of the order of less than 1 ppb, which only double OCXOs can approach.

Application and Principle

The use of OCXOs is recommended for highly precise frequency references. This technique ensures that the piezoelectric resonator - a quartz crystal of the highest quality - will operate at a constant temperature. If the oven operating temperature coincides with the inversion point of the quartz, and the inversion point is higher than the permissible ambient temperature, the influence of the external temperature on the oscillator and, in particular, the quartz is minimized. The inversion point is the maximum value of a third-degree polynomial, describing the dependence of the frequency on the temperature. This means that small temperature fluctuations around the inversion point result in relatively small frequency shifts. Although these fluctuations are actually relatively slight compared to the overall temperature, they become quite important with regard to the required frequency stability. This condition is shown in Figure 1 , which represents the inversion point.

At the inversion point of 98.15° C, a temperature change of 0.3° C at the crystal would cause a frequency shift of approximately 10 ppb, exceeding the stability requirements.

Thermal Consideration

This frequency shift vs. temperature of the crystal clearly indicates that the quality of an OCXO depends decisively on how constant the temperature is that the quartz blank is experiencing. Therefore, as a rule, the quartz housing is heated and temperature regulated by means of a suitable heating element. Naturally, the electrical components (active and passive components), over the temperature range, have a considerable influence on the overall performance. This is why heating is provided.

Through the use of multiple temperature sensors and their optimal placement, it is possible to achieve such low temperature gradients, which can be verified through the use of the b-mode of the quartz (temperature-sensitive oscillation mode with the SC section) and temperature sensors. A locked feedback loop for the interior and exterior assemblies helps ensure that a tolerance window of ±0.5 ppb can be maintained within the expanded ambient temperature range of -10° C to 70° C, as shown in Figure 2 .The laws of thermodynamics dictate that, in order to maintain a constant temperature, the thermal energy flowing out into the environment must always be balanced by thermal energy inputs. The better the object to be heated is insulated from the outside world, the smaller the energy outflow becomes. In order to increase the insulation, a solid material (such as urethane foam), a vacuum or gases (air, in the simplest case) are used. A vacuum offers the best insulation, due to its low heat conductivity (dependent on vacuum level), but is more difficult to handle from a technical, production viewpoint, as it requires a sealed oscillator housing without leaks that will remain over the years. Therefore, most manufacturers are switching to air as an insulator. Once convection (air movement due to rising warm air) is eliminated as much as possible, this insulation method is usually sufficient with more expensive heating implementations. Thus it becomes clear why OCXOs with small mechanical dimensions are unable to achieve the same thermal quality as large housings. "Thermal quality" is defined as the relationship between the fluctuations of the ambient temperature and the component temperature (here the quartz blank temperature). In order to increase this quality, it is necessary to develop an improved heating concept, primarily by using double OCXOs (DOCXOs). This means that the quartz and the temperature-sensitive components are heated in accordance with proven procedures and then the entire assembly is also provided with secondary heating and regulation. With such an approach, thermal qualities can be achieved in the range of 1400, for example. Thus the change in the temperature of the quartz blank is approximately 0.05 K for each 1 K of outside temperature change (relative to the specified temperature range of 0° to 70° C).

Switch Design

Since load fluctuations can negatively influence the behavior of the oscillator, a sufficient output decoupling (buffer) must be undertaken. The simplest option for this decoupling is a suitable, basic transistor circuit. Once this circuit is optimized, frequency changes caused by load fluctuations cannot occur.The block circuit diagram of a double oven-controlled crystal oscillator consists mainly of the voltage regulation, oscillator, buffer level and heating controls. An example of such a diagram can be seen in Figure 3 .

Other disruptive influences on the frequency behavior of the oscillator, resulting from changes in the power supply, can be prevented through the use of suitable voltage regulators. Because of this requirement, a cascaded voltage regulation is used. The oscillating tendency and the phase noise introduced into the overall oscillator behavior through these additional components must be considered.

In addition to these environmental influences the long- and short-term behavior of the quartz crystal suggests a further optimization approach. The long-term behavior depends on the aging behavior of the external components (Q of the resonant circuit, influence of capacitors, transistors, inductors), as well as on the aging of the quartz resonator itself. In order to minimize this effect with regard to the quartz, cold-welded housings are used and the entire quartz assembly is pre-aged.

The aging process of the quartz also depends strongly on the way the quartz is being operated. While a high quartz load has a positive effect on the phase noise, it will also result in a stronger aging. A compromise must be sought to meet both these demands, which is determined by the specification of the OCXO. If it is possible to adjust the oscillator by means of an external synchronization during its deployment, its cost can be reduced with regard to aging and the design focus can more strongly be placed toward phase noise reduction.

Phase noise and aging requirements make it necessary to limit the oscillation amplitude in the oscillator externally, which is obtained through an automatic gain control (AGC) circuit. If this measure is not taken, then limiting effects occur through the transistor in the oscillator circuit, which create distortions and a worsening of the phase noise. The short-term behavior of the quartz resonator is, however, primarily influenced by the phase steepness, which influences the quartz quality factor Q. If quartz resonators with low Qs are used, no low phase noise requirements may be met in the near-carrier range less than 1 kHz. It is for this reason, among others, that quartz resonators are used with a Q greater than one million when used in DOCXO. Thus, in addition to the high frequency stability, a minimal phase noise behavior in the near-carrier range (around 100 Hz) is observed, as shown in Figure 4 . This high Q quartz operation occurs because of physical conditions that are present only through the use of overtone quartz crystals in a narrow frequency range of approximately 5 to 10 MHz and is proportional to the overtone used. However, increasing mechanical dimensions with an ever-smaller (fundamental tone) frequency create difficulties.

By combining the aforementioned procedures, in conjunction with an almost completely automated manufacturing process, a double oven has been sufficiently developed to meet the highest temperature and environmental demands. It is shown in Figure 5 and its characteristics are given in Table 1 .Considering the main points raised, it would seem to be a simple task to manufacture OCXOs or DOCXOs. It should be noted, however, that highly precise oscillators have only come about after years of experience and expertise gained in construction and switching technology. The quartz resonator itself introduces another problem and a higher cost factor, since it must be selected to meet the highest demands. Due to the high demand for precision, only one SC section can be used for the quartz resonator, which, because of its double-turned cutting angle, already results in higher costs and brings along undesired additional modes. All physical and electrical effects are dependent on the exact observance of this angle; even small dips have a cost-boosting effect. Depending on the temperature, sudden increases in resistance can occur in the proximity of the operating temperature. Such resonators are not useable and must be excluded during selection. A similar situation arises from other distorting physical effects or contamination of the quartz blank. Only very few quartz manufacturers are in a position to take all these demands into account and deliver consistent quality.

Timo Reinhardt has studied electronics at the University of Applied Science, Heilbronn, Germany, and received an academic degree of diplom-ingenieur. Since 2000 he has been employed as a development engineer in the department of oven-controlled crystal oscillators at FOQ Piezo Technik GmbH, Bad Rappenau, Germany.