Various factors of instability contribute to the overall accuracy of the OCXO and one of the most important accuracy factors is the phase noise.
Typical phase noise curve
Phase noise is important because it determines the frequency and stability of the oscillator. The high stability of an OCXO is attributable to the steepness of the phase slope at the frequency of oscillation. The phase slope of an OCXO is determined by the crystal’s unloaded Q-value which should be as high as possible.
A high crystal Q-value is achieved by optimizing the crystal design. Since the Q-value is proportional to the crystal mass, it is easier to create a crystal with high Q-value and subsequent low phase noise by simply making it big enough. As the crystal thickness determines the frequency, the Q value will decrease in inverse proportion to the frequency if the radial dimensions are constant. The best Q value is obtained around 5 to 10 MHz. The challenge lies in keeping the small crystal size, so that it fits into a small surface mounted oscillator packaging. Many OCXO manufacturers make surface-mounted oscillators in which there is room only for small crystals. OEM system designers should look for OCXO manufacturers that have managed to use as large a mass as possible to obtain high Q values and thus low phase noise. Quartz Pro has managed to fit a large crystal into a small package.
Frequency Aging
Frequency aging (or long-term frequency shift) is also a main concern for mobile base station manufacturers. Aging refers to the change in OCXO frequency with time, all other parameters held constant. The crystal must be manufactured for extremely good long-term stability so that the OCXO will feature very little frequency shift as time progresses. For oscillators in a master-slave configuration, frequency re-calibration eases aging requirements and OCXO cost but brings high maintenance costs. But, in the case of macro base stations, every re-calibration of the oscillator "steels" bandwidth that could have been used by subscribers and billed by the operator. There are several ways to compensate for aging. The preferable way is of course to use a sufficiently stable OCXO so that as little correction of frequency as possible is needed. Oscillators that are calibrated need a tuning range that covers aging and gives the necessary pull-in range.
Typical aging curve
There are a number of reasons why the frequency may increase or decrease over time. Perhaps the greatest cause of aging can be explained with the same method as that used by the crystal manufacturer to adjust the frequency of the crystals. An oscillating crystal can be compared to a pair of scales, although much more sensitive than a pair of scales for weighing people. If the mass increases, the frequency decreases and vice-versa. In order to keep the frequency stable over time, it is, therefore, necessary, among other things, for all impurities on the surfaces of the crystal and its holder and enclosure walls to be completely eliminated. Otherwise, they can "wander" between various parts of the enclosure and inevitably lead to aging. Typical requirements for long-term stability involve the change in mass being restricted to less than fractions of a layer of atoms. The sensitivity to change in mass is proportional to the frequency squared, which is one of the reasons why the best long-term stability is achieved with relatively low frequencies (less than approximately 15 MHz). The Quartz Pro patented crystal process minimizes impurities in and on the crystal surface.
Two additional factors that influence aging are 1) Mechanical stresses in the crystal material and deposited electrodes and 2) trapped gases that escape. To obtain a precision OCXO with long calibration cycles, Quartz Pro deploys crystals that are compensated for mechanical stress.
Retrace
Maintenance of the network requires that the base station is switched off from time to time. After the power cut, it is important that the base station is up and running at full operating capacity as fast as possible to minimize the cost of lost usage time. Quartz Pro's unique crystal and oscillator design results in a clock with excellent retrace characteristics. That is, the frequency quickly stabilizes back on operational frequency after a power cut.
Typical retrace curve
Involve your oscillator supplier
The complexity of a high performance OCXO design is substantial and there are considerable system gains to be made with the right design for a particular implementation. The difficulty is often accentuated by the fact that it is a complex task to understand how the different performance parameters of an OCXO interact. For example, an excessive pulling range (being able to adjust the operating frequency) usually results in a worse phase noise performance. Thus it is not possible to obtain both a large pulling range and good phase noise. It can also be difficult to find the right balance between cost and performance. Therefore, it is important to involve specialist knowledge, often found with the crystal oscillator manufacturer, in an early project stage. By doing so, the mobile system manufacturer increases the possibility to improve system performance, and cuts costs and development time. Shorter development time is particularly important in the telecom industry where time to market is considered to be one of the major competitive advantages. Considering a high-end crystal oscillator’s performance and cost, it often represents a great value to any system. Almost all mobile systems that use high-end crystal oscillators require specialized performance from the oscillator.