Oscillator basics for telecommunications applications

The very basic building block of digital communication is synchronisation. Synchronisation has many perspectives. In digital transmission, synchronisation is about managing transmission and reception without errors by managing an average transmission and reception rate across nodes. In cellular communication, synchronisation enables the user equipment to work reliably while in motion and when moving from one cell to the next. In advanced networks like 5G, synchronisation is fundamental to the operation and high-performance of the networks.

Digital network synchronisation is hierarchical; an atomic source clock from the core of the network distributes synchronisation across all the nodes. Each node has a servo system that will acquire synchronisation information from the higher level and steer the local timing reference oscillator in the system. Depending on the level of accuracy required by the system, the servo system will average the incoming synchronisation information. The more averaging, the better output accuracy becomes. However, during this averaging time, the output clock is controlled by the local reference. Therefore, for a particular level of output accuracy, a specific level of stability of the local reference oscillator is required.

Oscillators provide the ticks for digital circuits to operate. The ticks progress the digital states and make the digital transitions possible. Oscillating circuitries are made possible using several technologies, of which quartz crystal-based oscillators are the most popular. Quartz oscillators are based on the piezoelectric effect. A voltage is applied to a piezoelectric material, forcing the material to have mechanical deformation. When the voltage is removed, the material gets back into its original form and creates a reverse voltage across it. When this technique is applied across an oscillating circuitry, it creates a sustained oscillation.

For a given oscillator, there are two key parameters to reference; accuracy and stability. The accuracy of an oscillator is the ability to provide a frequency with the same as an ideal absolute reference frequency. For atomic clocks acting as the master sources of time, accuracy is essential. For clocks on the transport equipment, which always receive timing from the upper hierarchy, accuracy is not essential. Stability is the feature of the oscillator to continually produce the same frequency without much change. This feature matches the requirement to work with servo systems, where the system can hold the output frequency or minimal deviation between the correction intervals.

Quartz material is grown in autoclaves, after which wafers are made out of the bars. The blanks are made at different ‘cut angles’, resulting in specific oscillation behaviours. The most popular cut types used in telecommunication equipment are the AT-cut and SC-cut crystals. The blanks are connected with electrodes to pass through electricity and are made into resonators. These are packaged in different ways to produce oscillators of different technologies and performance levels.