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Synchronous And Asynchronous Clocking
A wide range of timing solutions are available, including crystal oscillators (XO), voltage-controlled crystal oscillators (VCXO), and clocks. No one size fits all strategy applies when it comes to component selection. Picking the right device for a particular application is dependent on a number of factors, including whether or not the clocks must be synchronized to an externally provided reference clock, the system architecture of the processor and high speed serial data transmission ICs, and the frequency and jitter requirements of the end application. In high performance applications, low jitter and low phase noise are critical given that they have a direct impact on the bit-error rate in high speed serial data transmission applications and the signal-to-noise ratio of analog to digital data converters.
Hardware design in high performance applications such as networking, wireless RF transmission, broadcast video and test and measurement is becoming increasingly complex as hardware designers grapple with the need to support a growing number of standards, protocols, and specifications within a single hardware design. A few examples illustrate this trend. The latest networking gear is being designed to support not only SONET/SDH and Ethernet, but also high-definition video transmission. Next generation wireless infrastructure equipment is being designed to support both WiMAX and LTE (Long-Term Evolution). Broadcast video equipment, which spans a wide variety of functions including image capture, encoding, decoding, processing and video transport, must support NTSC and PAL standards to ensure worldwide compatibility. As the lines between traditionally different types of equipment become increasingly blurred, a separate challenge arises. Designing the most efficient timing architecture is paramount in these applications to minimize design time and BOM cost.
Asynchronous Clocking
The simplest clock generation source is an oscillator (XO), which generates a single output frequency for a single component. XOs are oftentimes used in asynchronous applications. Each oscillator provides a local reference to maintain two independent clock domains. System operation requires XO frequencies be close but not identical. This architecture is ideal for burst-mode traffic applications. Continuous communication requires bit or packet stuffing and FIFO management to prevent overflow/underflow conditions. Video processing equipment and 10/100/1000BaseT Ethernet are examples of applications that use asynchronous clocking. XO selection should be based on the frequency, jitter and stability requirements of the end application.
Synchronous Clocking
Synchronous clocking is most often used in applications that require continuous communication. Network latency and variability in latency must be minimized. To accomplish this, applications including SONET/SDH, Synchronous Ethernet (SyncE), wireless backhaul, and video transport require that the source and destination operate at the same frequency. On the transmit side, the clocks that provide timing for the transmit path of the high speed SerDes are locked to a highly accurate reference clock. Both primary and secondary reference clocks are supplied from a centralized timing source (e.g. GPS). A PLL is used to lock to this backplane reference, attenuate jitter on the clock signal to remove unwanted noise, and provide a low jitter output clock to the PHY. On the receive side, a Clock and Data Recovery (CDR) unit is used to recover the system clock. This CDR may be an external component or integrated within the PHY depending on the application. A XO may be used to center the CDR to provide fast acquisition and lock. The recovered clock passes through another PLL to divide the clock frequency down to a lower rate. Local timing can either be synchronized to this clock or to another local clock that is synchronized to a centralized timing source. Synchronization to the source ensures clock synchronization across all nodes in the network. Depending on system requirements, the PLLs used in this application may require a low loop bandwidth to filter unwanted clock jitter from the clock signal.
About the Author
Silicon Labs - Clock Generators and Optocouplers.
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