Optical modules enable high-speed data path connections within the data center switching network, as well as to the external core and metro telecommunications network. PAM-4 based DSPs are primarily used within the data center switching network as they are cost-optimized for shorter ranges, while coherent optics DSPs are better suited for longer distances. Module form factors have greatly reduced in size over the past 5 years, fueled by a high degree of component integration into the DSP. Selecting a high performance, low-jitter reference clock source to the DSP is essential to maximizing bandwidth speeds and minimizing bit error rate. Our portfolio of Ultra Series Oscillators is designed to meet the rigorous RMS phase jitter specifications required by the latest generation 112G PAM-4 SerDes and coherent optical DSPs, supporting wide frequency ranges and offered in small form factor packaging.
Reference clock frequency requirements vary by DSP vendor and platform. Silicon Labs' Si54x oscillators and Si56x VCXOs support the industry's widest frequency range of up to 3 GHz. Some DSP platforms benefit from using high-frequency reference clocks, oftentimes in excess of 2 GHz. Silicon Labs' Si54x products support up to 3 GHz output clocks in package sizes down to 2.5 mm x 3.2 mm, making them the smallest products in the market capable of reaching those frequencies.
RMS phase jitter performance is of the highest importance when designing 400G/800G products. Using a higher reference clock frequency provides for better jitter performance because there is less noise folding. Silicon Labs offers two levels of RMS phase jitter performance in our Ultra Series, capable of delivering performance below 100fs RMS. Most DSP vendors will specify a 156.25 MHz differential reference clock, however, it may be possible for the DSP to alternatively accept a 312.5 MHz differential clock, which could improve the overall system-level jitter margin budget. Silicon Labs makes it easy to see the actual phase jitter performance of any of our oscillators through our Phase Noise Look-Up Tool. Simply select the part number, frequency, and format and the tool provides a phase noise plot of the actual performance of the device.
Depending on the DSP design and implementation, coherent optics designs may benefit from an input reference clock that has granular frequency tuning capability across a set ppm range that may be required. Silicon Labs Si545/9 oscillators come equipped with an I2C interface, providing in-system frequency tuning capabilities with <1ppb resolution. Alternatively, the Si569 Ultra Series VCXO provides similar capabilities using a controlled voltage input reference.
Power supply and other board-level noise can significantly impact the RMS phase jitter performance of an oscillator output, oftentimes exceeding maximum limits set by optical module DSP vendors. Silicon Labs includes on-chip LDO's on all power pins of the entire oscillator portfolio, which suppress power supply or board-level noise and prevent it from affecting output clock jitter performance. Simple analysis performed on Silicon Labs' Si54x oscillators, as well as leading competitor products, clearly shows the advantages of having on-chip LDOs. The same level of noise injected into the power pin of all three devices significantly impacts the output clock jitter performance of the competing devices, exceeding the limits required by DSP vendors, while the jitter performance of the Silicon Labs' Si54x remains stable and within specification.