The deployment of 5G networks will enable tremendous advancements in communications – increasing bandwidth 10-fold and reducing latency by 50 times. To achieve such massive improvements, several technologies are pushed to evolve at a rapid pace including equipment and components used in datacenters. Optical transceivers, responsible for connecting and translating data carried over optical fiber into electrical signals within the datacenter, are one example.
To handle the vast increases in data traffic, the transmission rates of optical modules is doubling and in some cases quadrupling. In 2020, 100 gigabit per second (Gbps) data rate modules are commonly used. However, the use of 400 Gbps modules is rapidly increasing and 800 Gbps modules are now in development. Higher-capacity 400 Gbps and 800 Gbps networks are placing greater demands on optical modules and the oscillators within them. These devices must have greater functionality with denser designs, lower power per bit, and tighter jitter than their predecessors.
图1：光模块在光学骨干的每个点使用 - 从Fronthaul到回程 - 带有地铁网络和数据中心所需的高数据速率收发器
超奇数据中心是最大的光吞吐量驱动程序之一。5G需要传输和计算大量数据。为了适应这一点，数据中心必须采用更高容量的光学模块。但是，数据中心是空间有限且昂贵的扩展，这意味着光模块必须双人或四次数据速率，同时最小化额外的尺寸。
此外，操作数据中心所需的功率是非凡的。一些行业专家预计数据中心将占全球电力消耗的八个百分之一年[1]。预计光学模块将在吞吐量的巨大改进，需要额外的电力。除其他高带宽数据通信应用外，数据中心还在推动光学模块技术的边界和扩展，对振荡器技术施加更大的要求。
图2：光模块框图，带有寿势低抖动MEMS振荡器时钟PAM4重压器
The role of an optical module is to convert incoming optical signals into electrical signals, and conversely transform outgoing electrical signals back to the optical format for transport without introducing errors. This poses the complex problem of synchronizing the two time domains, that of the optical network and that of the chipset on the host board. This makes accurate timing one of the most critical factors within an optical module. The component that is responsible for bridging the timing gap, aptly named the retimer, requires a reference clock that must have increasingly lower jitter as the data-rate increments from 100 to 400 and 800 Gbps.
观看视频：光学模块的MEMS定时解决方案
As 400 gigabit modules are beginning deployment, phase jitter of the reference oscillator is becoming ever more critical. RMS phase jitter is typically computed by integrating phase noise over 12 kHz to 20 MHz offset frequencies. SiTime’sSIT9501差分振荡器从每赫兹的相位噪声开始，每赫兹的噪声噪声和每赫兹的170 dBc的噪音底板结束。集成时，紧密相位噪声转换为70 FemtoSeconds的RMS相位抖动，用于156.25 MHz时钟频率。振荡器RMS相位抖动量化时钟边缘的变化。参考时钟的RMS相位抖动驱动光学模块尤为重要，因为它在通过模块的串行数据流中增加了抖动，如果此抖动太大，则可以产生错误。由于吞吐量从400 Gbps加倍到800 Gbps，信号中的抖动应按两倍按比例减少两个以保持类似的时序余量。
图3：SIT9501 MEMS振荡器（RMS抖动：70.629FSEC;没有马刺）和基于石英PLL的振荡器（马刺）之间的相位噪声比较。
应力时另一个需要考虑的重要因素ing phase jitter is the presence of spurious noise (spurs) in the phase noise. Referring to figure 3, the phase noise plots seem comparable at first glance, but with a closer look the spurs in the quartz-based phase-locked loop (PLL) oscillator become apparent. The phase noise of the SiT9501 oscillator has no spurs, leading to an RMS phase jitter of just 70 femtoseconds. Conversely, the quartz oscillator has a total RMS phase jitter of 267 femtoseconds. When calculated without the spurs, the RMS phase jitter of the quartz oscillator is only 90 femtoseconds, meaning the spurs attribute to 60 percent of the total jitter. SiTime’s advanced integer-N PLL technology allows for tight phase noise and lower jitter without spurs.
Figure 4: Comparison of footprint and current consumption of traditional AC-coupled LVPECL layout with a 2520 oscillator (left) and layout of a 2016 SiT9501 device with integrated LVPECL source-bias resistors (right).
While optical modules are driven to increase data rates by two to four fold, the components included in the module need to deliver these improvements without increasing their footprint. SiTime’s SiT9501 differential oscillator is the optimal solution for 400 Gbps and 800 Gbps designs as it requires no compromise in performance for smaller size with only 70 femtoseconds of RMS phase jitter. Furthermore, the SiT9501 oscillator (in a 2.0 x 1.6 mm package) integrates source-bias resisters leading up to a 50 percent reduction in the total footprint compared to leading 2.5 x 2.0 mm quartz oscillators.
SIT9501振荡器还集成了片上电压调节器，滤波电源噪声并提高模块设计中的功率完整性。减少具有此类集成功能和小封装尺寸的定时占地面积很重要，因为在激光子组件和其相关电子器件中消耗了超过一半的光学模块，留下了用于信号处理和数据路径的小空间。任何空间节省允许模块制造商在其他功能中包装。
为了解决光学模块上施加的严格电流限制，两种偏置电阻器的移除导致32毫安降低电流消耗，具有交流耦合输出。SIT9501还介绍了FlexSWING™技术，可以独特地实现差动电压摆幅在工厂定制，以符合任何芯片组的差分输入 - 摆幅要求。FlexSwing允许工程师容纳具有非标准电压摇摆的低压芯片组。通过匹配芯片组的精确需求，可以消除典型的终端，通过DC耦合的LVPECL输出将功率降低到16毫安。
The evolution of optical modules to 400 Gbps and 800 Gbps data rates, driven by emerging technologies, demands leaps in performance without increases in size and current consumed. This in turn is driving oscillators to be more power efficient, consume less space, and provide lower jitter. With innovations such as integrated bias resistors and programmable voltage swing, SiTime’s SiT9501 differential oscillator delivers reductions in total footprint and current consumption, all with just 70 femtoseconds of RMS phase jitter. SiTime MEMS oscillators provide an innovative timing solution that meets the needs of optical module makers that must quickly scale performance to support rapid advancements in network equipment.
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参考：
[1]吉隆，尼古拉。“如何停止数据中心从吞噬世界的电力。”自然新闻，自然出版集团，2018年9月12日，www.nature.com/articles/d41586-018-06610-y..
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