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How to Select Crystal Oscillators for Optical Modules?

Frequency Precision Matching

The frequency shall be fully consistent with the reference input of DSP/SerDes chips. 156.25 MHz is the industry-standard reference frequency point. For high-speed applications of 800G and above, the 312.5 MHz native fundamental frequency solution is preferred to avoid extra spurious noise introduced by PLL multiplication.

Output Format Compatibility

  • LVDS: Balanced low power consumption and EMI performance, the mainstream option for 800G short-reach modules
  • LVPECL: Strong driving capability and fast edge rate, standard for 1.6T/3.2T long-reach coherent modules
  • HCSL: Low power consumption with small swing, suitable for 2016 ultra-small package high-density modules

Phase Jitter Hard Specifications

The integrated bandwidth follows the industry standard of 12 kHz ~ 20 MHz: ≤100 fs for 400G, ≤50 fs for 1.6T, and within 20 fs for 3.2T. Excessive jitter directly causes eye diagram closure and sharp BER rise, which cannot be compensated by software algorithms.

Temperature Range & Frequency Stability

Industrial-grade wide temperature range of -40℃ ~ +105℃ is mandatory; consumer-grade products with an upper limit of 85℃ are not acceptable. ±20 ppm frequency stability is selected for commercial data centers, upgraded to ±10 ppm for high-temperature computer rooms and industrial scenarios.

Package Size Compatibility

  • 3225 package for general mainstream applications, balancing performance and PCB footprint
  • 2520 package for QSFP-DD / OSFP high-density compact modules
  • 2016 package for ultra-small low-power modules, with footprint only 64% of the 2520 package

Single-ended crystal oscillator (CMOS): Outputs a single signal and transmits clock signals via 0/1 level transitions. At higher frequencies, edges become slower and noise immunity degrades.

Differential crystal oscillator (LVDS / LVPECL / HCSL): Outputs a pair of complementary signals; the receiver extracts the difference between the two signals. It features strong common-mode noise rejection and steeper edges, ideal for high-speed long trace routing.

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