Know Your XO: Four Questions for Picking the Right Oscillator

David McParland, Silicon Labs Product Manager, Oscillator Products

Crystal oscillators are often one of the last components a designer thinks about, but the wrong part can quickly kill a design. Searching through the wide variety of available oscillators and their capabilities can be confusing. Here are four key questions to help ensure your design’s requirements are met.

Four Key Questions to Ask:

  1. Do you need a crystal or an oscillator?
  2. What jitter performance is required?
  3. Will your frequency change?
  4. How important is frequency stability?

Do You Need a Crystal or an Oscillator?

While they may look identical and share many specs, crystals and oscillators are very different devices. A packaged crystal is a piece of quartz, cut and polished to resonate at a specific frequency with high Q. It does NOT contain the oscillator circuit that drives the quartz to produce a clock output. Instead, the drive circuitry resides inside the device to which the crystal is connected.

In contrast, a crystal oscillator (XO) is a complete device that contains the quartz crystal, oscillator circuit, output driver and potentially a phase-locked loop (PLL). An XO provides a clock output at a specified frequency and signal format (e.g. CMOS, LVDS, LVPECL). An oscillator is a complete, tiny, one-output clock generator that can either drive a chip directly or be fed through a buffer to provide multiple copies of a particular frequency.

Most consumer and battery-powered applications use system-on-chip (SoC) devices with integrated oscillator circuitry and a simple, low-cost crystal for clock synthesis. For higher end applications, including data center, telecom, industrial and audio/video, an external XO is typically used to provide reference timing for the SoC’s internal PLLs. Using an off-chip clock source is advantageous because it provides an independent, isolated reference clock optimized to provide low-jitter operation with minimal cross talk. As another benefit, state-of-the-art oscillators incorporate integrated power supply noise rejection to minimize the impact of board-level noise on clock jitter.

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