One Crystal Oscillator to Replace Many: I2C User-Programmable XO for Frequencies of 0.2 to 1500 MHzFebruary 05, 2018 by Nick Davis
A new high-performance I2C-programmable crystal oscillator (XO) that delivers an ultra-low-jitter output frequency.
Here's the breakdown of a new high-performance I2C-programmable crystal oscillator (XO) that delivers an ultra-low-jitter output frequency.
Silicon Labs recently announced their new Si549 Ultra Series I2C programmable crystal oscillator (XO). Silicon Labs refers to this device as an "Any (I2C) Frequency Ultra Low Jitter Crystal Oscillator" because its frequency can be changed, via I2C, to any value between 0.2 and 1500 MHz. In addition to having I2C pins, this crystal oscillator has an OE (output enable) pin and an FS (frequency select) pin (see the figure below), making it a 6-pin device.
Figure 1. The Si549 pinout, from the datasheet (PDF).
Available in Two Packages
The Si549 is available in two sizes: 5mm × 7mm (which is rather large) and 3.2mm × 5mm, which still isn't tiny. The figure below displays these two package outlines.
Figure 2. The two package options of the Si549, taken from the datasheet (PDF).
Setting the Desired Frequency
The datasheet discusses, at length, how to properly set the targeted frequency; it provides equations, example Frequency Planner VB Code, and a table—which includes the values of numerous I2C registers—for achieving common output frequencies (see below). This should, hopefully, make the task of setting a desired frequency a little easier.
Figure 3. The datasheet has provided this table to help users implement common frequency settings.
Frequency Signal Format
This configurable XO has eight signal formats to choose from, plus one custom format. These formats are specified via the device's part-numbering scheme (see table below). Various other options are also specified in the part number; for more information see Section 1 (Ordering Guide) of the datasheet.
Figure 4. There are nine frequency format options, including the custom option. Table taken from the datasheet (PDF).
As described in the datasheet, generating a low-jitter signal is, in part, achieved by the use of on-chip power supply filtering that claims to provide “industry-leading power supply noise rejection.” The figure below depicts the workings of the device, including the built-in power supply noise rejection block, which the datasheet specifically identifies as a remedy for the noisy supply rails generated by switching regulators.
Figure 5. Block diagram of the Si549, from the datasheet (PDF).
As you can see in the diagram, the Si549’s operation is based on one fixed-frequency crystal. The output frequency is generated by a phase-locked loop (PLL)—more specifically, it is a “DSPLL.” This refers to a technology developed by (and patented by) Silicon Labs; they describe it as a “revolutionary architecture” that offers smaller form factor, lower power, and better performance compared to “conventional cascaded PLL devices.” I’m no expert on DSPLL technology, but it does seem rather impressive that such a wide range of high-precision frequencies can be generated from one crystal.
The ultra-low jitter spec of 95 fs typical (150 fs maximum) is valid over the entire operating ambient temperature range of -40°C to 85°C, and it is also valid over the entire frequency range. The first version of this article reported that the lowest jitter spec applies only to a limited portion of the frequency range, but a representative from Silicon Labs kindly pointed out that this was a misinterpretation of the information presented in the following plot.
Figure 6. Phase jitter vs. output frequency, from the datasheet (PDF).
SiLabs explained to us that they measure the clock jitter for more than 700 popular carrier frequencies to show that the device can generate ultra-low jitter clocks across its entire operating range. In Figure 6, the x-axis stops at 800 MHz simply because most common frequencies fall in this range.
An Evaluation Kit
If you would like to test the Si549 before incorporating it into a design, Silicon Labs has released the Universal Oscillator Evaluation Board, which is also referred to as the Si5xxUC-EVB (see image below).
Figure 8. The Si5xxUC-EVB evaluation kit. Courtesy of the user's guide (PDF).
Have you had a chance to use this new I2C user-programmable crystal oscillator (XO) from Silicon Labs, or its evaluation kit? If so, leave a comment and tell us about your experiences.