Silicon Labs has two types of LVPECL drivers for Timing products. Please see below for the recommended LVPECL terminations for these two types of LVPECL drivers.

• Type1: Si534x/7x/8x/9x

These LVPECL outputs only support AC coupling but not DC coupling. Customers must use AC coupling for these LVPECL output terminations as shown below:

The termination at the receiver depends on the receiver’s requirements. The only termination requirement from Si534x/7x/8x/9x is seeing an equivalent differential load impedance of 100ohm for signal integrity.

• Type2: other Timing products

These LVPECL outputs support both DC and AC coupling. Please refer to the below information.

1. LVPECL DC coupled termination

1. LVPECL AC coupled termination

For AC coupling, please note we use different Rb values in different products as shown below:

Recommended Rb value for different Timing products and power supplies

The purpose of this KB is a reminder that different Timing products at Silicon Labs may have different LVPECL structures and therefore different termination requirements. Please refer to the product datasheets, reference manuals and application notes for more detailed termination information when starting designs with Silicon Labs Timing products.

If an input clock is a spread spectrum signal, the spreading function is achieved by adding jitter to the clock.  For low loop BW devices, this jitter will be attenuated and effectively removed.  A problem that can arise in that there can be false OOF and LOL assertions due to the frequency of the input clock changing over time. This is caused by the dithering of the clock input's spreading function.  In order to avoid these false OOF and LOL assertions, the thresholds for OOF and LOL need to be loosened to accommodate the spread. 

As an example, consider a 100 MHz spread spectrum clock that has a spectrum spread of ± 5 kHz.  Noting that 5 kHz is a change of 50 ppm for a 100 MHz clock, the OOF and LOL assertion thresholds should be set to 50 ppm or greater.