When terminating clock drivers keep in mind these guidelines for optimum phase noise and jitter performance:
- Transmission line theory should be followed for good impedance matching to prevent reflections.
- Clock drivers should be presented with the proper loads.
- LVDS outputs are current drivers and require a closed current loop.
- HCSL drivers are switched current outputs and
require a DC path to ground through 50-Ω
termination.
- LVPECL outputs are open emitter and require a DC path to ground.
- Receivers should be presented with a signal
biased to their specified DC bias level (common-mode
voltage) for proper operation. Some receivers have
self-biasing inputs that automatically bias to the proper
voltage level; in this case, the signal should normally be
AC coupled.
It is possible to drive a non-LVPECL or non-LVDS
receiver with a LVDS or LVPECL driver as long as the above
guidelines are followed. Check the data sheet of the receiver or
input being driven to determine the best termination and coupling
method to be sure the receiver is biased at the optimum DC voltage
(common-mode voltage).