• Meet or exceed the reference clock characteristics as specified in the Ult device data
sheets.
• Meet or exceed the reference clock characteristics as specified in the standard for which the
GTM transceiver provides physical layer support.
• Fulfill the oscillator vendor’s requirement regarding power supply, board layout, and noise
specification.
• Provide a dedicated point-to-point connection between the oscillator and GTM transceiver
Dual clock input pins.
• Keep impedance discontinuities on the differential transmission lines to a minimum
(impedance discontinuities generate jitter).
Reference Clock Interface
LVDS
The following figure shows how an LVDS oscillator is connected to a reference clock input of a
GTM transceiver.
Figure 53:
Interfacing an LVDS Oscillator to the GTM Transceiver Reference Clock
Input
LVDS Oscillator
0.01 µF
0.01 µF
GTM Transceiver
Reference Clock
Input Buffer
Internal to
Ult Device
X20934-053118
LVPECL
The following figure shows how an LVPECL oscillator is connected to a reference clock input of a
GTM transceiver.
Chapter 5: Board Design Guidelines
UG581 (v1.0) January 4, 2019
Virtex Ult GTM Transceivers
123