The GTH/GTY receiver reference clock, however, does not need an exact
relationship with the bit rate of the incoming SDI signals. This is because the clock
and data recovery (CDR) unit in the GTH/GTY receiver can receive bit rates that are up
to ±1,250 ppm (≤ 6.6 Gb/s) or ±200 ppm (> 8.0 Gb/s) away from the nominal bit rate as
set by the reference clock frequency. This allows the receiver reference clock to be
generated by local oscillators that have no exact frequency relationship to the incoming
SDI signal. The GTH/GTY receiver generates a recovered clock that is frequency-locked to
the incoming SDI bit rate. These clocks are output as rxusrclk and rxusrclk2 ports of the
Receiver User Clocking Network Helper Block from the GTH/GTY Wizard IP and are driven by
BUFG_GT. As is described in more detail later, rxusrclk and rxusrclk2 are true
recovered clocks when receiving all SDI line rates except when receiving SD-SDI signals.