DS125RT410 Datasheet, PDF (10/42 Pages) Texas Instruments – DS125RT410 Low Power Multi-Rate Quad Channel Retimer

DS125RT410
SNLS459 – APRIL 2011
www.ti.com
The CTLE is a four-stage variable boost high-gain amplifier with a quasi-high-pass characteristic. Each of the
four stages can be set to provide various amounts of high-frequency boost with the overall transfer function of
the CTLE set by the cascade of all four sections. The high-frequency boost of each CTLE stage is variable. The
optimum boost for each stage is one that causes the transfer function magnitude of the transmission channel and
the CTLE in cascade to be flat over a band of frequencies extending up to half the data rate which is commonly
referred to as the “Nyquist” frequency. In normal operation, the DS125RT410 sets the boost of the CTLE
automatically to approximate the optimum cascaded response.
Clock and Data Recovery
The DS125RT410 performs its clock and data recovery function by detecting the bit transitions in the incoming
data stream and locking its internal VCO to the clock represented by the mean arrival times of these bit
transitions. This process produces a recovered clock with greatly reduced jitter at jitter frequencies outside the
bandwidth of the CDR Phase-Locked Loop (PLL). This is the primary benefit of using the DS125RT410 in a
system. It significantly reduces the jitter present in the data stream, in effect resetting the jitter budget for the
system.
The DS125RT410 uses the 25 MHz reference to determine the coarse tuning setting for its internal VCO. On
power-up, on CDR reset, and when the DS125RT410 loses lock and cannot re-acquire lock after four attempts,
the 25 MHz reference is used to calibrate the VCO frequency. The required VCO frequency is set by using the
rate/subrate settings (see Table 1) or by manually setting the PPM count and divide ratio. To calibrate the VCO
frequency, the DS125RT410 searches through the available VCO coarse tuning settings and counts the divided
VCO frequency using the 25 MHz reference as a clock source. The VCO coarse tuning setting which provides
the VCO frequency closest to the required frequency is stored, and this coarse tuning setting is used for
subsequent operation. This produces a fast, robust phase lock to the input signal.
Output Driver
The DS125RT410 is commonly used in applications where lossy transmission media exist both at the input and
the output of the DS125RT410. The CTLE compensates for lossy transmission media at the input to the
DS125RT410. The output de-emphasis compensates for the lossy transmission media at the output of the
DS125RT410.
When there is a transition in the output data stream, the output differential voltage reaches its configured
maximum value within the configured rise/fall time of the output driver. Following this, the differential voltage
rapidly falls off until it reaches the configured VOD level minus the configured de-emphasis level. This
accentuates the high-frequency components of the output driver signal at the expense of the low-frequency
components. This pre-distorted signal, with its high-frequency components emphasized relative to its low-
frequency components, travels down the dispersive transmission media at the output of the DS125RT410 with
less inter-symbol interference than an undistorted signal would exhibit.
The output driver is capable of driving variable output voltages with variable amounts of analog de-emphasis.
The output voltage and de-emphasis level can be configured by writing registers over the SMBus. The
DS125RT410 cannot determine independently the appropriate output voltage or de-emphasis setting, so the user
is responsible for configuring these parameters. They can be set for each channel independently.
An idealized transmit waveform with analog de-emphasis applied is shown in Figure 2.
1.0
0.5
0.0
-0.5
-1.0
0 1 2 3 4 5 6 7 8 9 10
TIME (UI)
Figure 2. Idealized De-Emphasis Waveform
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