gs9076 Gennum Corporation, gs9076 Datasheet - Page 18

• Manufacturer Part Number: gs9076
• Description: Sd-sdi Automatic Reclocker With Dual Differential Outputs
• Manufacturer: Gennum Corporation

4.4 Frequency Acquisition Loop — The Phase-Frequency Detector
4.5 Phase Acquisition Loop — The Phase Detector
An external crystal of 14.140 MHz is used as a reference to keep the VCO centered
at the last known data rate. This allows the device to achieve a fast synchronous
lock, especially in cases where a known data rate is interrupted. The crystal
reference is also used to clock internal timers and counters. To keep the optimal
performance of the reclocker over all operating conditions, the crystal frequency
must be 14.140 MHz, +/-50ppm. The GO1535 meets this specification and is
available from GENNUM.
The VCO is divided by a selected ratio which is dependant on the input data rate.
The resultant is then compared to the crystal frequency. If the divided VCO
frequency and the crystal frequency are within 1% of each other, the PLL is
considered to be locked to the input data rate.
The phase detector is a digital quadrature phase detector. It indicates whether the
input data is leading or lagging with respect to a clock that is in phase with the VCO
(I-clk) and a quadrature clock (Q-clk). When the phase acquisition loop (PA loop)
is locked, the input data transition is aligned to the falling edge of I-clk and the
output data is re-timed on the rising edge of I-clk. During high input jitter conditions
(>0.25UI), Q-clk will sample a different value than I-clk. In this condition, two extra
phase correction signals will be generated which instructs the charge pump to
create larger frequency corrections for the VCO.
OUTPUT DATA
Figure 4-2: Phase Detector Characteristics
When the PA loop is active, the crystal frequency and the incoming data rate are
compared. If the resultant is more that 2%, the PLL is considered to be unlocked
and the system jumps to the FA loop.
44617 - 1
WITH JITTER
INPUT DATA
RE-TIMED
q-clk
I-clk
i-PHASE ALIGNMENT
q-PHASE ALIGNMENT
January 2008
EDGE
EDGE
DATA RE-TIMING
EDGE
0.25UI
0.8UI
GS9076 Data Sheet
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