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Si5013
12
Rev. 1.6
4. Functional Description
The Si5013 integrates a high-speed limiting amplifier
with a multi-rate CDR unit. No external reference clock
is required for clock and data recovery. The limiting
amplifier magnifies very low-level input data signals so
that
accurate
clock
and
data
recovery
can
be
performed. The CDR uses Silicon Laboratories DSPLL
technology to recover a clock synchronous to the input
data stream. The recovered clock retimes the incoming
data, and both are output synchronously via current-
mode logic (CML) drivers. Silicon Laboratories’ DSPLL
technology ensures superior jitter performance while
eliminating the need for external loop filter components
found
in
traditional
phase-locked
loop
(PLL)
implementations.
The limiting amplifier includes a control input for
adjusting the data slicing level and provides a loss-of-
signal level alarm output. The CDR includes a bit error
rate performance monitor which signals a high bit error
rate condition (associated with excessive incoming
jitter) relative to an externally adjustable bit error rate
threshold.
The optional reference clock minimizes the CDR
acquisition time and provides a stable reference for
maintaining the output clock when locking to a reference
is desired.
4.1. Limiting Amplifier
The limiting amplifier accepts the low-level signal output
from a transimpedance amplifier (TIA). The low-level
signal is amplified to a usable level for the CDR unit. The
minimum input swing requirement is specified in
Table 2on page 7. Larger input amplitudes (up to the maximum
input swing specified in
Table 2) are accommodated
without
degradation
of
performance.
The
limiting
amplifier ensures optimal data slicing by using a digital
dc offset cancellation technique to remove any dc bias
introduced by the amplification stage.
4.2. DSPLL
DSPLL
technology
to
maintain
superior
jitter
performance while eliminating the need for external loop
filter
components
found
in
traditional
PLL
implementations. This is achieved using a digital signal
processing (DSP) algorithm to replace the loop filter
commonly found in analog PLL designs. This algorithm
processes the phase detector error term and generates
a digital control value to adjust the frequency of the
voltage-controlled oscillator (VCO). This technology
enables CDR with far less jitter than is generated using
traditional methods, and it eliminates performance
degradation caused by external component aging. In
addition, because external loop filter components are
not required, sensitive noise entry points are eliminated,
thus making the DSPLL less susceptible to board-level
noise
sources
and
making
SONET/SDH
jitter
compliance easier to attain in the application.
4.3. Multi-Rate Operation
The Si5013 supports clock and data recovery for OC-
12/3 and STM-4/1 data streams.
Multi-rate operation is achieved by configuring the
device to divide down the output of the VCO to the
desired data rate. The divide factor is configured by the
RATESEL
pin.
The
RATESEL
configuration
and
associated data rates are given in
Table 7.4.4. Operation Without an External Refer-
ence
The Si5013 can perform clock and data recovery
without
an
external
reference
clock.
Tying
the
REFCLK+ input to VDD and the REFCLK– input to
GND configures the device to operate without an
external reference clock. Clock recovery is achieved by
monitoring the timing quality of the incoming data
relative to the VCO frequency. Lock is maintained by
continuously monitoring the incoming data timing quality
and adjusting the VCO accordingly. Details of the lock
detection and the lock-to-reference functions while in
this mode are described in their respective sections
below.
Note: Without an external reference the acquisition of data is
dependent solely on the data itself and typically
requires more time to acquire lock than when a refer-
ence is applied.
4.5. Operation With an External Reference
The Si5013 can also perform clock and data recovery
with an external reference. The device’s optional
external reference clock centers the DSPLL, minimizes
the acquisition time, and maintains a stable output clock
(CLKOUT) when lock-to-reference (LTR) is asserted.
When the reference clock is present, the Si5013 uses
the reference clock to center the VCO output frequency
so that clock and data is recovered from the input data
stream. The device self configures for operation with
one
of
three
reference
clock
frequencies.
This
Table 7. Multi-Rate Configuration
RATESEL
SONET/SDH
1622.08 Mbps
0155.52 Mbps