GS2975A Datasheet, PDF (17/27 Pages) Gennum Corporation – HD-LINX® III Multi-Rate SDI Automatic Reclocker with Dual Differential Outputs

The S-PLL offers several advantages over the linear PLL. The Loop Bandwidth of an
S-PLL is independent of the transition density of the input data. Pseudo-random data has
a transition density of 0.5 verses a pathological signal which has a transition density of
0.05. The loop bandwidth of a linear PLL will change proportionally with this change in
transition density. With an S-PLL, the loop bandwidth is defined by the jitter at the data
input. This translates to infinite loop bandwidth with a zero jitter input signal. This
allows the loop to correct for small variations in the input jitter quickly, resulting in very
low output jitter. The loop bandwidth of the GS2975A’s PLL is defined at 0.2UI of input
jitter.
The PLL consists of two acquisition loops. First is the Frequency Acquisition (FA) loop.
This loop is active when the device is not locked and is used to achieve lock to the
supported data rates. Second is the phase acquisition (PA) loop. Once locked, the PA loop
tracks the incoming data and makes phased corrections to produce a re-clocked output.
4.2 VCO
The internal VCO of the GS2975A is an LC oscillator. It is trimmed at the time of
manufacture to capture all data rates over temperature and operation voltage ranges.
Integrated into the VCO is a series of programmable dividers used to achieve all serial
data rates, as well as additional dividers for the frequency acquisition loop.
4.3 Charge Pump
During frequency acquisition, the charge pump has two states, “pump-up” and
“pump-down,” which is produced by a leading or lagging phase difference between the
input and the VCO frequency.
During phase acquisition, there are two levels of “pump-up” and two levels of “pump
down” produced for leading and lagging phase difference between the input and VCO
frequency. This is to allow for greater precision of VCO control.
The charge pump produces these signals by holding the integrated frequency
information on the external loop-filter capacitor, C_LF. The instantaneous frequency
information is the result of the current flowing through an internal resistor connected to
the loop-filter capacitor.
GS2975A HD-LINX® III Multi-Rate SDI Automatic
Reclocker with Dual Differential Outputs
Data Sheet
41487 - 4
April 2009
17 of 27