CX28344 Conexant, CX28344 Datasheet - Page 182

• Manufacturer Part Number: CX28344
• Description: (CX2834x) Dual / Triple / Quad / Hex / Octal - Enhanced DS3 / E3 Framer
• Manufacturer: Conexant

Dejitter Circuit Analysis
A-6
When designing the circuit, there are several criteria to keep in mind:
1. The loop gain
2. Natural Frequency (
3. Damping factor ( )
The whole premise of the dejitter circuit as implemented in the CX2834x device is to
provide a clock-to-data edge relationship that is stable and consistent.
It should be noted that when using the CX28344 with an LIU, there is a receive clock
that is available even when there is no signal coming into the LIU. Therefore, the
system will be locked to this clock and the PLL circuitry will not be hunting. The
system should be optimized for step response inputs. The implication is the system
will be limited by the ability of the PLL circuitry in the LIU.
A high loop gain is desired so the loop will remain in lock with the incoming
clocks. A higher loop gain allows a greater change in frequency before too much
phase error is introduced. The gain is derived from the phase detector and the
VCO.
The phase detector is set within the framer itself at approximately 3/2 v/rad. The
VCO is selected by the designer. The gain from the VCO is in the form 2
rad/v-s.
The hold-in range is approximately plus/minus the product of the phase detector
and the VCO or
±3 f / v (/s)
The natural frequency is determined by the passive components R1, R2, and C1.
This is also the characteristic frequency of the loop where the -3 dB corner is
located. The tradeoff is that with a high natural frequency more low frequency
jitter is allowed to pass, but the system will react faster to step inputs.
The damping factor should be chosen to allow the fastest excursion of the system
output without the system becoming unstable. When the system frequency
approaches the natural frequency, if underdamped, there is a tendancy for the
system to become unstable, i.e. lock will be lost. Overshoot of the output signal
will not settle to a steady state and ring. If overdamped, the system will have
trouble obtaining lock, i.e. the system will not track. The problem with
overdamped systems is getting to the steady state condition.
The goal is to allow some overshoot without allowing the system to be become
completely unstable or to ring too long. In general, the ideal damping factor is
0.707.
Mindspeed Technologies™
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CX28342/3/4/6/8 Data Sheet
28348-DSH-001-B
f / v