VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC8116

ATM/SONET/SDH 622/155Mb/s Transceiver

Mux/Demux with Integrated Clock Generation

Page 6

© VITESSE SEMICONDUCTOR CORPORATION

G52220-0, Rev 4.1

741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896

1/8/00

Clock Multiplier Unit

The VSC8116 uses an integrated phase-locked loop (PLL) for clock synthesis of the 622MHz high speed

clock used for serialization in the transmitter section. The PLL is comprised of a phase-frequency detector

(PFD), an integrating operation amplifier and a voltage controlled oscillator (VCO) configured in classic feed-

back system. The PFD compares the selected divided down version of the 622MHz VCO (CMUFREQSEL

selects divide-by ratios of 8 or 32, see Table 2) and the reference clock. The integrator provides a transfer func-

tion between input phase error and output voltage control. The VCO portion of the PLL is a voltage controlled

ring-oscillator with a center frequency of 622MHz.

The reactive elements of the integrator are located off-chip and are connected to the feedback loop of the

amplifier through the CP1, CP2, CN1 and CN2 pins. The configuration of these external surface mounted

capacitors is shown in Figure 6. Table 1 shows the recommended external capacitor values for the configurable

reference frequencies.

Good analog design practices should be applied to the board design for these external components. Tightly

controlled analog ground and power planes should be provided for the PLL portion of the circuitry. The dedi-

cated PLL power (VDDA) and ground (VSSA) pins should have quiet supply planes to minimize jitter genera-

tion within the clock synthesis unit. This is accomplished by either using a ferrite bead or a C-L-C choke (

π

filter) on the (VDDA) power pins. Note: Vitesse recommends a (

π filter) C-L-C choke over using a ferrite bead.

All ground planes should be tied together using multiple vias.

Table 1: Recommended External Capacitor Values

Reference

Frequency

[MHz]

Divide Ratio

CP

CN

Type

Size

Tol.

19.44

32

0.1

0.1

X7R

0603/0805

+/-10%

77.76

8

0.1

0.1

X7R

0603/0805

+/-10%