ZL30101

Data Sheet

23

Zarlink Semiconductor Inc.

6.2

Master Clock

The ZL30101 can use either a clock or crystal as the master timing source. Zarlink application note ZLAN-68 lists a

number of applicable oscillators and crystals that can be used with the ZL30101.

6.2.1

Clock Oscillator

When selecting a clock oscillator, numerous parameters must be considered. This includes absolute frequency,

frequency change over temperature, phase noise, output rise and fall times, output levels and duty cycle.

The output clock should be connected directly (not AC coupled) to the OSCi input of the ZL30101, and the OSCo

output should be left open as shown in Figure 12.

Figure 12 - Clock Oscillator Circuit

6.2.2

Crystal Oscillator

Alternatively, a crystal oscillator may be used. A complete oscillator circuit made up of a crystal, resistor and

capacitors is shown in Figure 13. The Telcordia GR1244-CORE Stratum 3 requirements for holdover stability and

freerun accuracy may not be met with this crystal oscillator circuit.

The accuracy of a crystal oscillator depends on the crystal tolerance as well as the load capacitance tolerance.

Typically, for a 20 MHz crystal specified with a 32 pF load capacitance, each 1 pF change in load capacitance

contributes approximately 9 ppm to the frequency deviation. Consequently, capacitor tolerances and stray

capacitances have a major effect on the accuracy of the oscillator frequency.

The crystal should be a fundamental mode type - not an overtone. The fundamental mode crystal permits a simpler

oscillator circuit with no additional filter components and is less likely to generate spurious responses. A typical

crystal oscillator specification and circuit is shown in Table 6 and Figure 13 respectively.

1

Frequency

20 MHz

2

Tolerance

4.6 ppm

3

Rise & fall time

< 10 ns

4

Duty cycle

40% to 60%

Table 5 - Typical Clock Oscillator Specification

+3.3 V

20 MHz OUT

GND

0.1 µF

+3.3 V

OSCo

ZL30101

OSCi

No Connection