ZL30409/DDA ZARLINK [Zarlink Semiconductor Inc], ZL30409/DDA Datasheet - Page 16

• Manufacturer Part Number: ZL30409/DDA
• Description: T1/E1 System Synchronizer with Stratum 3 Holdover
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

The accuracy of a crystal oscillator depends on the crystal tolerance as well as the load capacitance tolerance.
Typically, for a 20MHz crystal specified with a 32pF load capacitance, each 1pF change in load capacitance
contributes approximately 9ppm to the frequency deviation. Consequently, capacitor tolerances, and stray
capacitances have a major effect on the accuracy of the oscillator frequency.
The trimmer capacitor shown in Figure 9 may be used to compensate for capacitive effects. If accuracy is not a
concern, then the trimmer may be removed, the 39pF capacitor may be increased to 56pF, and a wider tolerance
crystal may be substituted.
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. The crystal
specification is as follows.
Frequency:
Tolerance:
Oscillation Mode:
Resonance Mode:
Load Capacitance:
Maximum Series Resistance:
Approximate Drive Level:
e.g., R1B23B32-20.0MHz
(20ppm absolute,
TIE Correction (using PCCi)
When Primary Holdover Mode is entered for short time periods, TIE correction should not be enabled. This will
prevent unwanted accumulated phase change between the input and output.
For instance, 10 Normal to Holdover to Normal mode change sequences occur, and in each case Holdover was
entered for 2s. Each mode change sequence could account for a phase change as large as 350ns. Thus, the
accumulated phase change could be as large as 3.5us, and, the overall MTIE could be as large as 3.5us.
•
•
0.05ppm is the accuracy of Holdover Mode
50ns is the maximum phase continuity of the ZL30409 from Normal Mode to Holdover Mode
±
6ppm 0C to 50C, 32pF, 25
20MHz
As required
Fundamental
Parallel
32pF
35
1mW
1uH inductor: may improve stability and is optional
Ω
Phase hold
Phase state
Phase 10
Figure 9 - Crystal Oscillator Circuit
Ω
OSCo
OSCi
1MΩ
)
ZL30409
=
Zarlink Semiconductor Inc.
=
=
10
ZL30409
0.05ppm 2s
50ns
×
(
56pF
250ns
16
+
100Ω
200ns
20MHz
×
+
100ns
=
=
39pF
250ns
100ns
)
=
1uH
3.5us
3-50pF
Data Sheet