ZL30409 Data Sheet
16
Zarlink Semiconductor Inc.
Figure 9 - 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 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 39 pF capacitor may be increased to 56 pF, 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: 20 MHz
Tolerance: As required
Oscillation Mode: Fundamental
Resonance Mode: Parallel
Load Capacitance: 32 pF
Maximum Series Resistance: 35
Ω
Approximate Drive Level: 1 mW
e.g., R1B23B32-20.0MHz
(20 ppm absolute,
±6 ppm 0C to 50C, 32 pF, 25 Ω)
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 2 s. Each mode change sequence could account for a phase change as large as 350 ns. Thus, the
accumulated phase change could be as large as 3.5 us, and, the overall MTIE could be as large as 3.5 us.
OSCo
56pF
1MΩ
39pF
3-50pF
20MHz
ZL30409
OSCi
100Ω
1uH
1uH inductor: may improve stability and is optional
Phase
hold
0.05ppm 2s× 100ns==
Phase
state
50ns 200ns 250ns=+=
Phase
10
10 250ns 100ns+()× 3.5us==
