MT9040
Data Sheet
MT9040
OSCi
20 MHz
1 MΩ
56 pF
39 pF
3-50 pF
OSCo
100 Ω
1 uH
1 uH inductor: may improve stability and is optional
Figure 6 - 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 6 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
As required
Fundamental
Parallel
32 pF
Tolerance:
Oscillation Mode:
Resonance Mode:
Load Capacitance:
Maximum Series Resistance:
Approximate Drive Level:
e.g., R1B23B32-20.0MHz
35 Ω
1 mW
(20 ppm absolute, ±6 ppm 0C to 50C, 32 pF, 25 Ω)
11
Zarlink Semiconductor Inc.
XILINX [ XILINX, INC ]
