AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 19

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
AN2321/D
PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
The resonator Q and capacitors C1 and C2 form the resonant circuit. C1 and C2
represent the external capacitors and any stray capacitance in parallel. A
crystal or ceramic resonator has the small signal equivalent circuit shown in
Figure
18.
R
L
C
Q
C
0
Figure 18. Crystal Equivalent Circuit
R is called the series resistance, L and C are called the motional or series
inductance and capacitance, respectively. C
is the shunt capacitance, it
0
represents the sum of the low-frequency parallel plate capacitance of the
resonator and the stray capacitance of the crystal holder. Any additional stray
capacitance between the OSC1 and OSC2 pins should be included into this
value.
Values for R , L , C and C
for a particular crystal are specified in the crystal
0
manufacturer’s data sheet. In order to measure these values, the manufacturer
must apply a signal to the crystal, i.e. the values are obtained at a particular
level of power dissipation in the crystal. However, at the start-up of the
oscillator, the only signal across the crystal is due to thermal (Johnson) noise,
so the power dissipation in the crystal is extremely low. It is known that the
effective value of R may increase as the power dissipated in the crystal
decreases to low levels. The maximum value of R is therefore estimated by the
crystal manufacturer.
Since the value of the R
, C1 , and C2 not only depends on the inverter
0
characteristic of the MCU but also on the characteristic of the external crystal
or ceramic resonator, the exact component configuration should be deduced
from manufacturer’s data sheets.
MOTOROLA
Designing for Board Level Electromagnetic Compatibility
19
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