Freescale Semiconductor, Inc.
Appendix: CGM Practical Aspects
Theoretically speaking, nothing precludes the use of non-identical
values for C1 and C2. As this complicate a bit the management of the
final board device list, this is not recommended. However, if
asymmetrical capacitors are chosen, the value of C1 should be higher
than C2 (because the reflected loading is proportional to the square of
the impedance of C2).
21.3.2 DC Bias
Due to the nature of the translated ground Colpitts oscillator a DC
voltage bias is applied to the crystal.
Please contact the crystal manufacturer for specific DC bias conditions
and recommended capacitance value (if applicable).
21.3.3 What Is the Final Oscillation Frequency?
The exact calculation is not straightforward as it takes into account the
resonator characteristics and the loading capacitors values as well as
internal design parameters which can vary with Process Voltage
Temperature (PVT) conditions. Nevertheless, if L is the series
inductance, R is the series resistance, C is the series capacitance and
Cc the parallel capacitance of the crystal, we can then use the following
simplified equation:
1
2π
1
1
------
Fosc =
⋅
------- + ---------------------------------------------
||
LC L ⋅ (Cc + C1 C2)
C1=C2=Cl yields to
1
2π
1
1
------
Fosc =
⋅
------- + --------------------------------------
LC L ⋅ (Cc + Cl ⁄ 2)
21.3.4 How Do I Control The Peak to Peak Oscillation Amplitude?
The CGM oscillator is equipped with an Amplitude Limitation Control
loop which integrates the peak to peak ‘extal’ amplitude and in return
reduces the steady current of the transconductor device until a stable
Advance Information
388
68HC(9)12D60 — Rev 4.0
Appendix: CGM Practical Aspects
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