MC912DG128A Motorola, MC912DG128A Datasheet - Page 378

• Manufacturer Part Number: MC912DG128A
• Description: Microcontrollers
• Manufacturer: Motorola

Appendix B: CGM Practical Aspects
DC Bias
What Is the Final
Oscillation
Frequency?
How Do I Control
The Peak to Peak
Oscillation
Amplitude?
MC68HC912DT128A Rev 2.0
378
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).
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).
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:
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
quiescent point is reached. Controlling this final peak to peak amplitude
can be performed by three means:
1. Reducing the values of C1 and C2. This decreases the loading so
that the necessary gm value required to sustain oscillation can be
Appendix B: CGM Practical Aspects
Fosc
Fosc
=
=
----- -
2
1
C1=C2=Cl yields to
----- -
2
1
------- -
LC
1
------- -
LC
1
+
+
--------------------------------------------- -
L
-------------------------------------- -
L
Cc
Cc
+
1
1
+
C1 C2
Cl 2
MOTOROLA
2-cgmpa