
68HC(9)12DG128 Rev 1.0
MOTOROLA
Appendix A: CGM Practical Aspects
379
Appendix A: CGM Practical Aspects
Appendix A: CGM Practical Aspects
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .379
A Few Hints For The CGM Crystal Oscillator Application. . . . . . . . .379
Practical Aspects For The PLL Usage . . . . . . . . . . . . . . . . . . . . . . .382
Printed Circuit Board Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . .387
Introduction
This sections provides useful and practical pieces of information
concerning the implementation of the CGM module.
A Few Hints For The CGM Crystal Oscillator Application
What Loading
Capacitors To
Choose
First, from small-signal analysis, it is known that relatively large values
for C1 and C2 have a positive impact on the phase margin. However, the
higher loading they represent decreases the loop gain. Alternatively,
small values for these capacitors will lead to higher open loop gain, but
as the frequency of oscillation approaches the parallel resonance, the
phase margin, and consequently the ability to start-up correctly, will
decrease. From this it is clear that relatively large capacitor values
(>33pF), are reserved for low frequency crystals in the MHz range.
NOTE:
Using the recommended loading capacitor CL value from the crystal
manufacturer is a good starting point. Taking into account unavoidable
strays, this equates to about (CL-2pF).
1-cgmpa