MC912DG128ACPVE Datasheet, PDF (203/478 Pages) Freescale Semiconductor, Inc – Upward compatible with M68HC11 instruction set

Freescale Semiconductor, Inc.
Oscillator
MC68HC912Dx128C Colpitts Oscillator Specification
13.4.2.2 Gain Margin
The Gain Margin of the oscillator indicates the amount the gain of the
oscillator can vary while maintaining oscillation. Specifically, Gain
Margin is:
Gain Margin = MIN(gain/minimum required gain, maximum allowed
gain/gain)
Just like NRM, Gain Margin may be dominated by either too much or too
little oscillator gain and an increase in gain may not increase Gain
Margin. Gain Margin is theoretically related to NRM since the maximum
allowed gain is (approximately) inversely proportional to ESR, and the
minimum required gain is (approximately) proportional to ESR, leaving
Gain Margin (approximately) inversely proportional to ESR.
The preferred method for specifying the oscillator, given a set of load
capacitor values, is to determine the maximum allowed while
maintaining a worst-case Gain Margin of 2. Since Gain Margin is
proportional to ESR, this means the empirically measured NRM at the
worst-case point would be approximately twice the maximum allowed
ESR. However, since typical NRM is likely to be higher and most
measurement techniques do not account for ALC effects, actual NRM
measurements are likely to be much higher.
13.4.2.3 Optimizing Component Values
The maximum ESR possible (given a worst-case Gain Margin of 2) is not
the optimum operating point. In some cases, the frequency accuracy of
the oscillator is important and in other cases satisfying the traditional
NRM measurement technique is important.
If frequency accuracy is important, the total load capacitance (the
combination of the load capacitors and their associated parasitics)
should be equal (or close to equal) to the rated load capacitance of the
resonator. Provided the resultant load capacitors yield a maximum
allowed ESR greater than the maximum ESR of the crystal (while
maintaining the worst case Gain Margin of 2), this is an acceptable
operating point. If the maximum allowed ESR is not high enough, the
closest possible components with high enough ESR should be chosen.
MC68HC912DT128A — Rev 4.0
MOTOROLA
Oscillator
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Technical Data
203