Datasheet

Oscillator
Technical Data MC68HC912DT128A — Rev 4.0
202 Oscillator MOTOROLA
yield an unacceptable NRM for a worst-case situation (the slope of the
NRM vs. capacitance curve is very steep, indicating severe sensitivity to
small variations). If the NRM optimization happened to be performed on
a best-case sample set, there could be unexpected sensitivity at worst-
case.
NRM measurement techniques can also generate misleading results
when applied to Automatic Level Control (ALC) style oscillator circuits
such as the D60x/Dx128x. Many NRM methods slowly increase series
resistance until oscillation stops. ALC-style oscillators reduce the gain of
the oscillator circuit after start-up to reduce current, so if the oscillator
tends to have more gain than optimum it will be more tolerant of
additional resistance after start-up than it will during the start-up process.
This means that NRM figures may be optimistic unless the method
verifies the NRM value by attempting to start the oscillator with the
additional resistance in-place. Worse, this phenomenon exaggerates
the difference between best- and worst-case NRM curves.
Negative Resistance Margin vs. Capacitance
10
100
1000
68564739332722181513108
Capacitance
Negative Resistance Margin
WCS
TY P
TY P
BCS
Frees
cale Semiconductor,
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Freescale Semiconductor, Inc.
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