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1.12 Crystal Oscillator Consideration
Introduction
STDM110
1-46
Samsung ASIC
1.12.2.1 On-Chip Oscillator
In most cases, the on-chip amplifier with the appropriate external components
provides the most economical solution to the clocking problem. Exceptions may
arise in server environments when frequency tolerances are tighter than about
0.01%.
The external components that commonly used for CMOS gate oscillator are a
positive reactance (normal crystal oscillator), two capacitors, C1 and C2, and
two resistor Rf and Rx as shown in the figure below.
Figure 1-19.
CMOS Oscillator
1.12.2.2 Crystal Specifications
Specifications for an appropriate crystal are not very critical, unless the
frequency is. Any fundamental-mode crystal of medium or better quality can be
used.
We are often asked what maximum crystal resistance should be specified. The
best answer to that question is the lower the better, but use what is available.
The crystal resistance will have some effect on start-up time and steady-state
amplitude, but not so much that it can’t be compensated for by appropriate
selection of the capacitance, C1 and C2.
Similar questions are asked about specifications of load capacitance and shunt
capacitance. The best advice we can give is to understand what these
parameters mean and how they affect the operation of the circuit (that being the
purpose of this application note), and then to decide for yourself if such
specifications are meaningful in your frequency tolerances are tighter than about
0.1%.
Part of the problem is that crystal manufacturers are accustomed to talking
“ppm” tolerances with radio engineers and simply won’t take your order until
you’ve filled out their list of frequency tolerance requirements, both for yourself
and to the crystal manufacturer. Don’t pay for 0.003% crystals if your actual
frequency tolerance is 1%.
C1
C2
Rx
Rf
PADA
PADY
Feedback
Amplifier
Inside of a Chip