
CRYSTAL OSCILLATOR CONSIDERATIONS
INTRODUCTION TO STD80/STDM80
STD80/STDM80
1-30
SEC ASIC
Oscillator Design Considerations
ASIC designers have a number of options for clocking
the system. The main decision is whether to use the
“on-chip” oscillator or an external oscillator. If the
choice is to use the on-chip oscillator, what kinds of
external components are to use an external oscillator,
what type of oscillator would it be
The decisions have to be based on both economic and
technical requirements. In this section we will discuss
some of the factors that should be considered.
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 1-31.
CMOS Oscillator
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 capacitances, 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%.
OSCILLATION FREQUENCY
The oscillation frequency is determined 99.5% by the
crystal and up to about 0.5% by the circuit external to
the crystal.
The on-chip amplifier has little effect on the frequency,
which is as it should be, since the amplifier
parameterizes temperature and process dependent.
The influence of the on-chip amplifier on the frequency
is by means of its input and output (pin-to-ground)
capacitances, which parallel C1 and C2, and the
PADA-to-PADY (pin-to-pin) capacitance, which
parallels the crystal. The input and pin-to-pin
capacitances are about 7pF each.
Internal phase deviations capacitance of 25 to 30pF.
These deviations from the ideal have less effect in the
positive reactance oscillator (with the inverting
amplifier) than in a comparable series resonant
oscillator (with the non-inverting amplifier) for two
reasons: first, the effect of the output capacitor;
second, the positive reactance oscillator is less
sensitive, frequency-wise, to such phase errors.
C1
C2
Rx
Rf
PADA
PADY
Feedback
Amplifier
Inside of a Chip