CRYSTAL OSCILLATOR CONSIDERATIONS
C
values. The shunt load capacitance, C , presented
L L
across the crystal can be estimated to be:
The following options may be considered to provide a ref-
erence frequency to Motorola’s CMOS frequency synthe-
sizers.
C C
C1 • C2
in out
C =
L
+ C + C +
a o
C
+ C
C1 + C2
in
out
where
Use of a Hybrid Crystal Oscillator
C
= 5 pF (see Figure 11)
= 6 pF (see Figure 11)
C = 1 pF (see Figure 11)
in
out
a
O
C
Commercially available temperature–compensated crystal
oscillators (TCXOs) or crystal–controlled data clock oscilla-
tors provide very stable reference frequencies. An oscillator
capable of sinking and sourcing 50 µA at CMOS logic levels
C
= the crystal’s holder capacitance
(see Figure 12)
C1 and C2 = external capacitors (see Figure 10)
may be direct or dc coupled to OSC . In general, the highest
frequency capability is obtained utilizing a direct–coupled
in
C
a
square wave having a rail–to–rail (V
swing. If the oscillator does not have CMOS logic levels on
the outputs, capacitive or ac coupling to OSC may be used.
to V ) voltage
DD
SS
in
C
C
out
in
OSC , an unbuffered output, should be left floating.
out
For additional information about TCXOs and data clock
oscillators, please consult the latest version of the eem Elec-
tronic Engineers Master Catalog, the Gold Book, or similar
publications.
Figure 11. Parasitic Capacitances of the Amplifier
R
L
C
S
S
S
Design an Off–Chip Reference
1
2
1
2
The user may design an off–chip crystal oscillator using
ICs specifically developed for crystal oscillator applications,
such as the MC12061 MECL device. The reference signal
C
O
from the MECL device is ac coupled to OSC . For large am-
plitude signals (standard CMOS logic levels), dc coupling is
in
R
X
e
e
2
1
used. OSC , an unbuffered output, should be left floating.
out
In general, the highest frequency capability is obtained with a
direct–coupled square wave having rail–to–rail voltage
swing.
NOTE: Values are supplied by crystal manufacturer
(parallel resonant crystal).
Figure 12. Equivalent Crystal Networks
Use of the On–Chip Oscillator Circuitry
The on–chip amplifier (a digital inverter) along with an ap-
propriate crystal may be used to provide a reference source
frequency. A fundamental mode crystal, parallel resonant at
the desired operating frequency, should be connected as
shown in Figure 10.
The oscillator can be “trimmed” on–frequency by making a
portion or all of C1 variable. The crystal and associated com-
ponents must be located as close as possible to the OSC
in
and OSC
out
pins to minimize distortion, stray capacitance,
stray inductance, and startup stabilization time. In some
cases, stray capacitance should be added to the value for C
in
and C
.
out
Power is dissipated in the effective series resistance of the
FREQUENCY
R
f
SYNTHESIZER
crystal, R , in Figure 12. The drive level specified by the crys-
e
tal manufacturer is the maximum stress that a crystal can
withstand without damage or excessive shift in frequency. R1
in Figure 10 limits the drive level. The use of R1 may not be
necessary in some cases (i.e., R1 = 0 Ω).
To verify that the maximum dc supply voltage does not
overdrive the crystal, monitor the output frequency as a func-
OSC
C1
OSC
out
in
R1*
C2
tion of voltage at OSC . (Care should be taken to minimize
out
loading.) The frequency should increase very slightly as the
dc supply voltage is increased. An overdriven crystal will de-
crease in frequency or become unstable with an increase in
supply voltage. The operating supply voltage must be re-
duced or R1 must be increased in value if the overdriven
condition exists. The user should note that the oscillator
start–up time is proportional to the value of R1.
* May be deleted in certain cases. See text.
Figure 10. Pierce Crystal Oscillator Circuit
For V
DD
= 5.0 V, the crystal should be specified for a load-
ing capacitance, C , which does not exceed 32 pF for fre-
L
quencies to approximately 8.0 MHz, 20 pF for frequencies in
the area of 8.0 to 15 MHz, and 10 pF for higher frequencies.
These are guidelines that provide a reasonable compromise
between IC capacitance, drive capability, swamping varia-
tions in stray and IC input/output capacitance, and realistic
Through the process of supplying crystals for use with
CMOS inverters, many crystal manufacturers have devel-
oped expertise in CMOS oscillator design with crystals. Dis-
cussions with such manufacturers can prove very helpful
(see Table 1).
MOTOROLA
MC145151–2 through MC145158–2
29
MOTOROLA [ MOTOROLA ]
