ML145146
LANSDALE Semiconductor, Inc.
DESIGN CONSIDERATIONS
CRYSTAL OSCILLATOR CONSIDERATIONS
The following options may be considered to provide a refer-
ence frequency to Motorola’s CMOS frequency synthesizers.
The most desirable is discussed first.
Use of a Hybrid Crystal Oscillator
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
may be direct or DC coupled to OSCin. In general, the highest
frequency capability is obtained utilizing a direct coupled
square wave having a rail–to–rail (VDD to VSS) voltage
swing. If the oscillator does not have CMOS logic levels on the
outputs, capacitive or AC coupling of OSCin may be used.
OSCout, an unbuffered output, should be left floating.
For additional information about TCXOs and data clock
oscillators, please consult the latest version of the
eem
Electronic Engineers Master Catalog,
the
Gold Book,
or simi-
lar publications.
Design an Off–Chip Reference
The user may design and off–chip crystal oscillator using
ICs specifically developed for crystal oscillator applications,
such as the ML12061 MECL device. The reference signal from
the MECL device is AC coupled to OSCin. For large ampli-
tude signals (standard CMOS logic levels), DC coupling is
used. OSCout, an unbuffered output, should be left floating. In
general, the highest frequency capability is obtained with a
direct–coupled square wave having rail–to–rail voltage swing.
Use of the On–Chip Oscillator Circuitry
The on–chip amplifier (a digital inverter) along with an
appropriate 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 8.
For VDD = 5.0 V the crystal should be specified for a load-
,
ing capacitance. CL, which does not exceed 32 pF for frequen-
cies 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 variations stray in
IC input/output capacitance, and realistic CL values. The shunt
load capacitance, CL, presented across the crystal can be esti-
mated to be:
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 OSCin and
OSCout pins to minimize distortion, stray capacitance, stray
inductance, and startup stabilization time. In some cases, stray
capacitance should be added to the value for Cin and Cout.
Power is dissipated in the effective series resistance of the
crystal, Re. In Figure 10 The drive level specified by the crys-
tal manufacturer is the maximum stress that a crystal can with-
stand without damaging or excessive shift in frequency. R1 in
Figure 8 limits the drive level. The use of R1 may not be nec-
essary in some cases (i.e. R1 = 0 ohms).
To verify that the maximum DC supply voltage does not
overdrive the crystal, monitor the output frequency as a func-
tion of voltage at OSCout. (care should be taken to minimize
loading.) the frequency should increase very slightly as the dc
supply voltage is increased. An overdriven crystal will decrease
in frequency or become unstable with an increase in supply
voltage. The operating supply voltage must be reduced or R1
must be increased in value if the overdrive condition exists.
The user should note that the oscillator start–up time is propor-
tional to the value of R1.
Through the process of supplying crystals for use with
CMOS inverters, many crystal manufacturers have developed
expertise in CMOS oscillator design with crystals. Discussions
with such manufacturers can prove very helpful. See Table 1.
where
Cin = 5.0pF (See Figure 9)
Cout = 6.0pF (See Figure 9)
Ca = 1.0pF (See Figure 9)
CO = the crystal’s holder capacitance (See Figure 10)
C1 and C2 = external capacitors (See Figure 8)
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