
Design Considerations
MOTOROLA WIRELESS
RF PRODUCT DEVICE DATA
MC145170-2
4–21
4 Design Considerations
4.1 Crystal Oscillator Considerations
The following options may be considered to provide a reference frequency to Motorola's CMOS frequency
synthesizers.
4.1.1 Use of a Hybrid Crystal Oscillator
Commercially available temperature-compensated crystal oscillators (TCXOs) or crystal-controlled data
clock oscillators provide very stable reference frequencies. An oscillator capable of CMOS logic levels at
the output may be direct or dc coupled to OSC
in
. If the oscillator does not have CMOS logic levels on the
outputs, capacitive or ac coupling to OSC
in
may be used (see Figures 9 and 10).
For additional information about TCXOs, visit
motorola.com
on the world wide web.
4.1.2 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 20.
The crystal should be specified for a loading capacitance (C
L
) which does not exceed 20 pF when used
at the highest operating frequencies listed in Table 6,
Loop Specifications
. Larger C
L
values are possible
for lower frequencies. Assuming R1 = 0
, the shunt load capacitance (C
L
) presented across the crystal
can be estimated to be:
where
C
in
= 5.0 pF (see Figure 21)
C
out
= 6.0 pF (see Figure 21)
C
a
= 1.0 pF (see Figure 21)
C1 and C2 = external capacitors (see Figure 21)
C
stray
= the total equivalent external circuit stray
capacitance appearing across the crystal
terminals
The oscillator can be “trimmed” on-frequency by making a portion or all of C1 variable. The crystal and
associated components 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. Circuit stray capacitance can
also be handled by adding the appropriate stray value to the values for C
in
and C
out
. For this approach, the
term C
stray
becomes 0 in the above expression for C
L
.
A good design practice is to pick a small value for C1, such as 5 to 10 pF. Next, C2 is calculated. C1 < C2
results in a more robust circuit for start-up and is more tolerant of crystal parameter variations.
Power is dissipated in the effective series resistance of the crystal, R
e
, in Figure 22. The maximum drive
level specified by the crystal manufacturer represents the maximum stress that the crystal can withstand
without damage or excessive shift in operating frequency. R1 in Figure 20. limits the drive level. The use
of R1 is not necessary in most cases.
CL
Cin
Cout
+
---CinCout
Ca
Cstray
C1
C2
C2
¥
+
C1
+
+
+
=
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.