mc145170-2 Freescale Semiconductor, Inc, mc145170-2 Datasheet - Page 18

• Manufacturer Part Number: mc145170-2
• Description: Pll Frequency Synthesizer With Serial Interface
• Manufacturer: Freescale Semiconductor, Inc

Design Considerations
4
4.1
The following options may be considered to provide a reference frequency to our CMOS frequency
synthesizers.
4.1.1
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
outputs, capacitive or ac coupling to OSC
For additional information about TCXOs, visit www.freescale.com on the world wide web.
4.1.2
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
The crystal should be specified for a loading capacitance (C
at the highest operating frequencies listed in
for lower frequencies. Assuming R1 = 0 Ω, the shunt load capacitance (C
can be estimated to be:
where
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
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
term C
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.
18
stray
Design Considerations
C
C
C
C1 and C2 = external capacitors (see
C
in
out
a
stray
Crystal Oscillator Considerations
= 1.0 pF (see
Use of a Hybrid Crystal Oscillator
Use of the On-Chip Oscillator Circuitry
= 5.0 pF (see
becomes 0 in the above expression for C
= 6.0 pF (see
= the total equivalent external circuit stray capacitance appearing across the crystal terminals
Figure
Figure
Figure
C L
21)
21)
21)
=
--------------------------- -
C in
MC145170-2 Technical Data, Rev. 5
C in C out
+
in
C out
may be used (see Figures
Table
Figure
Figure
in
. If the oscillator does not have CMOS logic levels on the
+
6, Loop Specifications. Larger C
C a
20.
21)
L
.
+
C stray
L
) which does not exceed 20 pF when used
+
C1
-------------------- -
C1
9
×
+
in
in
and 10).
C2
C2
L
and OSC
and C
) presented across the crystal
out
. For this approach, the
out
Freescale Semiconductor
L
pins to minimize
values are possible