MC145170-1 Motorola, MC145170-1 Datasheet - Page 14

• Manufacturer Part Number: MC145170-1
• Description: PLL FREQUENCY SYNTHESIZER WITH SERIAL INTERFACE
• Manufacturer: Motorola

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4
CRYSTAL OSCILLATOR CONSIDERATIONS
erence frequency to Motorola’s CMOS frequency synthe-
sizers.
Use of a Hybrid Crystal Oscillator
oscillators (TCXOs) or crystal–controlled data clock oscilla-
tors 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 Figure 8).
oscillators, please consult the latest version of the eem Elec-
tronic Engineers Master Catalog, the Gold Book, or similar
publications.
Design an Off–Chip Reference
discrete transistors or ICs specifically developed for crystal
oscillator applications, such as the MC12061 MECL device.
The reference signal from the MECL device is ac coupled to
OSC in (see Figure 18). For large amplitude signals (standard
CMOS logic levels), dc coupling is used.
Use of the On–Chip Oscillator Circuitry
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 18.
(C L ) which does not exceed 20 pF when used at the highest
operating frequencies listed in the Loop Specifications
table. Larger C L values are possible for lower frequencies.
Assuming R1 = 0 , the shunt load capacitance (C L ) pres-
ented across the crystal can be estimated to be:
where
C1 and C2 =external capacitors (see Figure 18)
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. 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 .
crystal, R e , in Figure 20. The maximum drive level specified
MC145170–1
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The following options may be considered to provide a ref-
Commercially available temperature–compensated crystal
For additional information about TCXOs and data clock
The user may design an off–chip crystal oscillator using
The on–chip amplifier (a digital inverter) along with an ap-
The crystal should be specified for a loading capacitance
The oscillator can be “trimmed” on–frequency by making a
Power is dissipated in the effective series resistance of the
C stray = the total equivalent external circuit stray capaci–
C out = 6 pF (see Figure 19)
C L =
C in = 5 pF (see Figure 19)
C a = 1 pF (see Figure 19)
tance appearing across the crystal terminals
DESIGN CONSIDERATIONS
C in + C out
C in C out
+ C a + C stray +
C1 + C2
C1 C2
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by the crystal manufacturer represents the maximum stress
that the crystal can withstand without damage or excessive
shift in operating frequency. R1 in Figure 18 limits the drive
level. The use of R1 is not necessary in most cases.
cause the crystal to be overdriven, monitor the output fre-
quency at the REF out pin (OSC out is not used because load-
ing impacts the oscillator). The frequency should increase
very slightly as the dc supply voltage is increased. An over-
driven crystal decreases in frequency or becomes unstable
with an increase in supply voltage. The operating supply volt-
age must be reduced or R1 must be increased in value if the
overdriven condition exists. The user should note that the os-
cillator start–up time is proportional to the value of R1.
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 2).
To verify that the maximum dc supply voltage does not
Through the process of supplying crystals for use with
1
Figure 19. Parasitic Capacitances of the Amplifier
OSC in
NOTE: Values are supplied by crystal manufacturer
Figure 18. Pierce Crystal Oscillator Circuit
Figure 20. Equivalent Crystal Networks
* May be needed in certain cases. See text.
(parallel resonant crystal).
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C1
C in
OSC in
and C stray
1
1
C stray
R f
C2
C a
R S
R1*
R e
SYNTHESIZER
X e
C out
FREQUENCY
OSC out
C O
L S
MOTOROLA
2
C S
OSC out
2