mc145220 Freescale Semiconductor, Inc, mc145220 Datasheet - Page 17

• Manufacturer Part Number: mc145220
• Description: Dual Frequency Synthesizer
• Manufacturer: Freescale Semiconductor, Inc

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 REF in . If the oscillator does not have CMOS
logic levels on the outputs, capacitive or ac coupling to REF in
must 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
REF in . (See Figure 8.) For large amplitude signals (standard
CMOS logic levels), dc coupling may be 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 approximately 20 pF when used
near the highest operating frequency of the MC145220.
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)
either a portion or all of C1 variable. The crystal and associ-
ated components must be located as close as possible to the
REF in and REF out pins to minimize distortion, stray ca-
pacitance, 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 zero in the above
expression for C L .
crystal, R e , in Figure 20. The maximum drive level specified
by the crystal manufacturer represents the maximum stress
MOTOROLA
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
Power is dissipated in the effective series resistance of the
C stray = the total equivalent external circuit stray
C out = 6 pF (see Figure 19)
C in = 5 pF (see Figure 19)
C L = C in C out
C a = 1 pF (see Figure 19)
C in + C out
capacitance appearing across the crystal
terminals
+ C a + C stray + C1 C2
C1 + C2
DESIGN CONSIDERATIONS
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 (f R ) at Output A as a function of supply voltage.
(REF out is not used because loading impacts the oscillator.)
The frequency should increase very slightly as the dc supply
voltage is increased. An overdriven crystal decreases in fre-
quency or becomes unstable with an increase in supply volt-
age. The operating supply voltage must be reduced or R1
must be increased in value if the overdriven condition exists.
Note that the oscillator 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 4.
To verify that the maximum dc supply voltage does not
Through the process of supplying crystals for use with
REF in
1
NOTE: Values are supplied by crystal manufacturer
Figure 18. Pierce Crystal Oscillator Circuit
Figure 19. Parasitic Capacitances of the
Figure 20. Equivalent Crystal Networks
(parallel resonant crystal).
* May be needed in certain cases. See text.
2
C1
C in
Amplifier and C stray
REF in
1
1
C stray
R f
C2
Ca
R e
R S
R1*
C out
X e
SYNTHESIZER
FREQUENCY
REF out
C O
L S
2
MC145220
C S
REF out
2
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