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| 型號： | MC145173DW |
| 廠商： | MOTOROLA INC |
| 元件分類： | XO, clock |
| 英文描述： | Dual-Band PLL Frequency Synthesizer with ADC and Frequency Counter |
| 中文描述： | PLL FREQUENCY SYNTHESIZER, 130 MHz, PDSO24 |
| 封裝： | SOG-24 |
| 文件頁數(shù)： | 19/33頁 |
| 文件大小： | 277K |
| 代理商： | MC145173DW |
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MC145173
19
MOTOROLA
CRYSTAL OSCILLATOR CONSIDERATIONS
The following options may be considered to provide a ref-
erence frequency to Motorola’s CMOS frequency synthesiz-
ers.
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 CMOS logic levels at the output may be direct or
dc coupled to OSCin. If the oscillator does not have CMOS
logic levels on the outputs, capacitive or ac coupling to
OSCin must be used. (See Figure 12.)
For additional information about TCXOs and data clock
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
The user may design an off–chip crystal oscillator using
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
OSCin. (See Figure 12.) For large amplitude signals (stan-
dard CMOS logic levels), dc coupling may be used.
Use of the On–Chip Oscillator Circuitry
The on–chip amplifier (a digital inverter) along with an ap-
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 23.
The crystal should be specified for a loading capacitance
(CL) which does not exceed 20 pF when used at the highest
operating frequency. Larger CL values are possible for lower
frequencies. Assuming R1 = 0
, the shunt load capacitance
(CL) presented across the crystal can be estimated to be:
CinCout
Cin+ Cout
CL =
C1
C2
C1 + C2
+ Ca + Cstray +
where
Cin =6 pF (see Figure 24)
Cout =6 pF (see Figure 24)
Ca =1 pF (see Figure 24)
C1 and C2 =external capacitors (see Figure 23)
Cstray =the total equivalent external circuit stray capaci–
tance appearing across the crystal terminals
(see Figure 24)
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. Circuit stray
capacitance can also be handled by adding the appropriate
stray value to the values for Cin and Cout. For this approach,
the term Cstray becomes 0 in the above expression for CL.
Power is dissipated in the effective series resistance of the
crystal, Re, in Figure 25. 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 23 limits the drive
level. The use of R1 is not necessary in most cases.
To verify that the maximum dc supply voltage does not
cause the crystal to be overdriven, monitor the output
frequency at the OSCout pin. An active probe should be used
to minimize loading. The frequency should increase very
slightly as the dc supply voltage is increased. An overdriven
crystal decreases in frequency or becomes unstable with an
increase in supply voltage. The operating supply voltage
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.
Through the process of supplying crystals for use with CMOS in-
verters, many crystal manufacturers have developed expertise in
CMOS oscillator design with crystals. Discussions with such man-
ufacturers can prove very helpful. (See Table 5)
R1*
C2
C1
FREQUENCY
SYNTHESIZER
OSCout
OSCin
Rf
*May be needed in certain cases. See text.
Figure 23. Pierce Crystal Oscillator Circuit
Figure 24. Parasitic Capacitances of the Amplifier
and Cstray
Cin
Cout
Ca
OSCin
OSCout
Cstray
Figure 25. Equivalent Crystal Networks
Note: Values are supplied by crystal manufacturer
(parallel resonant crystal).
2
1
2
1
2
1
RS
LS
CS
Re
Xe
CO
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