參數(shù)資料
型號(hào)： MC145225
廠商： Motorola, Inc.
英文描述： Dual PLL Frequency Synthesizers With DACs and Voltage Multipliers(帶DACs和電壓乘法器的雙PLL頻率合成器)
中文描述： 雙鎖相環(huán)頻率合成器與DAC和電壓倍增器（帶數(shù)模轉(zhuǎn)換器和電壓乘法器的雙鎖相環(huán)頻率合成器）
文件頁(yè)數(shù)： 29/71頁(yè)
文件大?。?/td> 906K
代理商： MC145225
MC145225 MC145230
29
MOTOROLA RF/IF DEVICE DATA
7. APPLICATIONS INFORMATION
7A. CRYSTAL OSCILLATOR CONSIDERATIONS
The oscillator/reference circuit may be connected to
operate in either of two configurations. With the Mode pin
placed “high” and bit C6 programmed to 1, the
oscillator/reference circuit of the MC145181 will accept an
external reference input. The external reference signal
should be capacitive, connected to Osce with Oscb left
floating. Commercially available temperature compensated
crystal oscillators (TCXOs) or crystal–controlled data clock
oscillators provide a very stable reference frequency. For
additional information about TCXOs and data clock
oscillators, please consult the Electronic Engineers Master
Catalog, internet web page, or similar publication/service.
The on–chip Colpitts reference oscillator can be selected
by either tying the Mode pin low or by programming the C6 bit
to zero when Mode is high. The oscillator may be operated in
either the fundamental mode, as show by Figure 22, or as an
overtone oscillator. The “kick start” feature ensures reduced
“stalling” of hard–starting crystals.
Crystal Resonators
The equivalent circuit of a crystal resonator most
commonly used is shown in Figure 23. The crystal itself is a
specially cut (usually AT for overtone operation) block of
quartz. The dimensions, (shape, thickness, length, and
width) determine the operating characteristics of the crystal.
When deformed and allowed to return naturally to its resting
shape, it is observed to oscillate. This oscillation has the
typical characteristics of a damped oscillation and an
equivalent electrical signal can be found on the surface of the
crystal. In addition, if an equivalent electrical signal is applied
to the crystal, it will be observed to oscillate. The equivalent
values for Rs, Ls, Cs, and Co can be used to predict the
operation of the crystal when used as an electronic oscillator.
Due to the series/parallel arrangement of the equivalent
components, the crystal exhibits two resonances. The first,
sometimes just called resonance, is the series resonance of
the Rs, Cs, Ls branch. The other, sometimes called the
anti–resonance, is the parallel resonance including Co. For
the series resonance the formula is
fs =
2
π
Ls Cs
1
.
For parallel resonance, the formula is
fp =
2
π
1
Ls Cs Co
Co + Cs
.
As can be seen from this equation, the anti–resonant
frequency is higher than the series resonant frequency. The
ratio between the resonant and anti–resonant frequency can
be found using the formula
f
f
2 (Co + Cs)
where
f =
fs – fp
and
fs + fp
f =
Cs
=
.
2
By exploiting this characteristic, the crystal oscillator
frequency can be tuned slightly. If a capacitor is connected in
series with the crystal operating in the resonance mode, the
frequency will shift upward. If a capacitance is added in
parallel with a crystal operating in an anti–resonant mode, the
frequency will be shifted down.
Figure 22. Fundamental Mode Oscillator Circuit
0
X1
0
C3
C2
C1
R1
R2
Q1
M1
+V
Oscb
Osce
Frequency Synthesizer
I1
200/800
μ
A
M2
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