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74HC4046 参数 Datasheet PDF下载

74HC4046图片预览
型号: 74HC4046
PDF下载: 下载PDF文件 查看货源
内容描述: 锁相环 [Phase-Locked Loop]
分类和应用:
文件页数/大小: 16 页 / 293 K
品牌: ONSEMI [ ONSEMI ]
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MC74HC4046A  
Phase Comparators  
All three phase comparators have two inputs, SIG and  
outputs of these comparators are essentially standard 74HC  
outputs (comparator 2 is TRI–STATEABLE). In normal  
IN  
COMP . The SIG and COMP have a special DC bias  
operation V  
and ground voltage levels are fed to the loop  
IN  
IN  
IN  
CC  
network that enables AC coupling of input signals. If the  
signals are not AC coupled, standard 74HC input levels are  
required. Both input structures are shown in Figure 6. The  
filter. This differs from some phase detectors which supply  
a current to the loop filter and should be considered in the  
design. (The MC14046 also provides a voltage).  
V
CC  
V
CC  
SIG  
IN  
14  
PC2  
OUT  
13  
V
CC  
COMP  
3
IN  
PCP  
1
OUT  
PC3  
OUT  
15  
PC1  
2
OUT  
Figure 6. Logic Diagram for Phase Comparators  
Phase Comparator 1  
two input signals must be in phase. When the input  
frequencyisf ,theVCOinputmustbeV andthephase  
detector inputs must be 180 degrees out of phase.  
This comparator is a simple XOR gate similar to the  
74HC86. Its operation is similar to an overdriven balanced  
modulator. To maximize lock range the input frequencies  
must have a 50% duty cycle. Typical input and output  
waveforms are shown in Figure 7. The output of the phase  
detector feeds the loop filter which averages the output  
voltage. The frequency range upon which the PLL will lock  
onto if initially out of lock is defined as the capture range.  
The capture range for phase detector 1 is dependent on the  
loop filter design. The capture range can be as large as the  
lock range, which is equal to the VCO frequency range.  
To see how the detector operates, refer to Figure 7. When  
two square wave signals are applied to this comparator, an  
output waveform (whose duty cycle is dependent on the  
phase difference between the two signals) results. As the  
phase difference increases, the output duty cycle increases  
and the voltage after the loop filter increases. In order to  
achieve lock when the PLL input frequency increases, the  
VCO input voltage must increase and the phase difference  
max CC  
SIG  
IN  
COMP  
IN  
PC1  
OUT  
V
CC  
VCO  
IN  
GND  
Figure 7. Typical Waveforms for PLL Using  
Phase Comparator 1  
The XOR is more susceptible to locking onto harmonics  
of the SIG than the digital phase detector 2. For instance,  
IN  
a signal 2 times the VCO frequency results in the same  
output duty cycle as a signal equal to the VCO frequency.  
The difference is that the output frequency of the 2f example  
is twice that of the other example. The loop filter and VCO  
range should be designed to prevent locking on to  
harmonics.  
between COMP and SIG will increase. At an input  
IN  
IN  
frequency equal to f  
, the VCO input is at 0 V. This  
min  
requiresthe phase detector output to be grounded; hence, the  
http://onsemi.com  
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