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SL74HC4046D 参数 Datasheet PDF下载

SL74HC4046D图片预览
型号: SL74HC4046D
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: 12 页 / 136 K
品牌: SLS [ SYSTEM LOGIC SEMICONDUCTOR ]
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SL74HC4046  
the capacitor. The output from the internal logic is then  
taken to VCO output (Pin4).  
DETAILED CIRCUIT DESCRIPTION  
Voltage Controlled Oscillator/Demodulator Output  
The VCO requires two or three external  
components to operate. These are R1, R2, C1. Resistor  
R1 and Capacitor C1 are selected to determine the  
center frequency of the VCO (see typical performance  
curves Figure 12). R2 can be used to set the offset  
frequency with 0 volts at VCO input. For example, if R2  
is decreased, the offset frequency is increased. If R2 is  
omitted the VCO range is from 0 Hz. By increasing the  
value of R2 the lock range of the PLL is increased and  
the gain (volts/Hz) is decreased. Thus, for a narrow  
lock range, large swings on the VCO input will cause  
less frequency variation.  
The input to the VCO is a very high impedance  
CMOS input and thus will not load down the loop filter,  
easing the filters design. In order to make signals at the  
VCO input accessible without degrading the loop  
performance, the VCO input voltage is buffered  
through a unity gain Op-amp, to Demod Output. This  
Op-amp can drive loads of 50K ohms or more and  
provides no loading effects to the VCO input voltage  
(see Figure 10).  
An inhibit input is provided to allow disabling  
of the VCO and all Op-amps (see Figure 5). This is  
useful if the internal VCO is not being used. A logic  
high on inhibit disables the VCO and all Op-amps,  
minimizing standby power consumption.  
Internally, the resistors set a current in a current  
mirror, as shown in Figure 5. The mirrored current  
drives one side of the capacitor. Once the voltage  
The output of the VCO is a standard high speed  
CMOS output with an equivalent LS-TTL fan out of 10.  
The VCO output is approximately a square wave. This  
across the capacitor charges up to V of the  
ref  
comparators, the oscillator logic flips the capacitor  
which causes the mirror to change the opposite side of  
output can either directly feed the COMP of the  
IN  
phase comparators or feed external prescalers  
(counters) to enable frequency synthesis.  
Figure 5. Logic Diagram for VCO  
System Logic  
Semiconductor  
SLS