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74VHC4046MX Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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74VHC4046MX Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Detailed Circuit Description (Continued) the capacitor once the capacitor charges up to the thresh- old of the schmitt trigger the oscillator logic flips the capaci- tor over and causes the mirror to charge the opposite side of the capacitor The output from the internal logic is then taken to pin 4 The input to the VCO is a very high impedance CMOS input and so it 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 a source follower transistor is provided This transistor can be used by con- necting a resistor to ground and its drain output will follow the VCO input signal An inhibit signal is provided to allow disabling of the VCO and the source follower This is useful if the internal VCO is not being used A logic high on inhibit disables the VCO and source follower The output of the VCO is a standard high speed CMOS output with an equivalent LSTTL fanout of 10 The VCO output is approximately a square wave This output can ei- ther directly feed the comparator input of the phase compar- ators or feed external prescalers (counters) to enable fre- quency synthesis PHASE COMPARATORS All three phase comparators share two inputs Signal In and Comparator In The Signal In has a special DC bias network that enables AC coupling of input signals If the signals are not AC coupled then this input requires logic levels the same as standard 74VHC The Comparator input is a stan- dard digital input Both input structures are shown in Figure 3 The outputs of these comparators are essentially standard 74VHC voltage outputs (Comparator II is TRI-STATE) TLF11675-19 FIGURE 3 Logic Diagram for Phase Comparator I and the Common Input Circuit for All Three Comparators TLF11675 – 20 FIGURE 4 Typical Phase Comparator I Waveforms 9 |
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