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SL74HC4046D Datasheet(PDF) 8 Page - System Logic Semiconductor |
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SL74HC4046D Datasheet(HTML) 8 Page - System Logic Semiconductor |
8 / 12 page SL74HC4046 System Logic Semiconductor SLS 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. 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 across the capacitor charges up to Vref of the comparators, the oscillator logic flips the capacitor which causes the mirror to change the opposite side of the capacitor. The output from the internal logic is then taken to VCO output (Pin4). 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. 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 output can either directly feed the COMPIN of the phase comparators or feed external prescalers (counters) to enable frequency synthesis. Figure 5. Logic Diagram for VCO |
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