mm74hc4046 Fairchild Semiconductor, mm74hc4046 Datasheet - Page 9

• Manufacturer Part Number: mm74hc4046
• Description: Cmos Phase Lock Loop
• Manufacturer: Fairchild Semiconductor

Detailed Circuit Description
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 connecting 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
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 74HC. The Comparator input
FIGURE 3. Logic Diagram for Phase Comparator I and the common input circuit for all three comparators
FIGURE 2. Logic Diagram for VCO
(Continued)
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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
either directly feed the comparator input of the phase com-
parators or feed external prescalers (counters) to enable
frequency synthesis.
is a standard digital input. Both input structures are shown
in Figure 3.
The outputs of these comparators are essentially standard
74HC voltage outputs. (Comparator II is 3-STATE.)
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