74vhc4046 National Semiconductor Corporation, 74vhc4046 Datasheet - Page 9

• Manufacturer Part Number: 74vhc4046
• Description: Cmos Phase Lock Loop
• Manufacturer: National Semiconductor Corporation

Detailed Circuit Description
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
FIGURE 3 Logic Diagram for Phase Comparator I and the Common Input Circuit for All Three Comparators
FIGURE 4 Typical Phase Comparator I Waveforms
(Continued)
9
3
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
The outputs of these comparators are essentially standard
74VHC voltage outputs (Comparator II is TRI-STATE )
TL F 11675 – 20
TL F 11675-19