The HC297 and CD74HCT297 are high-speed silicon gate
CMOS devices that are pin-compatible with low power Schottky
These devices are designed to provide a simple, cost-effective
solution to high-accuracy, digital, phase-locked-loop applications.
They contain all the necessary circuits, with the
exception of the divide-by-N counter, to build first-order
Both EXCLUSIVE-OR (XORPD) and edge-controlled phase
detectors (ECPD) are provided for maximum flexibility. The
input signals for the EXCLUSIVE-OR phase detector must
have a 50% duty factor to obtain the maximum lock-range.
Proper partitioning of the loop function, with many of the building
blocks external to the package, makes it easy for the
designer to incorporate ripple cancellation (see Figure 2) or to
cascade to higher order phase-locked-loops.
The length of the up/down K-counter is digitally programmable
according to the K-counter function table. With A, B, C and D
all LOW, the K-counter is disabled. With A HIGH and B, C and
D LOW, the K-counter is only three stages long, which widens
the bandwidth or capture range and shortens the lock time of
the loop. When A, B, C and D are all programmed HIGH, the
K-counter becomes seventeen stages long, which narrows
the bandwidth or capture range and lengthens the lock time.
Real-time control of loop bandwidth by manipulating the A to
D inputs can maximize the overall performance of the digital
The HC297 and CD74HCT297 can perform the classic first
order phase-locked-loop function without using analog components.
The accuracy of the digital phase-locked-loop
(DPLL) is not affected by VCC and temperature variations but
depends solely on accuracies of the K-clock and loop propagation
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