BT261 datasheet(41/56 Pages) ETC

41 / 56 page

Brooktree®

L261_H

APPLICATION

INFORMATION

Phase Locking With the 74HC4046

Applications that call for multiple resolutions with clock rates in the 9–18 MHz

range may employ the circuit (Figure 10) shown with a 74HC4046 and a passive

loop ﬁlter. Residual jitter can be less than 30 ns, which is subpixel but not as good

as can be achieved with LC-tuned VCOs or VCXOs. These have narrower tuning

ranges. The loop parameters were obtained from a design program provided for the

74HC4046 with the Bt261’s phase comparator emulating the type 2 phase detector

with 4

π/VCC gain (Note 1). The type 1 second-order loop is designed for 10 dB

ripple suppression in closed-loop response with a critically damped response and

a tracking range of 4 percent, which is adequate for most stable sources (e.g.,

broadcast, camera, and videodisc). Nonstandard video sources (e.g., heterodyne

VCRs or ﬂoppy disk ESP cameras) may require a preﬁlter PLL for adequate track-

ing. Third-order loop ﬁlters may yield faster loop response rolloff, which reduces

jitter but can prolong loop settling time.

The same circuit can be extended to 26 MHz for genlocking to higher raster fre-

quency sources (such as noninterlaced VGA) or adapted to crystal-based operation

up to 10 MHz. The VCO timing-capacitor value must be reduced to 40 pF for the

former higher frequency case or replaced with a parallel resonant crystal in the lat-

ter case. Loop ﬁlter values will vary slightly in conjunction with modiﬁcation of

R1 and R2 VCO gain-setting resistors.

In general, VCO capture range must be limited to one octave to prevent harmon-

ic lockup. With the low-input bias current of the 74HC4046, passive ﬁlters are sim-

pler than active ﬁlters and can better accommodate large-value polarized

capacitors. Bipolar VC(X)O implementations that use the MC4024, 74LS624, or

MN3106 require active buffers with bias currents less than 10

µA to limit control

voltage droop across the 64

µs interval between phase-comparison pulses.

Low-absorption Mylar or Teﬂon capacitors in the loop ﬁlter can minimize jitter

caused by capacitance modulation.

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