PCB LAYOUT AND POWER SYSTEM BYPASS

RECOMMENDATIONS

Circuit board layout and stack-up for the CLC021 should be

designed to provide noise-free power to the device. Good

layout practice also will separate high frequency or high level

inputs and outputs to minimize unwanted stray noise pickup,

feedback and interference. Power system performance may

be greatly improved by using thin dielectrics (4 to 10 mils) for

power/ground sandwiches. This increases the intrinsic ca-

pacitance of the PCB power system which improves power

supply filtering, especially at high frequencies, and makes

the value and placement of external bypass capacitors less

critical. External bypass capacitors should include both RF

ceramic and tantalum electrolytic types. RF capacitors may

use values in the range 0.01 µF to 0.1 µF. Tantalum capaci-

tors may be in the range 2.2 µF to 10 µF. Voltage rating for

tantalum capacitors should be at least 5X the power supply

voltage being used. It is recommended practice to use two

vias at each power pin of the CLC021 as well as all RF

bypass capacitor terminals. Dual vias reduce the intercon-

nect inductance by up to half, thereby reducing interconnect

inductance and extending the effective frequency range of

the bypass components.

The outer layers of the PCB may be flooded with additional

V

isolation as well as increase the intrinsic capacitance of the

power supply plane system. Naturally, to be effective, these

planes must be tied to the V

quent intervals with vias. Frequent via placement also im-

proves signal integrity on signal transmission lines by pro-

viding short paths for image currents which reduces signal

distortion. The planes should be pulled back from all trans-

mission lines and component mounting pads a distance

equal to the width of the widest transmission line or the

thickness of the dielectric separating the transmission line

from the internal power or ground plane(s) whichever is

greater. Doing so minimizes effects on transmission line

impedances and reduces unwanted parasitic capacitances

at component mounting pads.

In especially noisy power supply environments, such as is

often the case when using switching power supplies, sepa-

rate filtering may be used at the CLC021’s VCO and output

driver power pins. The CLC021 was designed for this situa-

tion. The digital section, VCO and output driver power supply

feeds are independent (see pinout description table and

pinout drawing for details). Supply filtering may take the form

of L-section or pi-section, L-C filters in series with these V

DD

inputs. Such filters are available in a single package from

several manufacturers. Despite being independent feeds, all

device power supplies should be applied simultaneously as

from a common source. The CLC021 is free from power

supply latch-up caused by circuit-induced delays between

the device’s three separate power feed systems.

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FIGURE 12. Jitter Plots

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