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EL4585CS Datasheet(PDF) 11 Page - Intersil Corporation |
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EL4585CS Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 15 page 11 FN7175.4 September 3, 2009 . PCB Layout Considerations It is highly recommended that power and ground planes be used in layout. The oscillator and filter sections constitute a feedback loop and thus care must be taken to avoid any feedback signal influencing the oscillator except at the control input. The entire oscillator/filter section should be surrounded by copper ground to prevent unwanted influences from nearby signals. Use separate paths for analog and digital supplies, keeping the analog (oscillator section) as short and free from spurious signals as possible. Careful attention must be paid to correct bypassing. Keep lead lengths short and place bypass caps as close to the supply pins as possible. If laying out a PCB to use discrete components for the VCO section, care must be taken to avoid parasitic capacitance at the OSC pins 3 and 5, and FILTER out (pin 7). Remove ground and power plane copper above and below these traces to avoid making a capacitive connection to them. It is also recommended to enclose the oscillator section within a shielded cage to reduce external influences on the VCO, as they tend to be very sensitive to “hand waving” influences, the LC variety being more sensitive than crystal controlled oscillators. In general, the higher the operating frequency, the more important these considerations are. Self contained VCXO or VCO modules are already mounted in a shielding cage and therefore do not require as much consideration in layout. Many crystal manufacturers publish informative literature regarding use and layout of oscillators which should be helpful. LC LOOP FILTER COMPONENTS (APPROXIMATE) FREQUENCY (MHz) R2 (k Ω) R3 (k Ω) C3 (µF) C4 (µF) 26.602 100 39 0.01 0.001 27.0 100 39 0.01 0.001 29.5 100 43 0.01 0.001 35.468 100 51 0.01 0.001 21.476 100 30 0.01 0.001 24.546 100 36 0.01 0.001 28.636 100 43 0.01 0.001 FIGURE 12. TYPICAL LOOP FILTER XTAL LOOP FILTER COMPONENTS (APPROXIMATE) FREQUENCY (MHz) R2 (k Ω) R3 (M Ω) C3 (pF) C4 (pF) 26.602 100 4.3 68 6.8 27.0 100 4.3 68 6.8 29.5 100 4.3 68 6.8 35.468 100 4.3 68 6.8 21.476 100 4.3 68 6.8 24.546 100 4.3 68 6.8 28.636 100 4.3 68 6.8 EL4585 |
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