LV Intel
®
Pentium
®
III Processor 512K Dual Processor Platform
Design Guide
37
5.5.2.2
Filter Specification
The function of the filter is to protect the PLL from external noise through low-pass attenuation.
The low-pass specification, with input at V
TT
and output measured across the capacitor, is as
follows:
•
< 0.2 dB gain in pass band
•
< 0.5 dB attenuation in pass band (see DC drop in next set of requirements)
•
> 34 dB attenuation from 1 MHz to 66 MHz
•
> 28 dB attenuation from 66 MHz to core frequency
The filter specification is shown in Figure 22.
Other requirements:
•
Use shielded type inductor to minimize magnetic pickup
•
Filter should support DC current > 30 mA
•
DC voltage drop from V
TT
to PLL1 should be < 60 mV, which in practice implies series
R < 2
Ω
. It also means pass band (from DC to 1Hz) attenuation is < 0.5 dB for VCC = 1.1 V,
and < 0.35 dB for VCC = 1.5 V.
Figure 22. PLL Power Low Pass Filter Response
