Re: High speed capacitors

["Peter Marek" <peter.marek@marekmicro.de>]

High speed decoupling is considered to be black magic. There are lots of
different approaches out there, some work in practice, other do not.
Some just raise production costs.

The high speed caps recommended in the PCI spec are intended to produce an
AC/high frequency short between the Power supplies and GND. This is
neccessary to make the power supply pins lokk like an AC reference plane for
the high speed signals. This helps to maintain a sound characteristic
impedance acrosss the connector which in turn increases signal integrity on
PCI.

Long story short: "high speed caps" usually means surface mount ceramic
caps, no leaded caps or even electrolytic caps. The type of ceramic material
(COG/NPO/X7R) is the area where things start to be magic and religion. I
would recommend using X7R, since the higher damping factor at higher
frequencies (beyong signal bandwidth) eliminates unwanted high frequency
noise (it doesn't damp the signal). That's my personal religion.

Regards,



Peter Marek
General Director
MarekMicro GmbH
Kropfersrichter Str. 6-8
D-92237 Sulzbach-Rosenberg
Germany
Phone: 049 - 9661 - 908 - 210
Fax:      049 - 9661 - 908 - 100
----- Original Message -----
From: Eric de Jong <info@simultime.nl>
To: <pci-sig@znyx.com>
Sent: Tuesday, June 06, 2000 4:27 PM
Subject: High speed capacitors


> >From the book PCI hardware & software:
>
> "However, the PCI bus specification does state that twelve 0.01 uF high
> speed capacitors must be evenly spaced between the 3.3 volt and ground
> layers."
>
> What does the spec. mean with "high speed" capacitors?
> e.g. the philips databook has the folowing capacitor ranges:
> Class 1 NPO , Class 2 X7R, Y5V, Z5U
>
> types of dielectric EIA/IEC, CECC:
> COG/NPO/CG,
> X7R/ 2R1,
> Y5V/2F4,
> Z5U/2E6
>
> Who can shine a bit of light on this matter?
>
> Eric de Jong.