[SI-LIST] Re: PI filter
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, "SI-List" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 28 Mar 2006 09:40:15 -0700
If you're referring to the pi filters use on video cards
for PCs...
I think this is a throw back from old emissions problems.
The pi-filter was used (originally) to reduce the high
frequency components from early interface ICs and poor montor cable
design.
(Hence also the addition of a ferrite on the cable shield).
Perhaps someone from the PC industry can comment on the=20
"state of the art" of video card design.
Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Pager/Short Message: 3032042974@xxxxxxxx
Email: charles.grasso@xxxxxxxxxxxx;
Email Alternate: chasgrasso@xxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Ingraham
Sent: Tuesday, March 28, 2006 9:28 AM
To: SI-List
Subject: [SI-LIST] Re: PI filter
> Does anyone know why do we use PI filter for RGB and TV signal.
> In my opinion, the front capacitor(which the signal arrive first) is
not
> very necessary.
I'm guessing you're confused because you think the filter is being
driven by
a voltage source, and wondering, why does a Pi filter even exist?
Passive filter characteristics depend on their source and load
impedances.
Passive filters are almost never driven by an ideal voltage source (zero
Thevenin equivalent resistance). There is always some source
resistance,
which is an essential part of how the filter works. For video circuits
it
is often 75 ohms, and the load impedance on the output side may also be
75
ohms.
A properly designed passive filter takes all elements into consideration
(source resistance, load resistance, and all reactive elements) to give
the
desired response. Change any of these, and the filter's response
changes.
Without any source resistance, the filter's transfer function loses one
of
its poles. If you wanted (say) a 3-pole Bessel filter and you took a Pi
filter designed for 75 ohms and drove it with a 0 ohms source, it won't
have
that Bessel shape anymore. In that specific case, the first capacitor
would
be ineffective. But nobody would choose a Pi filter driven by an ideal
voltage source.
Regards,
Andy
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