[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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