john wilkinson wrote:
Hi,
When I was looking at an IF amp in the "Experimental mathods in RF", I
noticed a filter I have not seen before.
I will try to describe:

200 Ohm i/p impedance feeding 1 cap. The othe side of the cap feeds
another cap down to ground and an inductor. The other side of the inductor
has a cap down to ground. The junction of the ind and cap, has another cap
feeding a load of 100 Ohm.

When I simulate this it is a bandpass filter.

What do you call this filter, and where can I find some info about it?

Thanks,
John

Andy writes:

This is a 3 pole low pass filter. The type ( butterworth,
chebychev, bessel, etc)
is determined by the component values, as are the frequency , skirt
selectivity and impedance.
Look in the ARRL handbook for typical configurations and
values....This
filter can also be used to transform impedances, although when that is
done
you leave the "type" classification and become more just a low pass
impedance transformer.....

It ain't rocket surgery, but if you aren't into LaPlace transforms,
you need
to stick with the tables.... or the simulator.....

The cap connected to the 200 ohm output is just for coupling and
is
usually really big, but, in some cases , may be made small enough to
provide a small breakpoint for high pass but at a frequency
considerably
lower than the low pass passband.....

If you use PSPICE , or equiv, to simulate it, you can learn a great
deal...

Andy W4OAH