On Fri, 09 Apr 2004 18:38:30 +0100, Paul Burridge
wrote:

Hi all,

On page 57 of RF circuit Design, Chris Bowick sets out a filter design
example. I've posted this to a.b.s.e under the same subject header. He
claims that the filter in question - a low pass Butterwoth - matches
50 ohms source to 500 ohms load. However, having checked this out with
the aid of a Smith Chart, it appears there is some capacitive
reactance present that would require the addition of a shunt inductor
to neutralize. However, this would of course totally screw up the
filter's characteristics. Upon closer examination, it appears
impossible that this type of arrangement could ever be designed
without introducing some reactance into the signal path. Or am I nuts?
I'd always thought of these kind of filters as being purely resistive
overall at Fo but is that really the case? It don't look like it...

Design criteria:

Centre frequency: 35Mhz
Response -60dB at 105Mhz
zero ripple(!)
Rs 50 ohms
Rl 500 ohms

Most LC filters are designed to produce the advertised frequency
response, given a resistive generator and a resistive load, but they
don't generally look like nice resistors themselves, at either port.

There are filters designed to look like a resistor, or nearly so,
wideband. Picosecond Pulse Labs, among others, sell such. We had a
thread on s.e.d. a while back, and Jeroen Belleman posted some nice
work, some original techniques to make constant-resistance filters.

John