Another issue you'll have to deal with if you don't do some scaling of
some sort is that the inductors that go with the fractional-pF
capacitors will have significant parasitic capacitance...you're likely
going to get self-resonant frequencies not far above the filter
stopband frequency (or worse). It would help a lot if you could make
the filter stopband considerably wider. Seems like with a 10MHz
stopband, you're asking for filter performance not too unlike a
duplexer.

You mentioned the need for the filter to be flat down to low
frequencies, but what about higher frequencies? Are the repetitions
which are common in distributed (transmission-line) filters really a
problem? Or do you need to pass all the higher frequencies too?

Cheers,
Tom


"Joel Kolstad" wrote in message ...
I was curious if anybody had ideas as to how to deal with the fractional pF
capacitors that come out of 'first pass' filter design programs such as
Elsie for filters up in the 70cm (and slightly shorter) range (50 ohm
terminations on both ends, LC style)? I'm attempting to build a bandstop
filter up there (20dB attenuation, 10MHz bandwidth), and I'm aware of the
usage of microstrip traces as equivalent capacitors (and inductors), but the
filter needs to be (approximately) flat all the way back down to the ~30MHz
and using, e.g., lamba/8 microstrip sections would create repetition in the
filter's response every quadrupling of the center frequency.

So... other than attempting to be clever and transforming the filter such
that capacitance is increased... or scaling the system impedance (to... 20
ohms? Then I'm still stuck with 2-3pF values)... are there any good methods
of implementing such a wideband filter design at these frequencies?
Pointers to books, magazine articles, etc. would be appreciated.

Thanks,
---Joel Kolstad