What order filter (how many capacitors)? What sort of power level?
Guess I'd look at trimming the capacitances. I have a bunch of
0.5-5pF trimmer caps that I might use for such a filter. Or, I'd get
a big pile of 1pF, 1.2pF, 1.5pF, ...etc and select caps that measured
correctly. Since 10MHz really isn't a very big percentage bandwidth,
I'd probably go for trimmer caps, and tune the whole filter using a
network analyzer to monitor its response. Does your filter design
program give you the resonant frequencies of each resonator? -- Does
your design not have some really small inductances, 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

Hi Tom,

"Tom Bruhns" wrote in message
m...
What order filter (how many capacitors)?

Looks like I can get away with a third order; just a pi or tee.

What sort of power level?

1W.

Does your filter design
program give you the resonant frequencies of each resonator?

Yes, or at least I can calculate it readily enough.

Does
your design not have some really small inductances, too??

They're in the 'hundreds of nanohenries' range. With them I've been more
worried about obtaining decent Q's than the actual values, in that
parasitics would seem to be much more comparable to the size of the caps
than the inductors.

Thanks for your advice,
---Joel