"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 Joel,
Perhaps you can give us some additional number? What sort of attenuation
level do you require? Where are the -3dB corners? What is the tolerbale
passband return loss?
You mention the filter needing to be good down to 30MHz, but no mention of
characteristics above 70cM. A 10MHz wide bandstop filter with decent
attenuation at 70cM is going to be complex. Would perhaps an elliptic LPF
also work? Do you need the full 10 MHz of stop frequency?
I use the Genesys simulator/synthesizer package which is highly flexible,
allowing real time filter tuning- but ALWAYS try to avoid a wide spread in
component values and very small capacities.

73,

Dale W4OP