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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 |
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