It's been a while since I've done this. But one way to realize a band
stop filter is to begin with a lowpass prototype of the desired response
(Butterworth, Chebyshev, etc.). Then add a capacitance in parallel with
each inductance of the lowpass, of value 1/(w0^2 * L), and an inductance
in series with each capacitance of the network, of value 1/(w0^2 * C).
Beginning with a common series L - shunt C lowpass, this results in
series-connected parallel resonant circuits with shunt series resonant
circuits in between. Intuitively, the topology is correct in that you'll
have zeros due to both the series and shunt networks, and the response
will be unity at DC and infinite frequency. Another topology which would
work is alternating shunt series resonant circuits and series parallel
resonant circuits. The topology you describe can't possibly work since
the coupling capacitor prevents unity response at DC.

This methodology doesn't account for inductor loss, as Wes' analysis
does. You'll find the necessary information about this in Zverev, and
making use of it will give you a real appreciation for the programs now
available for the purpose.

Roy Lewallen, W7EL

Joel Kolstad wrote:

I've been reading Wes Hayward's articles on coupled resonator filters, and
I've simulated some of the results and they're what I'd expect. However,
his examples are all bandpass filters... and I need a bandstop filter.
Chanigng the parallel LC shunt resonators to series LC series resonators
doesn't really work at all... although there's a perfect notch at the
desired center frequency, the passband response is very low (tens of dB
insertion loss) and recovers very, very slowly. It seemed as though I just
needed a different size coupling capacitor, but playing around some doesn't
improve the response significantly (for instance, if you take the example on
page 85 of Introduction to Radio Frequency Design and change the parallel
resonators to series resonators, even if you change the 7.2pF top coupling
capacitor to something astronomical such as 1nF the response is still poor).

I've ordered a copy of Zverev's book (and Zverev w/Mathei), where the theory
of all this comes from, but in the interim... can anyone suggest what the
fundamental problem might be? Interesting, from looking at a few pages of
Zverev on Amazon.Com he does have schematics of what I think I'm after (top
capacitor coupled series resonators to create bandstop filters).

Thanks,
---Joel Kolstad