Combline tunable filter with negative resistance circuit

High, I have to build a tunable filter with narrrow band and I realized that it is very difficult to find a tunable capacitor with a desirible Q factor to get the required bandwith. I had the idea to introduce a negative resistance circuit in serie with each combline oscillator to get rid of this problem. Does anyone here have alredy tried such a thing? Where can I find paper that can help me with this project?

Thank you

Hi
the number of active filter publications were in 90yers
this is one of