Voltage controlled filter concept........

[Learner]

Hi guys,
I was wondering if anyone can direct a source of learning about voltage controlled filters?

I understand that op amp filters ultilize capacitors and resistors to create phase shift between current and voltage of different frequency to generate the filter output response, however I havent been able to find books about using voltage to change the filter cut off frequency.

Can anyone shed a light or direct a link to this?

Thanks!

 

[Johnson]

Based on freq. range the Switch Capacitor is another way to have voltage controlled filters.