MOSFET-C filter can be tuned by voltage. The cutoff frequency of this type filter is determined by the resistor of the mos transistor working in the linear region. You can change the cutoff frequency by changing the control voltage
Hi
You can use switched capacitors filters
The cut-off frequency is tuned by the clock frequency
To generate the frequency you can chose any VCO
So you obtain a variable cut-off frequency active filter
good luck
Voltage Control Amplifier (VCA) configured as closed loop unity gain can function as VCF
Changing the open loop amplification change the amplifier –3db cutoff point
Because the system configured as closed loop unity gain, the amplification stays constant, the change is only the –3db cutoff point
See National Semiconductor Application Note 386 Audio Noise Reduction
System