Posted on Jul 22, 2012 4704
The control voltage Vc easily sets the cutoff frequency w0 of this state-variable filter to any desired value, from about 1.7 MHz up to 5 MHz, with a BB 204 varicap and R = 100 KO. Vc can range from 0 to 28 V. This range changes the capacitance of the varicap from about 4 to 12 pF. The circuit consists of input summing circuit A1 and two noninverting integrators, A2 and A3. Both the integrators and the summing-amplifier circuits use CA3450 op amps. With them, cutoff frequencies up to 200 MHz are possible.
The circuit"s cutoff frequency, it"s Q-factor, and gain G are simply: w0 = 2/CR, Q = R,!R4, and G = R4/R1 For a given value for R4, say 10 KO, Q depends only upon the resistance of R3. The Q can be any value, even 100, independently of both w0 and G. Similarly, the gain then depends only on the resistance of R1 and can also be set as high as 100.