12 dB VCF - voltage controlled Butterworth low-pass filter

The circuit diagram shown here is a National Semiconductor application of the LM/XR 13600, in this case used as a kind of state-variable filter. The circuit contains a selective filter output (u1) and a low-pass filter (u2). The centre frequency of the selective fillter and the cutoff frequency of the low-pass filter can be influenced by the control voltage level uc. Both integration capacitors C determine the range in which these frequencies can be varied. The corresponding formulas are: u1 Li; 42 pr 462 ID" r2 +21 pr + 1 voltage controlled filter p = j0..); T = S; 2—Rc; = 19.2 • IABc; Rc = 15 kS2 selective (bandpass) filter U2 2 - = ui 462 p2r2 + 21 pr + 1 low-pass filter "ABC "7 " using the super OTA 13600 Cutoff frequency and central frequency respectively are: 1 21 T • KM UC Literature: When is an OTA not an OTA?, and IA 0—A IV 00 (10k.10k) 0 0-0 -0- r2nk (106.10k) CAN (106.10k) CAlm 720p 7720p Low-Pass •00 Band pass oinui 15 V "The 13600, a new OTA, C both publis...