Modified SVF for 2-scroll operation. * One of the opamps is used as a comparator to implement the 3rd state variable as a Schmitt trigger. * The reverse parallel diodes D1/D2 are used to make the output level of the 3rd state variable relatively independent of supply voltage. * The capactor C3 is (probably optional). It adds a switching delay. The charge current depends on R10 and Vcc and is fairly constant if |Vsupply - Vref| > 0.7V. * For the switchable variable, we take the integrator directly after the inverting amplifier. This gives us more flexibility as both positive and negative scaling are available.