I was trying to simulate the simplified model of RDC (AD2S90). The issue is to design proper compensator for this model to track constant velocity with zero velocity lag. As suggested in the data sheet (page No.9) I tried to add a compensator (K/s * (s+a)/(s+b)) between demodulator and the VCO. But from simulation(step response), it appears to oscillate. Where am I going wrong? Almost all the data sheets that I came across recommend the compensator with the above mentioned transfer function, but in simulation, it does not work. How to proceed? When I provide velocity input, surprisingly the circuit works, gives no velocity lag forever. Reference signal is 50Hzer!