I was trying to simulate the Bode plot for a Sallen Key low pass filter (R= 10k , C= 1nF) . The filter was designed for a cut off frequency of 15.9 kHz. The simulated filter response however shows a 3db frequency of 10 kHz .

Could any one help explain why the simulated frequency response is not as expected and what I am likely doing wrong ?.

I have attached the schematic and the response obtained.

Answering my own question here. The 3 db point for this case is sqrt(sqrt(2) -1) / (2*pi*R*C) which works out to 10 kHz. The simulation is correct. fc in this case is not the 3 db frequency but called the natural frequency.