Van Der Pol Oscillator

StreamPlot[{v, -(x x - 1) v - x}, {x, -3, 3}, {v, -3, 3}]

A = NDSolve[{x”[t] + 3 (x[t]^2 - 1) x'[t] + x[t] == 0, x[0] == 3, 
   x'[0] == 0}, x, {t, 0, 200}]; 
Plot[Evaluate[x[t] /. A], {t, 0, 30}]