In short, your sinusoidal signal is undersampled. There are only 5 samples in one cycle.
ts = 1/600 % sample time
t = 0:ts:0.04;
f = 150;
P = 1/150 % Period (1 cycle)
sam = P/ts + 1 % number of samples in 1 cycle
y = sin(2*pi*f*t);
plot(t, y), grid on, xlabel('t')
However, the curve looks smoother if you decrease the sample time by 100 times, as there are 401 samples (> 360 samples) in 1 cycle.
ts = ts/100 % sample time
sam = P/ts + 1 % number of samples in 1 cycle
t = 0:ts:0.04;
y = sin(2*pi*f*t);
plot(t, y), grid on, xlabel('t')
