How to create a wide-sense stationary time series with a frequency of 40 Hz?
I'm not an expert in this area but I can think of 


Add random noise to a 40-Hz sine wave: ts = std*randn(1,N) + sin(2*pi*40*t)
Add a random phase: phi = std*randn(1,N); ts = sin(2*pi*40*t + phi)