How to generate a noise which sums up as zero?

I have a sinusoidal downsampled signal. the size of the singal is 1*64. I need to add noise to that signal where sum of noise would be zero. How can I do that? The varience would be 0.001. I am new in this and don't have idea about this. I tried with randn command but I could get that? I tried the follwoing noise:

noise=randn(1,64)*sqrt(0.001)*240.

240 is the peak value of the singal and it has to be there.

Answer 1

[David Goodmanson]

Hi Ananta,

noise=randn(1,64)*sqrt(0.001)*240
noise = noise - mean(noise);

if you subtract off the mean, then the new sum of the noise = 0.

Ordinarily you would add noise (variance=.001 in this case) to the signal. If you multiply the noise by 240, now its variance is up around 58 which may or may not be what you want.