Hi,
I'm trying to simulate a 3rd-order SDM using Simulink. I followed the approach Schreirer used in his text "Understanding Delta-Sigma" to calculate the coefficients by simply comparing the powers of z of the required NTF, which is (1-z^-1)^3 in my case, and the general NTF of a 3rd order CIFB structure given by eq. (4.33) on page (116).
My calculated coefficients are:
a1 = 1, a2 = 3, a3 = 3
b1 = 1, b2 = 3, b3 = 3, b4 = 1 (to give an STF = 1)
Input is a sine-wave with frequency = 812.5Hz, amplitude = 0.5v, OSR = 32 and 1-bit quantizer.
However, it appears to be unstable for an unknown reason to me and I'm really confused, any suggestions?
Thanks