Dear All,

I built a 3rd order feedforward 1-bit DT sigma delta modulator in simulink with ideal integrator and quantizer. When I applied small input signal (-40dB), 2nd order distortion appears at the output spectrum as shown in attached fig. When I apply large input signal (-3dB), usually I can only see 3rd order distortion, which comes from the nonlinearity of 1-bit Qunatizer. Can anyone give some insight about why 2nd distortion shows up when small signal is applied?

[img][/img]

Thanks,
xlowen

Yes, especially one-bit quantizer is quite nonlinear. In fact I am expecting some tones show up which relates with sampling frequency, instead of signal dependent distortion. It really surprised me!