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Design >> Analog Design >> 2nd order distortion in a ideal sigma delta modulator
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Message started by xlowen on Jul 9^th, 2015, 11:20am

Title: 2nd order distortion in a ideal sigma delta modulator
Post by xlowen on Jul 9^th, 2015, 11:20am

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

Title: Re: 2nd order distortion in a ideal sigma delta modulator
Post by RobG on Jul 10^th, 2015, 8:10am

100dB SFDR and 145 dB down... maybe somebody sneezed during the simulation :D

I don't think the white noise approximation for quantization noise is as accurate for small signals. Perhaps that is it, but it is a really small error.

Title: Re: 2nd order distortion in a ideal sigma delta modulator
Post by xlowen on Jul 11^th, 2015, 1:43am

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! :o