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https://designers-guide.org/forum/YaBB.pl Design >> Mixed-Signal Design >> Feedforward Sigma-Delta Modulator problem https://designers-guide.org/forum/YaBB.pl?num=1181915566 Message started by caosl_zju on Jun 15th, 2007, 6:52am |
Title: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 15th, 2007, 6:52am Hi, all I was simulated two second-order feedforward sigma delta modulators recently. the model shown in sdm_ff.jpg, the spectrum shown in fft_sdm_ff.jpg, the result is correct. When i add the input signal feedforward path to the quantizer shown in sdm_ffxin.jpg, the result can not display the second order noise shaping effect. But this structure can usually be seen in papers or other situations. But why the result is so unreasonable?? All comments are welcome. thanks in advance. |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 15th, 2007, 6:55am fft_sdm_ff.jpg |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 15th, 2007, 6:56am sdm_ffxin.jpg |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 15th, 2007, 6:57am fft_sdm_ffxin.jpg |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by email_gz on Jun 16th, 2007, 1:48am 1: Check if overload occur for 1/(1-Z) which map to SW-OP output and comparator/AD ? 2: scaling you input signal to be a small range . 3: calculate you NTF , stability? find good scaling factors for both in-band SNR requirment and stability. 4: use multilevel comparator/AD. |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 16th, 2007, 3:24am hi, email_gz Thanks for your reply. The swing of two integrators are very small, this is the reason why i choose this architecture for saving of power consumption. The input to comparator is not large too. The STF and NTF i've caculated, no problems have found, confusing!!depressed!! |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by email_gz on Jun 16th, 2007, 11:34pm If no overload occur , STF and NTF have not stability problem and also have resonable target in-band SNR, but then you get bad simulink result, maybe you should check your fft window function and more fft caculation points. |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 17th, 2007, 9:12pm The only difference between two models is whether the input signal participate in feedforward adding. After adding input signal's feedforward, the STF is changed to 1, but the feedback loop which deals with quantization noise is not changed, so the NTF is the same between two behavior models. The input signal's amplitude is not changed from two spectrum figure, but noise floor was go up. Could you simulate these two models if this won't take up you much time. thank you! |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by Berti on Jun 17th, 2007, 11:07pm Hi, I think the coefficient of your direct-feed-forward path (according to your figure equals one) is wrong. When multiplying the gains of the integrators I obtain 1/15. For a loop-gain of 1 your quantizer-gain should be 15. Well, the quantizer-gain of a single-bit quantizer isn't defined. Nevertheless I think you have a coefficient of 1/15 for your direct feed-forward path. And your are right, the additional path doesn't affect the NTF at all, and therefore also the stability should be unchanged. |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by caosl_zju on Jun 19th, 2007, 5:05am hi, Berti But why it works fine if i take the input signal feedforward path off ? Is it 1 bit quantizer's gain so important ? |
Title: Re: Feedforward Sigma-Delta Modulator problem Post by Berti on Jun 20th, 2007, 12:25am The quantizer gain has an influence on the transfer functions (NTF/STF). Unfortunately, the gain of a 1bit quantizer isn't well defined. But usually you derive you filter coefficients assuming that the quantizer gain is equal one. After dynamic-range scaling you quantizer gain will probably change. In order to have the proper STF (equals 1 in your case), the direct feed-forward path times the quantizer gain should be one. But I cannot predict from scratch how the loop stability is influenced in your case. Of course, without the input signal feedforward you get a well designed feedforward modulator, while with the input signal feedforward you STF with probably even amplify the input signal. I hope that helps, Regards. |
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