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Analog Verification >> Analog Performance Verification >> Sigma-Delta modulator: even order harmonic distortion
https://designers-guide.org/forum/YaBB.pl?num=1301391851

Message started by matteo.tonelli on Mar 29^th, 2011, 2:44am

Title: Sigma-Delta modulator: even order harmonic distortion
Post by matteo.tonelli on Mar 29^th, 2011, 2:44am

Hi,

I'm simulating a second order Sigma-Delta Modulator (Cascade of Integrators Feedback) with a 1.5 bit quantizer (3 quantization levels); a behavioural model has been implemented and all the simulations are performed in Simulink. The results are compared with the results obtained simulating Temes and Schreier toolbox and good agreement is proved.

The fft results in odd harmonics according to the odd quantization function and the noise floor is compatible with the expected value.
The fft calculation is performed using Hann windowing and coherent sampling (fin=fs*127/2^19, fft n.of points=2^19).

The power spectrum however includes unexpected even harmonics.

Since the model is perfectly balanced and ideal, why the modulator described above shows even harmonics? Is this problem intrinsic of the architecture?

Many works in literature show sigma-delta spectra with both even and odd harmonics but none explanation is reported.

Could someone help me to undertand the phenomenon?

Many thanks,
Matteo

Title: Re: Sigma-Delta modulator: even order harmonic distortion
Post by sheldon on Mar 29^th, 2011, 6:23am

Matteo,

Can you provide a plot of the FFT?

Best Regards,

Sheldon

Title: Re: Sigma-Delta modulator: even order harmonic distortion
Post by matteo.tonelli on Mar 29^th, 2011, 8:01am

Thank Sheldon for your attention,

the fft result (2^22 points, Hann windowing) is attacched.

Further details:

The oversampling ratio is 2048; the input sine wave is at fs*1023/2^22 where fs is the sampling frequency.

The 2nd order (cascade of integrators feedback) modulator is based on a 1.3 bits quantizer; in other terms the quantization curve performs a mid-tread characteristic with three quantization levels

Thanks in advance,
Matteo

Title: Re: Sigma-Delta modulator: even order harmonic distortion
Post by Praveen Prabha on Aug 18^th, 2012, 11:51am

Matteo ,

Are you applying a full scale sine wave as the input ?

Do you also see similar tones if you apply a smaller input , lets say -12dBFS ?

Regards,
Praveen