I'm designing a 3rd order digital modulator, using 1.5 bit quantization for a 3 level output audio Class D amplifier. The SD modulator uses a CIFF structure using coefficients calculated by Schreier's Matlab Toolbox.

Simulated with a 1kHz sinusoid, 13 bit resolution. Simulation results seem OK, integrator's never overflow.

The problem is the output spectrum shows some harmonics tones (-60dB), odd and even, within audio band. Sorry I couldn't post the image.

How can I improve the modulator's linearity?
What harmonic amplitude should I espect for this kind of modulator?
Can 1.5 bit quantization be related to this?