Delta-Sigma modulators are used in Fractional-N Frequency synthesizers to enable the synthesizer generate frequencies which are a fractional multiple of the reference frequency. Therefore, the output frequency can be tuned finely while the reference frequency is kept relatively high for stability and fast settling reasons. The performance of the Delta-Sigma modulator is a key factor in determining the quality of the synthesized frequency. A certain type of modulators called MASH is conventionally used in Frac.-N synthesizers because they are stable and simple to implement. We have developed a modified MASH modulator (HK-MASH) which always yields sequences with maximized lengths with constant inputs. The main advantage of our modulator is to achieve very long sequences with a small word-length without dithering. This only is possible when the modulator is dithered using a pseudo-random sequence with a very long length; however, dithering will add noise to the output spectrum of the modulator degrading the low frequency performance. The figure shown here compares the spectrum of our modulator with that of the conventional modulator dithered with a state of the art technique. Our modulator performs better at low frequencies giving a better signal to noise ratio.

References:

• K. Hosseini and M.P. Kennedy. Maximum Sequence Length Digital Delta Sigma Modulators, IEEE Trans. Circuits and Systems-Part I, Vol. 54, Issue 12, pages: 2628-2638, Dec. 2007
• K. Hosseini and M.P. Kennedy, Mathematical Analysis of a Prime Modulus Quantizer MASH Digital Delta-Sigma Modulator, IEEE Trans. Circuits and Systems-Part II, Vol. 54, Issue 12, pages: 1105-1109, Dec. 2007
• K. Hosseini and M.P. Kennedy. Architectures for Maximum Sequence Length Digital Delta-Sigma Modulators. IEEE Trans. Circuits and Systems-Part II, 55(11):1104-1108, Nov. 2008.
• K. Hosseini and M.P. Kennedy. Minimizing Spurious Tones in Digital Delta-Sigma Modulators. Springer, 2011. 160pp (in preparation).