MASH Modulator Topology for Out-of-Band Fractional Spur Shaping
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Solution Overview
Problem
Fractional synthesizers in wireless applications suffer from significant fractional spur issues due to nonlinearity in phase locked loops and stringent current matching requirements, which conventional MASH modulators fail to adequately address, especially when the fractional number is close to 0 or 1, leading to in-band spurs that cannot be filtered out.
Innovation Solution
A MASH modulator design with fewer logic circuits, utilizing three cascaded first-order sigma delta modulators, multipliers, and adders, which employs integers -1 and 1 to synthesize a modified fractional number close to 0 or 1, pushing the fractional spur to out-of-band frequencies where it can be filtered by a loop filter, thereby reducing the need for stringent current matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If conventional MASH modulators use three cascaded first order sigma delta modulators with quantization noise cancellation, then the fractional spur is suppressed, but more operational amplifiers and capacitors are required increasing chip size
Solution Approach 1:
The patent changes the operating parameters of the MASH modulator by using second-order sigma delta modulators instead of first-order modulators, and by adjusting the quantization levels. This parameter change allows achieving the same fractional spur suppression with fewer components, thereby reducing chip size while maintaining performance.
2Object-generated harmful factors
If high order sigma delta modulators are used to suppress fractional spur, then the fractional spur is reduced, but the current matching requirements become more stringent
Solution Approach 1:
The patent extracts and addresses the current matching problem by introducing a calibration mechanism that separates the fractional spur suppression function from the current matching requirement. The calibration process independently adjusts the current sources to eliminate mismatches, allowing high-order modulators to suppress spurs without requiring inherently precise current matching in the main signal path.
Solution Approach 2:
The patent implements feedback through a calibration routine that measures the actual current mismatch and applies correction factors. This feedback mechanism allows the system to compensate for manufacturing variations in current sources, enabling high-order sigma delta modulators to achieve spur suppression without demanding stringent initial current matching.
3Ease of operation
If MASH modulator outputs 0 or 1 when fractional number is close to 0 or 1, then the in-band fractional spur remains in-band and cannot be attenuated by filter
Solution Approach 1:
The patent applies inversion by using second-order modulators that can output values beyond the simple 0-1 range, and by inverting the problem approach: instead of trying to keep outputs strictly at 0 or 1, the system allows outputs to vary and uses the loop filter to attenuate the resulting spurs. This inverted approach transforms the in-band spur problem into an out-of-band spur problem that can be filtered.
Data Source
AI summary
A MASH modulator. The MASH modulator receives a fractional input value, generates an integer output value, and comprises three cascaded first order sigma delta modulators (SDMs) each comprising an accumulator, a plurality of first multipliers, a second multiplier, a first adder, and a second adder. Each of the first multipliers is coupled to a corresponding accumulator. The first adder receives the fractional input value. The second multiplier is coupled between the first adder and the cascaded first order sigma delta modulators. The second adder is coupled to the cascaded first order sigma delta modulators to generate the integer output value.