Frequency Synthesizer Dither Injection for Fractional Spur Control
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Solution Overview
Problem
Fractional-N synthesizers in wireless communication systems suffer from fractional spurs and phase noise due to channel spacing and stepping, which adversely affect transceiver performance.
Innovation Solution
A controller is used to generate and inject a noise portion or code word into the sigma delta modulator of a frequency synthesizer, mitigating fractional spurs by appending a multibit noise word or fixed code word to the input signal, thereby controlling the multi-modulus divider and reducing phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If sigma delta shaping is used in fractional-N synthesizers to reduce phase noise, then oscillator noise attenuation is improved, but fractional spurs are introduced due to channel spacing and stepping
Solution Approach 1:
The patent introduces dither noise, which appears to be an additional harmful factor, but this noise actually breaks the periodicity that causes fractional spurs. The dither noise converts the deterministic spur generation into a stochastic process, spreading the energy across the spectrum and eliminating the discrete spur tones while preserving the phase noise benefits of sigma delta shaping.
Solution Approach 2:
The dither noise acts as an intermediary element between the sigma delta modulator and the frequency output. By inserting this random noise component into the modulation process, it mediates the interaction between the quantization process and the output frequency, preventing the formation of coherent spur tones while maintaining the overall frequency synthesis function.
2Manufacturing precision
If higher order sigma delta modulator is used to shift noise energy out of PLL bandwidth, then phase quantization noise is reduced, but fractional spurs and sub-fractional spurs are introduced
Solution Approach 1:
The dither noise converts the harmful periodic fractional spurs generated by higher order sigma delta modulation into beneficial spread spectrum noise. By adding random variability to the modulation process, the energy that would otherwise concentrate at specific spur frequencies is distributed across a broader frequency range, eliminating the discrete harmful tones.
Solution Approach 2:
The patent applies periodic dither sequences that are synchronized with the sigma delta modulation process. This periodic action, when combined with the higher order modulation, creates a composite signal where the spurious components are pushed to higher frequencies or distributed more uniformly, allowing for their effective suppression while maintaining the noise shaping benefits.
3Measurement precision
If feedback frequency divider modulus is dynamically adjusted to create average multiplication factor, then frequency resolution is improved, but instantaneous phase noise is introduced in the feedback loop
Solution Approach 1:
The patent employs dynamic adjustment of the feedback divider modulus through sigma delta modulation, allowing the system to achieve high frequency resolution by varying the division ratio. The dynamic nature of this adjustment enables the synthesizer to achieve fine frequency steps while the associated phase noise is shaped and pushed to higher frequencies by the modulation process.
Solution Approach 2:
The invention changes the parameter of the feedback divider modulus dynamically according to the sigma delta modulator output. By varying this parameter in a controlled manner, the system achieves high frequency resolution while the parameter changes are designed to minimize instantaneous phase noise impact, with the noise being shaped and filtered by the PLL loop.
Data Source
AI summary
A method and system for generating noise in a frequency synthesizer are provided. The method includes generating a noise portion of an input signal within the frequency synthesizer and appending the noise portion to a control portion of the input signal.