Frequency Synthesizer Carry Sequence Randomization
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Solution Overview
Problem
Conventional frequency synthesizers generate spurious tones due to regular carry sequences, leading to non-essential spurious tones in the frequency spectrum, which are difficult to eliminate and increase system complexity and cost.
Innovation Solution
A frequency synthesizer with a fractional part accumulator and register unit that randomizes the carry sequence by using different random address sequences for writing and reading carry bits, breaking the regularity of the carry sequence and converting spurious tones into noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a conventional accumulator with constant fractional part value is used, then the carry sequence is regular and stable, but spurious tones are generated in the frequency spectrum
Solution Approach 1:
The patent changes the parameter of the fractional part value from constant to variable. Specifically, it uses a fractional part value that changes with the carry sequence (e.g., based on the least significant bit of the carry sequence), thereby breaking the regularity that causes spurious tones while maintaining stability.
Solution Approach 2:
The patent introduces dynamics into the accumulator by making the fractional part value time-varying rather than fixed. The fractional part value is dynamically adjusted based on the current carry sequence state, transforming the static accumulation process into a dynamic one that eliminates periodic spurious tones.
2Object-generated harmful factors
If a random number is added to the control character to break carry sequence regularity, then spurious tones are reduced, but system complexity and cost increase
Solution Approach 1:
The patent extracts only the necessary randomness from the carry sequence itself (specifically the least significant bit) and uses it to modulate the fractional part value. This eliminates the need to add external random numbers or complex random number generators, thereby reducing system complexity while still breaking the regularity that causes spurious tones.
Solution Approach 2:
The carry sequence itself serves the dual purpose of both generating the frequency control and providing the randomness needed to eliminate spurious tones. The least significant bit of the carry sequence is reused to determine the fractional part value, making the system self-sufficient without requiring additional random number generation hardware.
3Object-generated harmful factors
If a random number is added to break carry sequence regularity, then spurious tones are reduced, but hardware resources are wasted
Solution Approach 1:
The patent makes the fractional part value serve multiple functions: it provides the necessary randomness to eliminate spurious tones while also being derived from the carry sequence itself. This multi-functionality eliminates the need for separate random number generation hardware, thereby saving hardware resources.
4Speed
If a high-speed adder is used to add random numbers in high-speed operation mode, then random dithering is achieved, but hardware resources are wasted
Solution Approach 1:
The system uses the carry sequence bits themselves (specifically the least significant bit) as the fractional part value, eliminating the need for high-speed adders to combine random numbers. This self-service approach maintains high-speed operation capability while avoiding the hardware resource waste associated with high-speed adders.
Data Source
AI summary
One of the advantages of direct frequency synthesis technique (e.g., flying-adder architecture) is its capability of generating arbitrary frequency by utilizing the time-average-frequency concept. In the clock output of the direct frequency synthesizer, instead of one type of cycle, there are two types of cycles. Unlike the conventional one-type-cycle clock wherein clock energy is concentrated at its designed frequency, Time-Average-Frequency based clock spreads some of its energy into spurious tones, which could be harmful to certain applications. The spurious tones are caused by the periodic carry sequence generated from a fractional part accumulator inside the frequency synthesizer. The invention suggests a method and an apparatus to break this periodicity and convert the spurious tones into broadband noise.