Transferred Impedance Filter Clock Phasing for Odd-Harmonic Rejection
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Solution Overview
Problem
Existing signal filters, particularly those using transferred impedance filters (TIFs), face limitations in harmonic rejection and attenuation around odd harmonics of the clock signal frequency, which are not sufficient for applications like wireless local area network (WLAN) coexistence in cellular transceivers, especially when using fewer paths like 4-path TIFs, and increasing the number of paths like 8-path TIFs leads to reduced attenuation and increased oscillator frequency requirements.
Innovation Solution
A signal filter comprising two 4-differential-path transferred impedance filters (TIFs) coupled in parallel, with clock signals having non-overlapping phases where the phases of the second TIF are delayed by 45 degrees relative to the first TIF, allowing for reduced harmonic folding and improved attenuation around odd harmonics without increasing the oscillator frequency, and using differential signal paths to reject even harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the number of paths in TIF is increased from 4 to 8 to improve harmonic rejection, then harmonic rejection capability is improved, but attenuation at harmonics is reduced and maximum VCO frequency requirement increases
Solution Approach 1:
The invention segments the filtering function into two separate 4-path TIFs operating in parallel with different clock phases. Each TIF handles specific harmonic rejection tasks, and their combined output achieves the harmonic rejection performance of an 8-path TIF while maintaining the simpler 4-path structure. This segmentation allows the system to benefit from multiple filtering paths without the complexity and frequency requirements of a single 8-path TIF.
Solution Approach 2:
The invention merges the outputs of two 4-path TIFs that are clocked with phase-shifted clock signals. By combining the filtering results from both TIFs, the system achieves improved harmonic rejection and attenuation performance that would require an 8-path TIF, but with reduced complexity and lower VCO frequency requirements.
2Manufacturing precision
If switching duty cycle is reduced to improve bandpass frequency selectivity, then frequency selectivity is improved, but attenuation at odd harmonics is severely degraded
Solution Approach 1:
The invention introduces asymmetry by clocking the two TIFs with phase-shifted clock signals (one at 0 degrees, the other at 45 degrees). This asymmetric phase relationship causes the TIFs to have different frequency responses, with each TIF providing strong attenuation at different harmonic frequencies. The combined response achieves broad harmonic rejection while maintaining bandpass selectivity.
Solution Approach 2:
The invention uses periodic switching with specific duty cycles in both TIFs, where the switching is synchronized with phase-shifted clock signals. This periodic action at different phases creates complementary filtering effects that maintain both frequency selectivity and harmonic attenuation.
3Device complexity
If 4-path TIF is used to simplify circuit design, then device complexity is reduced, but harmonic rejection and attenuation are insufficient for WLAN coexistence applications
Solution Approach 1:
The invention merges two 4-path TIFs in parallel to achieve the harmonic rejection performance required for WLAN coexistence applications. Each TIF maintains its simple 4-path structure, but their combined output provides the necessary attenuation and rejection characteristics that a single 4-path TIF cannot achieve alone.
Solution Approach 2:
The parallel configuration of two 4-path TIFs provides multi-functionality: each TIF performs basic bandpass filtering while collectively they provide enhanced harmonic rejection, frequency selectivity, and attenuation. This universal approach allows the same basic 4-path TIF structure to serve multiple performance requirements simultaneously.
Data Source
AI summary
A signal filter (100) comprises a first transferred impedance filter, TIF, (TIFA) having four differential signal paths (PA,1, PA,2, PA,3, PA,4) and a second TIF (TIFB) having four differential signal paths (PB,1, PB,2, PB,3, PB,4)- A first differential signal port of the first TIF (32A) is coupled to a first differential signal port of the second TIF (32B). A first clock generator (12A) is arranged to provide first-TIF clock signals (CLKA,I+, CLKA,Q+, CLKA,I−, CLKA,Q−) having four non-overlapping phases for selecting the respective first-TIF differential signal paths (PA,1, PA,2, PA,3, PA,4), and a second clock generator (12B) is arranged to provide second-TIF clock signals (CLKB,I+, CLKB,Q+, CLKB,J−, CLKB,Q−) having four non-overlapping phases for selecting the respective second-TIF differential signal paths (PB,1, PB,2, PB,3, PB,4). The phases of the second-TIF clock signals (CLKB,I+, CLKB,Q+, CLKB,I−, CLKB,Q−) are equal to the phases of the first-TIF clock signals (CLKA,I+, CLKA,Q+, CLKA,I−, CLKA,Q−) delayed by 45 degrees. The first-TIF first, second, third and fourth clock signals (CLKA,I+, CLKA,Q+, CLKA,I−, CLKAQ−) and the second-TIF first, second, third and fourth clock signals (CLKB,I+, CLKB,Q+, CLKB,I−, CLKB,Q−) have a duty cycle in the range 16.75% to 25%.