Local Oscillator Pulse Selection for Harmonic Pulling Avoidance
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Solution Overview
Problem
Existing local oscillator generation schemes in wireless transceivers are susceptible to RF signal interference, particularly frequency pulling due to unwanted RF signals, which complicates the design and requires stringent filtering to avoid spectral emission mask violations and harmonic interference.
Innovation Solution
The implementation of a local oscillator generation mechanism with a non-integer multiplication ratio between the local oscillator and RF frequencies, utilizing digital logic and all-digital phase locked loops to generate I and Q square waves, thereby avoiding frequency pulling and reducing the need for stringent band-pass filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If an integer multiplication ratio is used between local oscillator and RF frequencies, then the oscillator design is simplified, but frequency pulling occurs due to RF signal interference
Solution Approach 1:
The patent changes the multiplication ratio parameter from an integer to a non-integer value (e.g., 3/2, 5/3, 7/5). This parameter change eliminates frequency pulling by ensuring the local oscillator frequency does not coincide with RF harmonics, while still maintaining a rational ratio that simplifies oscillator design compared to irrational ratios.
2Object-affected harmful factors
If a non-integer multiplication ratio is used between local oscillator and RF frequencies, then frequency pulling is avoided, but the oscillator design becomes more complex
Solution Approach 1:
The patent selects specific non-integer rational ratios (3/2, 5/3, 7/5) that balance the need to avoid frequency pulling with the desire to maintain relatively simple oscillator design. These ratios are simple enough to implement while effectively separating the LO frequency from RF harmonics.
3Manufacturing precision
If stringent band-pass filtering is applied to avoid spectral emission mask violations, then spectral purity is improved, but device complexity and cost increase
Solution Approach 1:
The patent prevents spectral interference problems before they occur by using a non-integer multiplication ratio that inherently avoids generating LO tones at RF harmonic frequencies. This preliminary prevention eliminates the need for complex post-processing filtering to remove spectral spurs and harmonics.
Solution Approach 2:
The patent converts what would normally be a harmful frequency relationship (integer multiples causing harmonic interference) into a beneficial arrangement where the non-integer ratio naturally spacing LO tones away from RF harmonics provides inherent spectral filtering without requiring additional filtering components.
4Reliability
If harmonic interference is prevented through complex filtering, then spectral emission compliance is improved, but device simplicity is reduced
Solution Approach 1:
The patent ensures spectral emission compliance by design through the use of non-integer multiplication ratios that prevent harmonic interference at the source. This approach guarantees compliance without requiring complex filtering structures that would add device complexity.
Data Source
AI summary
A novel and useful apparatus for and method of local oscillator (LO) generation with non-integer multiplication ratio between the local oscillator and RF frequencies. The LO generation schemes presented are operative to generate I and Q square waves at a designated frequency while avoiding the well known issue of harmonic pulling. The input signal is fed to a synthesizer timed to a rational multiplier of the RF frequency L/N fRF. The clock signal generated is divided by a factor Q to form 2Q phases of the clock at a frequency of L(N*Q)fRF, wherein each phase undergoes division by L. The phase signals are input to a pulse generator which outputs a plurality of pulses. The pulses are input to a selector which selects which signal to output at any point in time. By controlling the selector, the output clock is generated as a TDM based signal. Any spurs are removed by an optional filter.