Two-Point Modulation PLL Calibration Using Periodic Pre-Distortion
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Solution Overview
Problem
Two-point modulation PLLs face signal quality degradation due to non-linearities in the path from the second injection point to the output, particularly in periodic modulation scenarios, where existing calibration methods for random data modulation systems are ineffective.
Innovation Solution
A method involving observing the loop control signal over at least one period to generate a distortion profile, which is then applied to the second periodic control signal before injection, utilizing principles of digital data pre-distortion to mitigate non-linearity effects, and an iterative calibration process to achieve modulation accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a two-point modulation PLL is used for periodic frequency modulation, then the PLL can generate desired frequency modulation patterns (chirps), but signal quality degrades due to non-linearities in the path from the second injection point to the output
Solution Approach 1:
The patent applies pre-distortion to the second periodic control signal before injection into the PLL. The distortion profile is generated in advance based on observed loop control signal characteristics, and this pre-computed distortion is applied to counteract the non-linearities that will occur during modulation, thereby maintaining signal quality while enabling frequency modulation
Solution Approach 2:
The patent implements an iterative calibration process where the loop control signal is observed, a distortion profile is generated from this observation, and the distortion profile is applied to the control signal. This feedback loop continues iteratively to refine the distortion profile and achieve optimal modulation accuracy
2Ease of manufacture
If existing calibration methods for random data modulation systems are applied to periodic modulation scenarios, then the calibration process can be implemented, but the methods are ineffective for periodic modulation scenarios
Solution Approach 1:
The patent adapts the calibration approach specifically for periodic modulation by generating a distortion profile tailored to periodic signals. Instead of using a generic calibration method, the system observes the loop control signal during periodic modulation and creates a distortion profile specifically suited to this modulation type, making the calibration effective for its intended application
3Ease of operation
If the second periodic control signal is injected directly without pre-distortion, then the injection process is simple, but non-linearities cause output distortion and information loss
Solution Approach 1:
The system performs pre-distortion of the second periodic control signal before injection by applying a distortion profile that is generated in advance. This preliminary modification of the control signal compensates for the non-linearities that will occur during the injection and modulation process, preventing output distortion while maintaining operational simplicity
Data Source
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AI summary
The method of calibrating a two-point modulation phase locked loop (PLL) comprises observing, between the loop filter and the second injection point, the loop control signal over at least one period of the first periodic control signal; generating, from the observed loop control signal, a distortion profile; and applying the distortion profile to the second periodic control signal before injecting the second periodic control signal in the PLL. Since, in the case of non-linearity in the controlled oscillator, the PLL output deviates from the ideally expected one, cancellation through the first injection point becomes imperfect disturbing the loop. This error pattern can be observed on the loop filter which allows to generate a distortion profile to distort the second periodic control signal for the next period of the modulation. This will mitigate the effects of the non-linearity of the oscillator.