Two-Point PLL Calibration Using Modulation Pre-Distortion
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Solution Overview
Problem
Two-point modulation phase-locked loops (PLLs) face signal quality degradation due to non-linear capacitance-to-frequency conversion, leading to distorted output signals in periodic modulation applications, such as FMCW radar, where existing calibration methods are inadequate for periodic signals and can negatively affect performance.
Innovation Solution
A calibration method that observes the loop control signal over a period, generates a distortion profile, and applies it to the second periodic control signal before injection, using principles of digital data pre-distortion to mitigate non-linearity effects, thereby ensuring accurate frequency modulation patterns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two-point modulation PLL is used for periodic frequency modulation, then the modulation capability is improved, but non-linear capacitance-to-frequency conversion causes signal quality degradation
Solution Approach 1:
The patent applies pre-distortion to the modulation signal before injection into the PLL. A calibration process measures the actual capacitance-to-frequency conversion characteristics and generates correction values that are applied in advance to the modulation signal, compensating for non-linearities before they occur during normal operation
Solution Approach 2:
The patent changes the parameters of the modulation signal based on measured distortion characteristics. By adjusting the injection signal parameters according to calibration data, the system compensates for non-linear capacitance-to-frequency conversion and maintains signal quality
2Measurement precision
If existing calibration methods are applied to periodic modulation signals, then calibration is performed, but performance is negatively affected
Solution Approach 1:
The patent applies preliminary anti-action by measuring distortion during calibration and applying corrective pre-distortion to counteract the measured non-linearities. The calibration process identifies errors and the system applies opposite corrections to compensate for capacitance-to-frequency conversion non-linearities
Solution Approach 2:
The patent implements feedback by using the loop filter output during calibration to measure actual distortion characteristics. This measured information is used to generate correction values that are applied to subsequent modulation signals, creating a closed-loop calibration and correction mechanism
3Measurement precision
If loop filter output is observed for calibration, then distortion profile can be generated, but additional observation circuitry is required
Solution Approach 1:
The patent applies self-service by using the existing loop filter output, which is already present in the PLL circuit for normal operation. This existing signal is repurposed for calibration and distortion measurement without requiring separate observation circuitry, reducing additional complexity
Solution Approach 2:
The patent makes the loop filter output serve multiple functions: it acts as the control signal for the VCO during normal operation and simultaneously serves as the measurement signal for calibration and distortion analysis, eliminating the need for separate measurement circuitry
Data Source
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.