Time-Interleaved Digitizer Calibration Using Harmonic Mixing
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Solution Overview
Problem
Test and measurement instruments with built-in calibration oscillators that do not span the entire range of potential signal sources face challenges in calibrating systems, as they require access to a signal source that covers the full frequency range for accurate calibration, which is not always feasible, especially under varying environmental conditions.
Innovation Solution
The implementation of a compensation oscillator that is tunable over a wider frequency range than the internal ADC channel process uncertainty, allowing for real-time calibration and correction of phase and amplitude errors, and the use of a Look Up Table (LUT) to store filter coefficients for correcting hardware errors based on measured clock delay and skew values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a built-in calibration oscillator is used for calibrating the test and measurement instrument, then calibration can be performed, but the oscillator does not span the entire frequency range of potential signal sources, limiting calibration accuracy
Solution Approach 1:
A compensation oscillator is introduced as an intermediary component with a tunable frequency range that overlaps with the input bandwidth. This compensation oscillator serves as a mediator to provide calibration signals across the full frequency range, enabling accurate calibration without requiring the built-in calibration oscillator to span the entire input bandwidth.
Solution Approach 2:
The calibration function is segmented into two parts: the built-in calibration oscillator provides calibration for its limited frequency range, while the compensation oscillator provides calibration for the remaining frequency ranges. This segmentation allows each oscillator to operate within its optimal frequency range, collectively covering the entire input bandwidth.
2Measurement precision
If hardware adjustments are made for mixing clock amplitude and phase, then interleave mismatch spurs can be minimized, but the system remains sensitive to environmental conditions such as temperature and humidity causing performance drift
Solution Approach 1:
The system performs preliminary calibration using the compensation oscillator to characterize hardware errors (amplitude and phase mismatches) before actual signal processing. This preliminary characterization allows the system to pre-compute correction coefficients that compensate for hardware imperfections, reducing sensitivity to environmental variations during operation.
Solution Approach 2:
The system implements a feedback mechanism where the compensation oscillator continuously monitors and characterizes hardware errors, and the measured errors are used to adjust correction coefficients in real-time or near-real-time, maintaining calibration accuracy despite environmental changes.
3Measurement precision
If a linear time-varying correction filter is used to cancel interleave spurs, then calibration accuracy can be improved, but the filter requires characterization over the full frequency range which is not feasible with limited calibration oscillators
Solution Approach 1:
The compensation oscillator acts as an intermediary that provides the necessary full-frequency-range calibration signals, enabling accurate characterization of the linear time-varying correction filter without requiring external signal sources or complex calibration setups.
Data Source
AI summary
A test and measurement instrument includes a coefficient storage facility coupled to a programmable filter. The coefficient storage facility is configured to store at least two pre-determined filter coefficient sets, and configured to pass a selected one of the at least two pre-determined filter coefficient sets to the filter based on a measurement derived using a compensation oscillator. The measurement may include clock delay and clock skew. In some examples the test and measurement instrument may additionally adjust clock delay and/or clock skew in addition to selecting appropriate filter coefficients.