Pulse Current Sensor with Ferrite Cores for High-Fidelity Measurement
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Solution Overview
Problem
Current current sensors for coaxial transmission lines face challenges in accurately measuring high-frequency RF signals and fast pulses over a dynamic resistance range, particularly in ESD protective circuit testing, due to limitations in rise time, accuracy, and distortion caused by parasitic inductances and capacitances, as well as noise interference.
Innovation Solution
The use of ferrite cores to isolate the coaxial current sense resistor from DC ground connections and the implementation of magnetic isolation elements allows for the transportation of undistorted, high-speed current signals to a remote location, with tapered ferrite toroids and a straight coaxial sensing resistor assembly minimizing capacitance effects and maintaining constant impedance to achieve fast rise time measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current transformers are used to measure current in coaxial transmission lines, then current measurement capability is provided, but parasitic inductances and capacitances in the windings limit accuracy over the time periods needed for ESD measurements
Solution Approach 1:
The patent extracts the sensing function from the traditional current transformer winding structure and places a high-value sensing resistor directly in the outer conductor of the coaxial transmission line. This eliminates the parasitic inductances and capacitances of windings while maintaining current measurement capability through voltage measurement across the sensing resistor.
Solution Approach 2:
The patent introduces ferrite beads as intermediary elements placed on the outer conductor at specific locations. These ferrite beads provide magnetic isolation and control impedance transitions, enabling accurate high-frequency current measurements by managing the electromagnetic field interactions without introducing significant parasitic effects.
2Measurement precision
If Hall-effect sensors are used in a gap of a magnetic core, then current measurement is enabled, but the sensor response time is too slow (slightly faster than one microsecond) for faster pulse current measurements
Solution Approach 1:
The patent replaces the mechanical/magnetic Hall-effect sensing mechanism with an electrical measurement approach using a high-value sensing resistor and voltage measurement. This substitution enables sub-nanosecond response times by directly measuring the voltage drop across the resistor, which instantly follows the current changes, eliminating the microsecond-scale response limitation of Hall-effect sensors.
3Measurement precision
If a split ring ferrite core is used to concentrate the field for Hall-effect sensing, then field measurement is improved, but the current flowing inside the coaxial transmission line is distorted
Solution Approach 1:
The patent removes the split ring ferrite core from the measurement system entirely. Instead of using magnetic field concentration, the measurement is made directly in the electrical domain by placing a sensing resistor in the outer conductor and measuring the voltage across it, thereby avoiding any distortion of the current waveform that would result from magnetic core insertion.
4Ease of operation
If voltage measurement on the coaxial line is used to calculate current, then voltage measurement is easily accomplished, but the accuracy requires measurement of voltage values over a narrow resistance range (immediate vicinity of 50 ohms)
Solution Approach 1:
The patent makes the sensing system universal by placing a high-value sensing resistor directly in the outer conductor, which allows direct current measurement (converted from voltage across the resistor) independent of the characteristic impedance of the transmission line. This approach works for any resistance value in the protected device, not just those near 50 ohms, because the measurement is made in series with the current path rather than relying on impedance matching.
5Speed
If sensors with 20- to 30-picosecond rise times are used to match digitizer capabilities, then optimal use of digitizer capabilities is achieved, but measuring pulse time durations of up to one microsecond or longer becomes challenging
Solution Approach 1:
The patent achieves both fast rise time response and long pulse duration measurement by carefully selecting and positioning ferrite beads with specific magnetic properties and dimensions. The ferrite beads provide magnetic isolation and impedance control that enable sub-nanosecond response while the high-value sensing resistor and coaxial geometry maintain signal integrity for microsecond-scale pulses, effectively expanding the measurable time range.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables accurate current measurements with sub-nanosecond rise times and minimal distortion, effectively addressing the limitations of existing sensors by maintaining signal integrity and accuracy over a wide resistance range.
Implementation Method 1
The use of ferrite cores to isolate the coaxial current sense resistor from DC ground connections
Implementation Method 2
tapered ferrite toroids and careful dimensioning of conductors to minimize capacitance and magnetic losses
Implementation Method 3
reducing noise interference, thus improving the accuracy and reliability of ESD protective circuit testing
Data Source
AI summary
A sensor measures the current in high frequency pulses (e.g., pulses having fast rise times) that are transported on coaxial transmission lines. The sensor includes an entrance coaxial conductor and an exit coaxial conductor that are interconnected by a continuous inner conductor. The outer conductors of the two coaxial conductors are interrupted and are interconnected by a sensing resistor with a substantially constant resistance. An output sensor coaxial conductor has an inner conductor electrically connected to a first end of the sensing resistor and has an outer conductor connected to a second end of the sensing resistor. Tapered ferrite cores are placed around the three coaxial conductors proximate the connections to the sensing resistor. Preferably, the sensing resistor is a tubular resistor formed on a dielectric cylindrical tube. The sensor is enclosed within a continuous conductive housing.