Enhancing Precision in Broadband Current Measurement Using Coaxial Shunt Technology

1. Introduction

In broadband current measurement scenarios, such as the calibration of power standard sources, harmonic sources, and power quality standard sources, measuring broadband currents below 100 A and 100 kHz is often required. When using a digital multimeter for such calibrations, several challenges frequently arise:

1) Limited Range: Most digital multimeters can measure a maximum current of only 20 A, with some models limited to as little as 2 A.

2) Low AC Accuracy: For example, with the Keysight 3458A, the best achievable accuracy for a 1 A AC current measurement is approximately 0.1%.

3) Reliability: The impact resistance and durability of current measurements in high-precision digital multimeters are generally much lower than their voltage measurement capabilities.

Therefore, it becomes essential to use a conversion method to reduce the high current into a smaller current or voltage signal before measurement. Commonly used conversion methods include the resistive method, transformer method, and comparator method.

This article focuses primarily on the resistive method.

2. Problems In Measuring High-Frequency Current Using Conventional Standard Resistors

The working principle of the resistance method is Ohm's law (see Figure 1). The resistance is converted into voltage after passing the measured current, and is connected to the voltage terminal of the digital multimeter for measurement. At this time, the measured current is calculated according to formula (1).

The resistance of a resistor is usually calibrated using a standard DC bridge. However, when measuring AC current, since the resistor itself has a certain distributed inductance and capacitance (see Figure 2), the equivalent impedance of the resistor under AC measurement can be calculated according to formula (2)

As shown in formula (2), the resistance value of the resistor varies with frequency. Additionally, the voltage across the resistor experiences a phase shift relative to the current, which significantly hinders the accurate measurement of AC current and power.

Firstly, the impedance of a resistor varies with frequency, meaning the measured amplitude of the current will differ at different frequencies. Typically, the AC-DC difference is used to express the variation between the resistor's impedance and its DC resistance across different frequencies.

Secondly, there is a phase shift between the voltage across the resistor and the current. According to the AC power formula P = U x I x cosφ, any deviation in the phase measurement of the current will significantly affect the accuracy of the power measurement. The power error caused by phase shifts at different phase differences Δφ is illustrated in Table 1. The table shows that when the phase difference is large, meaning the power factor is very low, even a small phase error can lead to a substantial power error.

Table 1. Power Error Introduced by Phase Under Different Phase Differences

3. Squirrel Cage Coaxial Structure and Its Advantages

TUNKIA has carefully analyzed the issues associated with using standard resistors and has successfully minimized the effects of AC-DC differences and phase shifts on precision measurements. This was achieved through the implementation of a squirrel-cage coaxial structure design and the use of high-quality resistive components.

The structure and appearance of the coaxial shunt are illustrated in Figure 3. It is built using high-precision resistive components and PCB printed circuit boards. An N-type coaxial connector is mounted on disk A, where the current is input through the center of the connector. The current then flows radially from the center of one side of board A to the high-potential side of each strip on the PCB. After passing through the resistive elements, the current returns from the other side of the PCB strip to the low-potential side of board A, and finally returns to the current source through the low end of the coaxial connector. The shunt's output potential is drawn from the C-plate, which is also equipped with a coaxial connector.

Since the current flows in equal and opposite directions on both sides of the parallel PCB, the inductance of the loop is significantly reduced. Additionally, the voltage and current loops are nearly perpendicular to each other, resulting in almost zero mutual inductance. This design allows for much smaller AC-DC differences and phase displacements during high-frequency measurements, enhancing measurement precision.

4. Typical Applications and Precautions for Coaxial Shunts

4.1 Typical Applications of Coaxial Shunt

For broadband current current measurement

For broadband power/energy measurement

For broadband transformer ratio difference and angle difference calibration

4.2 Precautions of Using Coaxial Shunt

In actual measurement, the measured value of AC current is calculated according to formula (4).