Broadband Current Measurement: Resistance Method and Coaxial Shunts

1.   Introduction

    In broadband current measurement scenarios such as the calibration of power standard sources, harmonic sources and power quality standard sources, it is often necessary to measure broadband currents below 100 A and 100 kHz. If a digital     multimeter is used directly when calibrating, there are often problems in the following aspects:

    1) Limited range: Digital multimeters can only measure a maximum current of 20A in general, and some models can even measure only 2 A;

    2) Low AC accuracy: Taking Keysight 3458A as an example, when the measured AC current is 1A, the best accuracy is only about 0.1%;

    3) Reliability: Usually the impact resistance and durability of high-precision digital meter current measurement are much lower than that of its voltage measurement.

    Therefore, it is necessary to use the conversion method, that is, to convert the high current into a small current or small voltage signal before measurement. Currently commonly used methods include resistance method, transformer method, comparator method, etc.

    This article mainly discusses the resistance 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).

    It can be seen from formula (2) that the resistance value of the resistor changes with frequency, and at the same time, the voltage at both ends of the resistor has a phase shift relative to the current, which is very detrimental to the precise measurement of AC current and power.

    First of all, the impedance value of the resistor changes with frequency, so the measured amplitude of the measured current at different frequencies will also be different. Usually the AC-DC difference is used to represent the difference between the impedance and DC resistance of the resistor at different frequencies.

    Secondly, there is a phase shift between the voltage at both ends of the resistor relative to the current. From the AC power P=U×I×cosφ, it can be known that if there is a deviation in the phase measurement of the measured current, it will cause greater interference to the power measurement. The power error caused by the phase under different phase differences Δφ is shown in Table 1. It can be seen from the table that when the phase difference is large, that is, the power factor is very low, even a small phase error can bring about a very large power error.

Table 1. Power error introduced by phase under different phase differences

3. Squirrel-Cage Coaxial Structure And Its Advantages

    TUNKIA has analyzed the above-mentioned problems in the use of standard resistors, and effectively reduced the impact of AC and DC differences and phase differences on precision measurements through squirrel-cage coaxial structure design and selection of high-quality resistive components.

    The structure and appearance of the coaxial shunt is shown in Figure 3. It is constructed of high-precision resistive components and PCB printed circuit boards. An N-type coaxial connector is installed on disk A. The current is input from the center point of the coaxial connector of board A, and then flows radially from the center of one side of board A to the high potential side of each strip PCB. 

    After flowing through the resistive element, it returns to the low-potential end surface of the A board from the other side of the strip PCB and eventually the current source is returned through the low end of the coaxial connector. The output potential of the shunt is brought out from the C-plate 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 greatly reduced. In addition, the voltage loop and the current loop are nearly perpendicular to each other, and the mutual inductance between them is almost zero. Therefore, smaller AC and DC differences and phase displacements can be obtained during high-frequency measurements.

4. Typical Applications And Precautions For Coaxial Shunts

4.1 Typical Applications

4.2 Precautions For Use

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