Test and Measurement Precise impedance characterisation
Measuring the impedance of passive components is relevant for electronics developers and component manufacturers. Rohde & Schwarz’s (R&S) LCX LCR meters generate test signals of up to 10 MHz and offer customized measurement functions for transformers
D
esigners of isolated switching mode power supplies (SMPS) need suitable high-frequency transformers. Adapting a design is always a trade-off. Greater
magnetizing inductance tends to increase leakage inductance, especially when different forms of isolation barriers are planned between the primary and secondary winding. This undesired effect may cause power losses and electromagnetic compatibility (EMC) problems from increased interference emissions, especially at higher converter switching frequencies.
Characterise transformers quickly Transformers need to be precisely characterised to estimate their impact on the effi ciency, maximum voltage and EMC
behaviour of switching elements. Besides leakage inductance, other parameters such as primary and secondary inductance, winding capacitance and winding resistance are relevant and necessary for high-quality designs. LCR meters from the R&S LCX instrument family are ideal for such measuring tasks. The meters also provide other parameters typical for transformers that users would have to calculate themselves, such as mutual inductance and turns ratio.
Two models cover basic and advanced measurement requirements
The LCR meters were developed to precisely measure all standard passive components. They use auto-balancing bridge technology to provide accurate impedance measurements of passive components. The instruments have a user-friendly 5-inch colour LCD touchscreen. Extensive remote control functions enable use in automated systems and operation via a web interface.
Fig. 1. The high-resolution display is used for graphical presentations. This example shows traces for impedance measurements of a capacitor.
The instruments come in two models, providing all the modern measurement functions needed in research, development or production. The R&S LCX100 has a test signal frequency range from 4 Hz to 300 kHz for basic development lab applications. The base configuration of the R&S LCX200 has an upper frequency limit of 500 kHz that can be extended to 1 MHz or 10 MHz to determine the self-resonance frequency of components. Both instruments have an impedance measurement range from 10 mΩ to 100 MΩ and a basic impedance measurement accuracy of ±0.05 per cent. Comprehensive analysis functions and versatile selection of test fixtures enable the meters to be used to measure a broad range of components.
Fig. 2. Transformer parameters (turns ratio, mutual inductance, phase angle) on the display with the R&S LCX- Z5 test fi xture.
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Fully automated component sorting LCR meters open the door to process automation. The R&S LCX-K107 option expands the meter interfaces with a trigger input and a digital I/O port for eight data lines. This allows components to be sorted in up to eight classes (binning) based on their measurement values. One criterion could be a tolerance value that separates high-precision capacitors from standard ones. A handling robot can insert the components into test fi xtures to automate component production. As part of a larger test system, the rack adapter (R&S ZZA-GE23 option) can
transform the desktop device into a 19" rack unit. The standard logging function records ten measurement values per second and can transfer them to the instrument memory, a USB fl ash drive or to an external PC via a LAN, USB or optional GPIB interface. The fast (≤ 15 ms), medium (≤ 100 ms), and slow (≤ 500 ms) measurement times allow fl exibility. Users can then determine the best trade-off between required measurement accuracy and available test speed.
Advanced analysis functions An LCR meter is mostly used for measuring impedance values for a component with a fi xed frequency and level. However, beyond the measurement point, impedance can vary depending on the DUT, making it necessary to perform measurements at various frequencies and levels.
The R&S LCX-K106 option can perform local dynamic impedance measurements, where test signals with different frequencies are generated. The R&S LCX sweep tool can also be embedded. The software runs on an external measurement PC and enables sweep measurements with an R&S LCX LCR meter. The frequency or level of the test signal or the DC bias voltage can be selected as sweep parameters. The tool plots the measurement results in tables or graphs, such as a Nyquist diagram. The LCR meter can generate test signals from 100 mV to 10 V and DC bias voltage up to 10 V and up to 40 V with the R&S LCX-K108 option using an external source.
Summary
The R&S LCX LCR meters precisely measure the impedance characteristics of passive components such as capacitors, inductors, resistors and transformers. The R&S LCX100 and R&S LCX200 comply with all standard requirements for laboratory component testing, high-end research and production applications. Numerous measurement and analysis functions along with user friendliness make them unique, according to Rohde & Schwarz. The LCR meters can measure various transformer properties to ensure the quality of customized transformer designs.
www.rohde-schwarz.com Components in Electronics April 2023 49
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