Wed. Sep 18th, 2019


Shenango Valley Makers

Use an STM32 MCU to Build a Low-Cost FFT LCR Meter

London-based electrical engineer Adil Malik needed a good LCR meter for his lab at home, but instead of buying one, he create his own for around $67, a fraction of the cost for a decent meter.

“The approach I took was a mixed-signal one where a capable analog front end would be paired up with a beefy DSP processor to compute the Impedance. Most importantly, in this scheme, the DSP is responsible for discriminating the phase between the sampled voltage and current waveforms; this approach is preferred because it leads to good accuracy and calibration stability.”

The FFT LCR Meter features a Nucleo-F446RE development board and a custom PCB to measure inductance, capacitance, and resistance of electronic equipment. (📷: Adil Malik)

On the DSP end, Malik went with ST’s Nucleo-F446RE, which packs an STM32 microcontroller in an LQFP64 package with Arduino Uno connectivity for adding additional expansion boards. The board has specific features that make it great for use as an LCR meter, including an external SMPS to generate Vcore logic supply, 24MHz HSE, and is Arm Mbed ready. On the analog side, Malik designed a custom PCB outfitted with four SMA connectors and a four-wire Kelvin adapter to measure leaded components.

The LCR meter with analog front-end wired to the Nucleo-F446RE with Kelvin adapter. (📷: Adil Malik)

The LCR meter measures current with a TIA (Tansimpedance Amplifier), which uses a four-to-one multiplexer (MUX) in the feedback loop, allowing users to select one of four different precision shunt resistors for the various ranges. Malik documented a great walkthrough for his Low-Cost High-Accuracy FFT LCR Meter on his website for those who would like to recreate his build. Unfortunately, there are no schematics or source code available at this time.

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Author: Cabe Atwell