2016/07/22

Hameg HM8118 LCR bridge

tl;dr: I made a control program for Hameg HM8118 LCR bridge. You can get it here: https://github.com/kaktus85/Hameg8118

Controlling Rohde & Schwarz Hameg 8118 LCR Bridge/Meter from PC


Introduction 


There's a lot of LCR bridges on the market from simple fixed-frequency el cheapo meters made in country where pandas kiss good night to high end thousands of dollars meters made in… well, the very same place.

Somewhere in the middle lies Hameg HM8118 LCR bridge from Rohde & Schwarz for about 2300 USD. While the less advanced models only have a couple of frequencies at best, this baby has 69 of them. That makes it suitable for measuring a frequency sweep and then fitting the data to a more complicated equivalent circuit than the two-component model you can use for a single frequency. The frequency range is 20 Hz to 200 kHz and the device has specified accuracy to 100 kHz so the frequencies between 100 kHz and 200 kHz are more of a "bonus". But that does not mean they are completely useless. Just that you can't expect them to be super-accurate.

Hameg HM8118

Operating principle


So, how does it work? Normally, the bridge measures the voltage and current through the device under test (DUT), which is typically a resistor, capacitor or inductor. Besides amplitudes, it also measures the phase and from the ratio of amplitudes of voltage and current and the phase difference, it then calculates the impedance. Either as magnitude-phase or resistance-reactance. It is just a different representation of the same quantity. Then, it can apply the knowledge of impedance together with the frequency on a simple two-component model such as a combination of resistor and capacitor or resistor and inductor. Plus, these components can be connected in series or in parallel. That is all math. The device calculates what would be the values of the resistor and capacitor/inductor if they've been connected in series or in parallel and the whole ciruit had the measured impedance at the set frequency. 

But as you only measure two quantities (voltage and current), you can only calculate two unknowns. If the circuit is more complicated, you need measurements at more frequencies. Preferably, you measure all the 69 available frequencies and then fit the experimental data to the equivalent circuit. Not a trivial task to do but possible.

This is the basic operating principle of this LCR bridge. It is a nice unit that, sadly, does not come with a desktop software so you have to rely on the front panel display and buttons which make complicated measurements tedious. Yet, you can connect the device via USB and switch it to a virtual COM (serial) port. Then send commands and read measured values.

Controlling from PC


I have used the serial port feature to create a simple control program in C# (.NET 4.6). It should work on any computer running Vista or higher. It is open source and released under GPL v.3. The project, which also contains the only executable file you need (Hameg8118.exe) is available on my GitHub account.

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