# Using QA403 to measure impedance of an inductor?

**URL:** <https://forum.quantasylum.com/t/using-qa403-to-measure-impedance-of-an-inductor/856>\
**Category:** QA40x\
**Created:** [April 23, 2023, 6:34pm UTC](https://forum.quantasylum.com/t/using-qa403-to-measure-impedance-of-an-inductor/856 "2023-04-23T18:34:08Z")\
**Posts on this page:** 1\
**Showing post:** 19

<div class="post-metadata">

**Author:** ![Dan](https://avatars.discourse-cdn.com/v4/letter/d/eada6e/32.png) [@Dan](https://forum.quantasylum.com/u/Dan)\
**Post date:** [May 22, 2024, 9:37am UTC](https://forum.quantasylum.com/t/using-qa403-to-measure-impedance-of-an-inductor/856/19 "2024-05-22T09:37:40Z")

</div>

Yes, have been using the QA403 to view the current waveform. It does indeed show a bit of distortion of the input sine. I have not yet tried with a sawtooth. Yes, I think there are a few more useful waveform shapes, a square wave should cause a triangular wave current waveform (remembering my SMPS stuff).

I have a very affordable discrete class-D from a member at diyaudio.

Here is a write-up of the method I’ve been using and it does seem to work well, nothing new here, but a bit easier than reading code I posed in the [other thread](https://forum.quantasylum.com/t/is-the-rest-api-phase-degrees-endpoint-working/1402).

First measure the time-domain voltage v(t) and current i(t) using the QA403. The current is measured by the voltage drop across a current sense resistor in series with the DUT. Shift into the frequency domain with an FFT,

V(f) = \text{FFT}(v(t)) \\ I(f) = \text{FFT}(i(t)) \\

The complex impedance is the ratio of the voltage V and current I (dropping the function-of notation, everything is now in the frequency domain),

Z = \frac{V}{I} = |Z|e^{\textbf{i}\phi}

Using,

\left| Z \right| = \sqrt{ R^2 + X\_L^2 }

\phi = \tan^{-1} \left( \frac{X\_L}{R} \right)

where R is the resistive component and X\_L is the inductive reactance. We have two equations and two unknowns, giving,

X\_L = \frac{\left| Z \right| \tan\left(\phi\right)}{\sqrt{\tan^2\left(\phi\right) + 1}}

R = \frac{\left| Z \right| }{\sqrt{\tan^2\left(\phi\right) + 1}}

therefore the inductance is,

L = \frac{X\_L}{2 \pi f}

---

_[View the full topic](https://forum.quantasylum.com/t/using-qa403-to-measure-impedance-of-an-inductor/856)._