THD testing is something that I have been eyeing for a while….

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You are more likely to see incremental updates and projects not even mentioned here (yet). Mainly, I post milestones on here, once fairly polished. If you want to see more of the inner-workings (i.e. screw-ups and then revisions) of my projects, check out the FB page too! I freely share my designs, to some extent.

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Now, on adding Total Harmonic Distortion measurements, I want to first mention a few things. You can achieve this in many ways, although many of them are very expensive (dedicated testers). Ones like what you see below, can be achieved for $30-$60 (depending on the capacitance range/s needed and THD measurement resolution). It can be higher, much higher, especially if you need to make a reference bridge for 10-1000uF and you pick x20 Black Gate™ NX, you will have hundreds of dollars invested. I’d be interested to try that range, but I’d probably use Panasonic FM.

To start out, I will likely do 0.01uF to 0.68uF, I’ll use 20-30 capacitors, wired in a parallel configuration (capacitances add in parallel) with SPDT or SPST switches, as shown below.

Example of a x15 capacitor quadrature bridge for THD measurements

MultiSim is the software you are seeing above,

MultiSim (By NI) is what most of professional engineers are using, or PSpice. I prefer MultiSim, because I have years of experience using it. I am still learning, don’t get me wrong, but I know how to do nearly all types of simulation testing/analysis types.

MultiSim is limited to -100dB or 0.001% THD and due to this, if I just wire up this circuit as intended, we’ll just see 0.000%. This isn’t very useful, as many things could happen and you’d still see that reading.

THD Circuit Simulation, without Noise added in (F-Gen off)
Testing THD with the Noise Source Disabled

To ensure things were functioning as I intended them to, I added a function generator. You need to put this directly on the output of the DUT arm, not before the capacitor (or else it will filter it and bypass the point of doing it). I set mine for 3kHz and about 500mV. Our source signal is +- 5V and 1kHz, so this was exactly on the 3rd Harmonic, normally, the most problematic in audio circuits (2nd is also common).

Quote of my FB post

Essentially, you are creating a near ideal capacitor (using many low THD, like C0G or PP Film) by using many caps in parallel. Along with a 100ppm (or less) resistors in series. The resistor arms use multiple resistors in series, to create the lowest THD as possible. Groner says the voltage coefficients of the resistor arms are negligible for all 100ppm types he tried.

“As there is some correlation between the power coefficient of R1 (ref arm) and R2 (DUT arm), the net distortion is even lower due to common-mode rejection.” – S Groner

Then you can isolate the difference between your reference arm (which is near ideal) and the DUT arm, by looking at the differential of the two signal

K. Hallman and S. Groner

Stay tuned as this design is constantly updated, until we have something that performs up to my desired quality level of -170 to -180dB. I will likely just update this post, so you’ll have to check back if interested in this. I’ll make a new post/page, when it’s completed.

Note: If all goes well, I will sell these boards (unpopulated). It will require different designs, for different capacitance ranges. This is due to the different capacitor sizes, lead spacings and lead diameters.

Join the discussion and let HL know what you think!

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