Distortions, Distortions, Distortions

I just bought a new MagCheck 95 single access AC magnetic field sensor. I am able to measure the AC characteristics of the diypemf magnetic fields coming out of the coils. The MagCheck plugs into my oscilloscope, and I can see the exact waveform coming out of the coils, with a frequency range of 25Hz to 3000Hz, and up to 50 gauss magnetic field strength.

There are a few factors that cause distortions of the input waveforms – too much volume, too low frequency for some waves, and an unbalanced waveform.

Here is what a clean 500Hz sine wave looks like (the volume on my iPod touch is at 8 out of 10):


When the volume is at to full, the sine wave looks as clipped as follows:


Because of the following attribute of the output of the headphone jack, where the output is decoupled from the headphones with a capacitor as in the following circuit, low frequency signals are distorted as seen in the following graphic:

Distorted Waveform

Here is an example of low frequency distortion as described in the above graphic. Here is a 25Hz square wave, distorted:


Here is a square wave at 500Hz, less distorted:


Here is the Lilly wave at 25 Hz:


Here is the Lilly wave at 500Hz, looking much better:


The capacitor coupled output also causes distortion of the pulse wave. The pulse wave is intended to be a positive offset only waveform.

The audio file for a pulse wave looks as follows:


However, when measured, the output looks as follows:


The capacitor in the output circuit balances the area above zero with the area below zero. Unfortunately, this makes having a north pole only magnetic field impossible unless one adds additional circuitry to add a positive Direct Current offset.

The other benefit of having this new magnetic probe is that I can measure magnetic strengths beyond the 100 milligauss limit of the TriField meter. In the above graphics, 1 millivolt is equal to 1 milligauss. In the clipped sine wave, the magnetic strength is around 300 milligauss, in the pulse wave above, the magnetic strength is around 250 milligauss.

[January 29, 2015 Update] I was measuring the various frequencies and waveforms I used on this blog post with the Spooky2-XM generator, and I noticed the same distortions on the low frequency waveforms. This got me wondering what the MP3 output looks like without the coils, by simply measuring the output out of the headphone jack with my oscilloscope. What I found was surprising, that the output waveforms are very clean even at a low frequency of 25 Hz. The following images are taken directly from the headphone output.

Very little distortion on a 25 Hz square wave:


Very little distortion on a 25 Hz Lilly wave:

25HzBareLillyThe 25 Hz pulse wave, though not perfect, is cleaner than the waveform coming through the coils:



This leads me to believe that the distortions are either coming out of the Alleva-Wave coils themselves, or the MagCheck 95 probe is introducing this distortion. The MagCheck 95 probe documentation states that the probe is highly accurate from 25Hz to 3000 Hz. So perhaps its the coils introducing this distortion.