# Common mode choke measurement – length matters

There must be thousands of Youtube videos of “how to measure a common mode choke” to give a picture of some sort of the test configuration… though most lack important detail… and detail IS important in this case. Likewise there are lots of web pages on the same subject, and some have pics of the test configuration, again mostly lacking important detail.

For the most part, these show test configurations or ‘fixtures’ that might be appropriate for audio frequencies, but are unsuitable at radio frequencies, even at HF.

Connecting wires at radio frequencies are rarely ideal, the introduce some impedance transformation that may or may not be significant to the measurement project at hand. Such connections can be thought of as transmission lines, often mismatched so they have standing waves (meaning the impedance of the load appears to vary along the line.

Let’s take the DUT in my recent article Baluns: you can learn by doing! as an example for discussion.

Let’s take the saved s1p file from a S11 reflection impedance measurement as the example.

Above is a plot of the common mode impedance of the choke, solid line is |Z|, dashed line is R, dotted line is X. This was measured with connecting wires <10mm, see the original article.

Now lets transform that to what we would see if just 100mm of 300Ω lossless line was used to connect the VNA to the balun.

The green curves are what would now be measured by the VNA. Observe the shift in the self resonant frequency (where X passes through zero), observe the shift in frequency of maximum |Z|, and the change in maximum |Z|.

These curves are like they were from different baluns.

Above is an example from a tutorial Youtube video by Fair-rite (a ferrite core manufacturer). Can you work out and draw a schematic of the test fixture? IIRC, the fixture itself was not calibrated, it cannot be because some of it is coax with the shield at one end disconnected.

A clear pic of the detail of all connections and how / where the fixture is calibrated is essential to interpreting any measurements.

A guide I often give people is this:

if you reduce the length of connections and measure a significant difference, then:

• they were too long; and
• they may still be too long.

Iterate until you cannot measure a significant difference.

Make some measurements with different fixture configurations, analyse the results and learn more about fixtures that you will glean from this or probably any written article or Youtube video.

If you believe s21 series through measurement technique or some other technique magically corrects poor fixtures, measure them and critically analyse the results… are they magic?