NanoVNA-H4 – inductor challenge – part 4

NanoVNA-H4 – inductor challenge – part 3 visited the basic model of an inductor as comprising a series resistance and inductance, and its failure above perhaps SRF/5, and proposed a simple extension that may give useful prediction of impedance up to SRF and a little above.

Importantly though is that it showed that measurement of Z departed from a frequency independent inductance \(X \propto f\) and some small frequency dependent resistance \(R \propto \sqrt f\) above perhaps 15% of SRF… and so we cannot simply infer the value of the underlying inductance from Z at an arbitrary frequency.

Where \(X \propto f\) we can say that \(L=\frac{X}{2 \pi f}\) (where X is the imaginary part of measured Z).

Let’s return to the plot of L from NanoVNA-APP’s interpretation of measured Z.

At lower frequencies where the plotted value of L is independent of frequency (ie a horizontal line) we can infer that the underlying inductance of the inductor is that value, 20µH in this case (an air cored solenoid).

Alternative instruments.

Cheap Chinese LC meter

This would appear to be a knock off of the AADE meter, cost $25 inc shipping some years ago.

Above, when properly calibrated, it reports 19.8µH, and a press of the F button reveals the measurement is made at 660kHz (in this case). At 660kHz the effects of self resonance are negligible. Displayed L is the underlying inductance.

DE-5000 LCR meter

This is a meter in the $150+ range, a traditional LCR meter.

Here the measurement is made at the selectede 1kHz frequency, so the effects of self resonance are negligible. Displayed L is the underlying inductance.

Summary

  •  The air cored inductor is more completely a resonator and can be approximated adequately by simpler representations at different frequency ranges.
  • There are many ways to “measure inductance”, but understanding the nature of an inductor is essential to properly interpreting measured values.
  • In some instances we want to know the underlying inductance, in some we want to know the equivalent series inductance or parallel inductance at some frequency… they are different, the application matters.

The next article will look at the concept / definition of Q, and measuring Q of an inductor. Is it simply the number read out from the NanoVNA?

Continued at NanoVNA-H4 – inductor challenge – part 5 .