Measuring Inductance of Planar Spirals

Hello,

I am currently measuring the inductance of spiral inductors on flexible plastic (no ground plane) using a VNA in the frequency range of 500KHz to 30MHz.

The values of inductance that are expected lie within the 100nH to 800nH range; the measured values are in within a reasonable error % (maximum 10%).

1. However, when I am trying to measure them at lower frequencies (around 500KHz) I am unable to obtain a constant measurement as the values of inductance jumping all over the place (say from 490nH to 520nH for one sample as an example). Is this normal?

2. I also observe that for each sample the inductance value (reactance) falls as frequency increases up to about the 10Mhz and then starts to increase again above 10MHz. I thought it was just meant to keep on falling. Is this right?


3. Is this a normal situation for measuring these type of structures?

4. Would I be better trying to measure the DC inductance for an absolute value?


Sorry for all the questions, I'm a bit stumped here.

Thanks for your time

Catalyst

Your VNA is not precise that low in frequency. Try using a bridge type LCR meter if available. Maybe turn on averaging or smoothing in the VNA but that is like averaging a bad measurement. The results are pretty but the measurment is garbage. Measure a lumped inductor of known value and you will probably get similar, inaccurate results.

The "measured" inductance should start off flat, then increase with frequency due to the self resonance with interwinding capaitance. In reality the indudctance pretty much stays flat, it only appears to change because of the capacitance. After the first self resonance (which is parallel), inductance will appear to decrease and you will hit the second, series resonance. You typically don't use inductors past the first self resonance so this data is typically not provided. Now that slight downward slope may be due to skin effect, I'm not sure.

You should measure a known, good inudctor to check the accuracy of your method.

Hi!

Could you describe coil geometry?
It is possible make a analytical analysis.

regards