Accurate measurement of inductors & capacitors, including their quality factor (Q)

The measurement accuracy of the secondary parameter is primarily based on phase shift accuracy, which remains roughly constant across the range. However, since Q is the tangent of the dissipation angle, phase measurement errors near 90° are greatly amplified.

It’s important to distinguish between immediate measurement accuracy and long-term drift. The latter affects the absolute measurement accuracy and explains the large tolerance range for the secondary parameter Q.

The effect is visible during warm-up: a test device with inductor 2 initially shows >450, but it slowly drops to 350–410 over the warm-up period, depending on the device’s condition. Afterwards it remains stable, changing only ±10 over an hour. This remaining variance is comparable to effects caused by cable positioning, contact points, or proximity to metal at high frequencies like 260 kHz.

Thus, short-term accuracy is much better than long-term accuracy—but it’s relative, not absolute. To leverage this, a reference standard optimized for actual usage—not just theoretical matching—is required.

Greater absolute accuracy is only achievable using specialized Q-measurement gear or classic “ring-down in a resonant circuit” methods.

Two approaches using a reference standard:

1.) Apply LOAD correction in addition to Open/Short

LOAD correction can only be done at specific frequency points. Set up SPOT No to 260 kHz, turn ON, perform MEAS OPEN and MEAS SHORT, then connect the test component and execute MEAS LOAD. The fields LOAD A/B show uncorrected values; enter correct REF A/B (e.g. measured inductance for REF A, expected value for REF B), then activate LOAD. The instrument will now display reference values within its relative accuracy.

2.) Switch to deviation measurement

On the ST2827 (see Section 3.5.8), go to MEASURE SETUP, measure REF A and REF B using MEAS, then move to DEV A/B and set to ΔABS or Δ%. The meter then displays absolute or relative deviation from the reference.

Note: To maintain test voltage, ALC must be ON in MEAS SETUP; otherwise effective voltage may drop at higher DUT impedance. Primary parameters (inductance) are less affected by drift than secondary parameters, giving better absolute accuracy, though measurement cables and metal proximity may still influence results, especially at high frequency.

See the illustrations for examples of how D and Q vary during sweep measurements of capacitors and inductors.

Tags: