1.6 Self Resonant Frequency
Practical inductive components are not perfect inductors; they have stray
resistances and capacitance associated with them.
For certain components, especially those with a low inductance value, the
impedance of the stray capacitance can become significant when compared to
that of the inductance.
XL = 2πf
XC = 1/(2πf)
At a sufficiently high frequency, the capacitive impedance can dominate,
making a measurement of the inductance impossible.
Under these circumstances, any measurement instrument may report negative
inductance values and measurement errors.
Should these symptoms be observed, reduce the test frequency to avoid
The frequency at which the inductive impedance equals the capacitive
impedance (XL = XC) is known as the self-resonant
frequency (SRF) of the component.
At this point, the phase angle of the impedance (which can be measured
using the ANGL test) is zero.
At test conditions where the frequency is low enough for problems with
capacitive impedance to be negligible, the phase angle will be positive
and close to 90 degrees.
Therefore an ANGL test can be used during program development to confirm
if measurement problems are due to the chosen test frequency approaching
the SRF of the part under test.
If the angle is significantly less than 90 degrees, consider reducing the
Note that stray fixture capacitance will add to the capacitance of the
component and reduce the SRF.
Performing compensation will remove the effect of stray fixture
capacitance on the measurement of capacitance, but cannot remove its
effect on SRF.