VOLTECH AT3600 USER MANUAL TRANSFORMER TESTS
TESTS AND TEST CONDITIONS VPN 98 - 024 PAGE 7.1.3.
* These tests require the use of one or more Voltech DC1000 Precision DC Bias Current Sources. Contact
your Voltech sales office for details.
SELF RESONANT FREQUENCY
Practical inductive components are not perfect inductors; they have stray resistances
and capacitances 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.
X
L
= 2πf
L
X
C
= ½ πf
C
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
problems.
The frequency at which the inductive impedance equals the capacitive impedance (X
L
= X
C
) 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 test frequency.
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.
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