Voltech AT5600, Руководство пользователя

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C H A P T E R 7 – T E S T S A N D T E S T C O N D I T I O N S
AT5600 User Manual 98-119 issue 14
Page 94
7.2. 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
= 2𝜋𝑓𝐿 (𝐼𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑒 𝐼𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒)
𝑋
𝐶
=
1
2𝜋𝑓𝐶
(𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑖𝑣𝑒 𝐼𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒)
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.