MIL-PRF-15305F
4.6.5.1.1.1 Effective inductance. Effective inductance shall be determined as follows when using test fixtures TF-A
or TF-B, as applicable on figure 3 or 4. The appropriate test fixture shall be inserted in the Q-meter coil terminals with
the side showing the test-fixture letter facing the capacitance terminals. The appropriate shorting bar conforming to
figure 5 or 6 shall be inserted in the clips of the test fixture in such a manner that the terminals rest firmly against the
stops, and so that the bar is centered between the test-fixture terminals. The Q-meter capacitance dial shall be set at
400 picofarads (pF) and the vernier capacitance dial at zero. The Q-meter shall then be resonated using the frequency
dial until a peak reading is obtained. This frequency shall be monitored in order to obtain an accuracy of 0.1 percent.
This resonant frequency value in MHz shall be recorded. The main capacitance dial shall be calibrated periodically in
accordance with a routine calibration program for test equipment. The sum of the residual inductance of the Q meter
and the inductance of the test fixture shall be calculated from:
1
LCF =
- LBAR
2
4š
f
2
c
Where: LCF = inductance in µH of the test fixture and residual inductance of the Q meter.
f
= frequency in MHz.
c
= capacitance in microfarads (µF).
LBAR = calculated inductance in µH of the shorting bar as determined from the following formula:
d
4
-1+
0.0022.303 log10
LBAR =
2
d
Where: ℓ = length of wire (cm).
d = diameter of cross section (cm).
The shorting bar shall then be removed from the test fixture and the Q meter frequency shall be set to the frequencies
specified in 4.6.5.1.1. The coil under test shall then be inserted in the test fixture in such a manner that the leads are
straight and rest firmly against the stops, and so that the unit is centered between the text-fixture terminals. The L-C
dial of the Q meter shall then be tuned until the resonance meter indicates a peak reading. The inductance (Ld) shall be
read directly on the L-C dial, using the inductance scale and the effective inductance (see 3.1), of the coil calculated
from the formula:
L = Ld - Lcf
Where:
L = effective inductance in µH of coils.
Ld = inductance dial reading in µH.
Lcf = correction factor for inductance of test fixture and residual inductance of the Q meter in µH.
4.6.5.1.2 Effective inductance for cylindrical coils (inductance greater than 10 µH). Test as specified in 4.6.5.1.1,
except that no allowance is made for residual inductance of Q meter and inductance of test fixture. Frequencies to be
used for testing various ranges of inductance shall be as follows:
Inductance range, µH
Reference test frequency (MHz) for 260 A
Above 10.0 to 100.0, inclusive
2.5
Above 100.0 to 1,000.0 inclusive
0.790
Above 1,000.0 to 10,000.0 inclusive
0.250
Above 10,000.0 to 100,000.0 inclusive
0.079
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