MIL-PRF-15305F
4.6.5.1.3 Effective inductance for radial lead coils (inductance 0.10 to 10.0 µH, inclusive). The test shall be
performed as specified in 4.6.5.1.1.1, except that test fixtures TF-C, TF-D, or TF-E, on figure 7, shall be used. The
shorting bar for use with these test fixtures shall be made of AWG size No. 18 solid copper wire approximately 1.25
inches long, and shall be bent as required.
4.6.5.1.3.1 Effective inductance for radial lead coils (inductance greater than 10 µH). The test shall be performed
as specified in 4.6.5.1.2.
4.6.5.2 Q of coils. The test shall be performed using a Q meter such as Hewlett Packard model 260A, HP4342A,
HP250RX meter or equivalent. Suitable means shall be used to calibrate the frequency dial of the Q meter and the Q-
standard within ±0.1 percent of the applicable test frequency. Frequencies to be used for testing the various ranges of
inductance shall be as specified in 4.6.5.1.1 and 4.6.5.1.2. The appropriate test fixture, as applicable, as shown on
figure 3, 4, or 7, shall be assembled to the coil terminals of the Q meter, with the side showing the test fixture letter
facing the capacitance terminals. The unit under test shall then be inserted into the test clip in such a manner that the
leads are straight and rest firmly against the stops, and so that the unit is centered between the terminals. The Q shall
then read on the Q voltmeter.
4.6.5.3 Self-resonant frequency. Unshielded coils shall be placed in the field of a variable-frequency oscillator,
such as Measurements Corporation megacycle meter, model 59, or equivalent. The oscillator shall include a device for
indicating the relative amount of power absorbed from the field (e.g., a grid-dip meter). The unit under test shall be
placed on the appropriate test fixture shown on figure 8 or 9. Units shall be suspended or supported a minimum of 1.50
inches from any surface other than the test-fixture supports or oscillator coil. The frequency of the oscillator shall be
varied through the frequency range near the self-resonant frequency specified (se 3.1 and 6.1). At any frequency in
the frequency range where an abrupt increase in power absorption is indicated, the coupling between the oscillator coil
and the unit under test shall be decreased, by increasing he separation between the coils, until a moderate dip in grid
current results when tuning to this resonance. This frequency shall be considered the self-resonant frequency for the
unit, and shall be accurately determined by suitable means to within ±0.2 percent. A check shall be made for spurious
indications due to a resonance not associated with the unit under test, by removing the unit from the field (at
frequencies below 2.5 MHz, any suitable method may be used). Coils which cannot be resonated in this manner,
however, when electrostatic shielded coils tested, the shield of the coil shall be grounded. The following method may
be used as an alternate method of measurement of the shielded coils (see 3.8.3).
4.6.5.3.1 Alternate test method. When unshielded coils under test cannot be resonated by the method specified in
4.6.5.3, the test shall be performed using the instruments specified in 4.6.5.2 or equivalent. The coils shall be mounted
in the appropriate test fixture, as applicable, as shown on figure 3, 4, or 7, with the test-fixture letter facing the
inductance terminals. The tuning capacitor of the q meter shall then be set to approximately 400 pF, and the Q circuit
shall be resonated by adjusting the oscillator frequency of the Q meter. The unit under test shall then be replaced with
a shielded comparison coil having an inductance about 1/25 that of the unit under test, or a coil that will resonate in the
Q circuit at a frequency about 10 times the initial resonant frequency. The Q meter shall then be set to a frequency
approximately 10 times the initial resonant frequency, and the Q circuit shall then be resonated at this new frequency.
(This factor of 10 is based on the distributed capacitance of the unit under test being in the region of 4 pF, which is
common for small coils. Higher distributed capacitances will lower the resonant frequency of the unit under test, and a
factor smaller than 10 will prevail.) The unit under test shall then be connected across the capacitance terminals of the
Q meter, taking care to avoid coupling between the unit under test and the comparison coil. The Q circuit shall then be
re-resonated by means of the Q-tuning capacitor or the vernier-tuning capacitor, observing whether the capacitance
has to be increased or decreased from its previous value, in order to restore resonance. If the capacitance has to be
increased, the oscillator frequency shall be increased by 10 to 20 percent. If the capacitance has to be decreased, the
oscillator frequency shall be decreased by the same amount. The unit under test shall then be disconnected from the Q
meter, and the Q circuit shall be resonated to the new frequency by means of the Q-tuning capacitor. The previous
procedure shall then be repeated, while at the same time changing the oscillator frequency by smaller increments as it
approaches the resonant frequency of the unit under test, until the frequency reaches a value at which the Q-circuit
capacitance is unchanged when the unit under test is connected or disconnected. The self-resonant frequency of the
unit under test will then be the frequency of the oscillator and shall be accurately determined to within ±0.2 percent
(see 3.8.3).
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