MIL-DTL-24640C
Where:
K1
=
The percent of capacitance unbalance of conductor No. 1 in relation to conductor No. 2.
K2
=
The percent of capacitance unbalance of conductor No. 2 in relation to conductor No. 3.
K3
The percent of capacitance unbalance of conductor No. 3 in relation to conductor No. 1.
=
The mutual capacitance and, if specified (see 3.1), the capacitance unbalance of every other transmission line shall
also be measured and calculated in an identical manner.
e. The mutual capacitance and, if specified (see 3.1), the capacitance unbalance of every transmission line
shall be determined, using the same method as used in (d) above, for specimens incorporating individually shielded,
insulated conductor triad transmission lines with the following change: the overall specimen braid (if any) and all
specimen conductors except the conductor triad under test shall be connected together electrically and considered to
be the shield of the transmission line under test.
4.8.2.4 Observation. Specimen failure shall be construed if the specimen exhibits a mutual capacitance or
unbalance capacitance which is greater than that specified (see 3.1).
4.8.2.5 Additional testing of specimen. If the characteristic impedance and/or mutual inductance tests are
specified (see 3.1), then the specimen shall be retained for use in those tests.
4.8.3 Characteristic impedance. This test shall determine the characteristic impedance encountered by signals
propagating along completed cable. The mutual capacitance (see 4.8.2) shall be calculated for each transmission
line within the completed cable prior to performing the characteristic impedance test. This is necessary since values
of mutual capacitance are used to calculate values of characteristic impedance.
4.8.3.1 Specimen. The specimen shall be that which was previously used to determine capacitance
characteristics (see 4.8.2.1).
4.8.3.2 Special apparatus. Apparatus shall include an instrument for measuring inductance (such as an
inductance bridge, Q meter, or other instrument) which shall support the requirements as specified in 4.8.3.3. This
apparatus shall have a two terminal measurement port, which shall use sinusoidal wave of the specified frequency
(see 3.1) for making measurements. Measurements shall be made within an accuracy of ±1 percent.
4.8.3.3 Procedure. Select and perform the appropriate characteristic impedance test method from the
following:
a. Specimens incorporating any number of individual shielded, insulated conductor transmission lines shall be
laid straight. At one specimen end the exposed conductor of each transmission line shall be electrically connected to
the pigtail of its associated shield by tightly twisting both together. At the opposite specimen end, one transmission
line shall be selected for measurement, which shall then be connected to the inductance measuring instrument. The
inductance of this transmission line shall be measured. In an identical manner, the inductance of every other
transmission line within the specimen shall also be measured. The characteristic impedance of each transmission
line shall then be calculated by using the following formula:
Characteristic impedance, in ohms = (105L/Cm)1/2
Where:
Cm
=
Mutual capacitance, in picofarads per foot, as calculated in 4.8.2.3
L
=
Measured inductance, in micro henries/10 feet.
b. Specimens incorporating any number of individually shielded or unshielded insulated conductor pair
transmission lines shall be laid straight. At one specimen end the two exposed insulated conductors of each
transmission line shall be laid adjacent to each other (to minimize superfluous inductance) and electrically connected
by tightly twisting both together. At the opposite specimen end, one transmission line shall be selected for
measurement, and its two conductors connected electrically to the inductance measuring instrument. Its shield (if
any) shall remain disconnected. The inductance of this transmission line shall be measured. In an identical manner,
the inductance of every other transmission line within the specimen shall also be measured. The characteristic
impedance of each insulated conductor pair shall then be calculated by using the formula shown in (a) above.
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