MIL-DTL-24643C
4.9.6 Mutual inductance. This test shall determine the ability of shielded, insulated conductor pairs within
completed cable incorporating both shielded, insulated conductor pairs and unshielded insulated conductors to
minimize crosstalk caused by the inductive effects of signal currents flowing in adjacent insulated conductors.
4.9.6.1 Specimen. The specimen shall consist of a 120±½ inch length of completed cable, which shall have all
overall components (such as armors, jackets, or overall binders) and all shields removed from each end for a
distance of 1½± inch, except that shields may be pushed back for this distance rather than removed. The
insulation shall then be stripped from each end of each insulated conductor for a distance of ½± inch, and both
insulated conductors within each shield shall be electrically connected by twisting each conductor pair tightly
together at one specimen end.
4.9.6.2 Special apparatus. Apparatus shall include the following:
a. A sinusoidal signal generator, which shall produce an unchanging signal at a constant frequency of
1000±200 Hz, which shall produce a signal current appropriate for use in the test as specified in 4.9.6.3 (a signal
current of approximately 1 A is typically found necessary), and the signal frequency and signal current shall be
known within an accuracy of ±2 percent.
b. A rms indicating microvoltmeter, intended for use at the signal generator frequency, exhibiting a signal
input impedance not less than 10,000 ohms, shall be used for the specimen measurements as specified in 4.9.6.3, and
shall exhibit a measurement error of not more than ±6 percent.
4.9.6.3 Procedure. One shielded, insulated conductor pair within the specimen shall be selected, and the
microvoltmeter shall be electrically connected across this conductor pair at its unshorted end. The shield shall
remain disconnected. The signal generator shall then be electrically connected across any two unshielded, insulated
conductors at the specimen end opposite that of the microvoltmeter connection. These same two unshielded,
insulated conductors shall be connected together electrically at the opposite specimen end (the end with the attached
microvoltmeter) by twisting their two conductors tightly together. The signal current from the signal generator shall
next be adjusted until a reasonable indication is noted on the microvoltmeter, and both the microvoltmeter reading
and the signal current magnitude shall then be recorded. In a similar manner, additional measurements shall be
taken such that microvoltmeter measurements are taken for each shielded, insulated conductor pair as the signal
generator is sequentially reconnected to circulate signal current in each possible pairing of unshielded, insulated
conductors. (Examples: cable Type LSMCOS-5 shall require three measurements, since it incorporates one
shielded, insulated conductor pair plus three unshielded insulated conductors, cable Type LSMCOS-6 shall require
two measurements, since it incorporates two shielded, insulated conductor pairs plus two unshielded, insulated
conductors.) The mutual inductance represented by each measurement shall then be calculated by using the
following formula:
Mutual inductance, in microhenries = V/2 * 3.14159 fI
Where:
V
=
Measured potential between two shielded, insulated conductors, in microvolts
f
=
Signal frequency, in Hz
I
=
Measured signal current, in A
4.9.6.4 Observation. Specimen failure shall be construed if any calculated value of mutual inductance is
greater than that specified (see 3.1).
4.9.7 Pulse response time. This test shall determine the ability of completed cable which incorporates one or
more shielded, insulated conductor pairs for pulse data transmission to propagate a square wave signal without
causing unacceptable signal distortion.
4.9.7.1 Specimen. The specimen shall consist of a piece of completed cable, which shall have a length of not
less than 200 feet. Individual transmission lines within this specimen (where each insulated conductor pair plus
surrounding shield is considered to be a transmission line) shall be selected for testing as follows:
a.
Specimens containing four or fewer transmission lines: every transmission line shall be tested.
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