MIL-DTL-24643C
b. Specimens containing more than four but fewer than 21 transmission lines: Four different transmission
lines shall be tested. No fewer than one of these transmission lines shall be selected from each concentric cabling
layer within the specimen.
c. Specimens containing 21 or more transmission lines: the number of transmission lines tested shall be equal
to the square root (rounded, if necessary, to the nearest whole number) of the total number of transmission lines.
Not fewer than one of these transmission lines shall be selected from each concentric cabling layer within the
specimen.
4.9.7.2 Special apparatus. Apparatus shall include the following:
a. Pulse generator: a signal generator which shall produce uniform square wave voltage pulses at a constant
rate of repetition, and which shall be used for the specimen measurements as specified in 4.9.7.3. The pulses
produced by this generator shall each exhibit a 10 to 90 percent voltage rise time of not more than 50 nanoseconds, a
90 to 10 percent voltage decay time of not more than 50 nanoseconds, and a pulse width of not less than 2
microseconds.
b. Oscilloscope: a dual beam oscilloscope shall have a band width of not less than 10 megahertz, and a time
base capable of producing a horizontal beam sweep rate of not slower than 0.1 milliseconds (ms) per centimeter.
This oscilloscope shall have provision for a high impedance, balanced or differential input on both signal channels,
and shall be used for the specimen measurements as specified in 4.9.7.3
c. Pulse transformers: two, winding transformers, each of which shall provide essentially distortion free
transmission of the voltage pulses from the pulse generator, and shall be used for the specimen measurements as
specified in 4.9.7.3. One of the pulse transformers (the "input" pulse transformer, see figure 17) shall have a
primary winding impedance equal to the nominal signal terminal impedance of the pulse generator; the secondary
winding of this transformer shall be centertapped, and shall have an impedance equal to the specified impedance of
the specimen (see 3.1). The other pulse transformer (the "output" pulse transformer, see figure 17) shall have a
primary winding which shall be centertapped, and shall have an impedance equal to the specified impedance of the
specimen (see 3.1); the secondary winding impedance of this transformer shall equal the nominal resistance of the
matching resistor (see d).
d. Matching resistor: an electrical resistor, shall be essentially non-inductive in construction (such as a carbon
type resistor), which shall have a power rating appropriate for its intended use (see 4.9.7.3), and shall exhibit an
electrical resistance appropriate for use with the output pulse transformer (see c).
4.9.7.3 Procedure. The test apparatus shall be electrically interconnected as shown on figure 17; one of the
selected transmission lines shall be connected as shown. The connections between this transmission line and the
pulse transformers shall be as short as practicable. In addition, both connecting cables between the transmission line
and the oscilloscope shall be of identical length and construction and shall not be longer than 5 feet each in order to
minimize pulse distortion. The pulse generator shall then be turned on, and both signal channels on the oscilloscope
shall be adjusted such that the pulse waveform displayed by each on the oscilloscope screen shall be of the same
height. The horizontal beam sweep rate of the oscilloscope shall then be increased, if necessary, and the time
required for the leading edge of each signal channel pulse to rise from 10 to 90 percent of its final value shall be
measured as shown on figure 18. The rise time of the transmission line shall then be calculated by using the
following formula:
Rise time, in ms per 500 feet = 500 (Yr2 - Xr2) 1/2/L
Where:
Xr =
Measured rise time from signal channel number one, in microseconds
Yr =
Measured rise time from signal channel number two, in microseconds
L
=
Measured specimen length, in feet; measurement accuracy shall be within ±1 percent
Following this, the time required for the trailing edge of each signal channel pulse to decay from 90 to 10
percent of its initial value shall be measured as shown on figure 19. The decay time of the transmission line shall
then be calculated by using the following formula:
Decay time, in ms per 500 feet = 500(Yd2 - Xd2) 1/2/L
Where:
Xd =
Measured decay time from signal channel number one, in microseconds
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