MIL-DTL-3890C
4.5.4 Characteristic impedance (see 3.7). The characteristic impedance of the lines shall be measured
with Time Domain Reflectometry (TDR) equipment or other methods that are approved by the procuring
activity. The TDR equipment shall have a total system response of 150 pico-seconds and shall provide
for a resolution of 0.1 ohm throughout the length of the line. The test results shall be displayed
on calibrated rectangular coordinates and a permanent record of the test results shall be produced. A
section of air line with a calibrated impedance of 50.0 ± 0.15 ohms is required as a reference.
4.5.5 Velocity of propagation (see 3.8). The velocity of propagation of the lines shall be determined
as follows:
4.5.5.1 Test equipment. The following test equipment shall be utilized:
a. A microwave sweep oscillator with a frequency range that covers the specified frequency range of
the assembly.
b. An analyzer with suitable readout features.
c. A phase bridge with a measurement accuracy of ± 0.1 degree.
d. A calibrated section of air line whose length equals the length of the line under test.
4.5.5.2 Test procedures:
a. Calibrate the system over that portion of the specified frequency range of the line that will be
within the limits of 4.5.5.1c with required adapters inserted in the leg of the phase bridge to be
used for the line.
b. Insert the line into the appropriate leg of the phase bridge and an equal length of air line in the
other leg. Note the length (L) of the line.
c. Using the phase bridge, adjust the electrical length of the air line leg to obtain a phase balance
across the frequency range of 4.5.5.2a. Note the added length (ĆL).
d. Calculate velocity of propagation (VP) of calibration line as follows:
�
� �100%
�� = �
� + �
e. Sweep over the specified frequency of the assembly. Note maximum deviation (φ in degrees)
from calibration line.
f. Calculate minimum velocity of propagation VP (min) as follows:
L
VP(min) =
( )
φc
L + ĆL +
360 f
where:
C is the velocity of propagation in free space
f is the frequency (in Hz) at which φ was measured
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