MIL-DTL-18266C
4.6.4 Frequency response (see 3.4.1). The frequency response of the transformer within the cord shall
be measured as follows:
a. Connect the tip and sleeve of the plug across the 600 ohms output terminals of the audio oscillator.
In the event the audio oscillator has no 600 ohms terminals, an impedance matching device may
be used, provided there is sufficient output to overcome the loss.
b. Connect a 15 ohm ± 2 percent (%) 1-watt resistor across each pair of earphone terminals.
c. Adjust the output from the audio oscillator to give 1.9 volts rms at 1,000 Hz, as measured by a
voltmeter at either pair of earphone terminals.
d. Maintain the audio oscillator output established for the 1,000 Hz reading and repeat the
measurement for each of the following frequencies:
1. From 200 to 1,000 Hz inclusive measure at each increment of 100 Hz.
2. From 1250 to 7,000 Hz inclusive measure at each increment of 250 Hz.
4.6.5 Distortion (see 3.4.2). The distortion introduced by the transformer in the cord shall be
determined as follows:
a. Connect the cord under test, audio oscillator and voltmeter outlined in 4.6.4 (a), (b), and (c).
b. Connect the distortion analyzer input in parallel with the voltmeter.
c. Measure the distortion introduced by the transformer for an output of 1.9 volts rms at 1,000 Hz.
d. Maintain the audio oscillator output established for the 1,000 Hz reading and repeat the distortion
measurements for each of the following frequencies:
1. From 200 to 1,000 Hz inclusive: Measure at each increment of 200 Hz.
2. From 2,000 to 7,000 Hz inclusive: Measure at each increment of 1,000 Hz.
4.6.6 Impedance (see 3.4.3). The impedance shall be measured at frequencies selected as necessary
to insure that the requirements are met. The voltmeter-ammeter method of measurement or other
method satisfactory to the responsible government inspector shall be used. The voltage applied across
the terminals during impedance measurements should be approximately the same as applied under
maximum output conditions. When the impedance of the primary is measured, the secondary shall be
terminated by a 7.5 ohm resistor and for the secondary measurement, the primary shall be terminated by
a 600 ohm resistor.
4.6.7 Overload (see 3.4.4). Audio power of 500 milliwatts rms at 1,000 Hz shall be applied through the
cord (transformer) to a dummy load for a period of 8 hours. During the last hour of the test, the cord
(transformer) shall be checked for signs of failure. Upon completion of the overload test, the frequency
response and distortion characteristics shall be measured as specified in 4.6.4 and 4.6.5.
4.6.8 Humidity (see 3.5.3). Each cord under test shall be subjected to an ambient humidity condition
of 95 percent, relative at 50 degrees C (122 degrees F) for a total of 144 hours. At the end of this period,
the frequency response measurement shall be as specified in 4.6.4.
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