Sec. 2.1513  Measurements of modulation characteristics.

    (a) Set-up. Test of modulation characteristics are to be performed 
in an RF shielded room.

    Step (1) Place the EPIRB directly on a metal ground plane, such as 
the shielded room floor.
    Step (2) Place a suitable receiving antenna at a convenient distance 
from the EPIRB and connect it to the input of the spectrum analyzer or 
receiver to observe the radiated signal from the EPIRB.
    Step (3) Set the spectrum analyzer or receiver controls as follows:
I.F. bandwidth: 300 kHz minimum
Video filter: OFF or as wide as possible
Amplitude scale: Linear
Frequency: 121.5 MHz
Scan width: 0 Hz
    Step (4) Connect the detected output of the spectrum analyzer or 
receiver to the input of the storage oscilloscope.
    Step (5) Set the oscilloscope controls as necessary to allow the 
demodulated waveform to be viewed. The input signal is to be DC coupled.

    (b) Measurement of Audio Frequencies.

    Step (1) Activate the EPIRB.
    Step (2) Trigger the oscilloscope and store at least one complete 
cycle of the audio waveform.
    Step (3) Measure the period (T) of the waveform. The period is the 
time difference between the half voltage points at the beginning and end 
of one complete cycle of the waveform. See Figure 2.
    Step (4) Calculate the frequency (F), where:

F=1/T.

    Step (5) Repeat Steps 2 through 4 until the highest and lowest audio 
frequencies are found.

    Note: The lowest and highest frequencies may occur several cycles 
before or after the transition from low to high frequency.)

    Step (6) Determine the audio frequency range (Frange), 
where:

Frange=Fhigh-Flow

    Step (7) Record instrument settings and the lowest and highest audio 
frequencies. Record the audio frequency range in Hertz.
    Step (8) Repeat Steps 1-7, above, for 243 MHz.

    (c) Modulation factor.

    Step (1) Activate the EPIRB.
    Step (2) Trigger the oscilloscope and store at least one complete 
cycle of the audio waveform. The input signal is to be DC coupled or 
erroneous results will be obtained.
    Step (3) Measure the maximum voltage (Vmax), and the 
minimum voltage (Vmin) for the cycle. The modulation factor 
(M) is calculated from the following formula:
[GRAPHIC] [TIFF OMITTED] TC03JN91.002

    See Figure 2.
    Step (4) Repeat Steps 2 and 3 until the lowest modulation factor is 
found.
    Step (5) Record instrument settings and the lowest modulation 
factor, expressed as a ratio between 0 and 1.
    Step (6) Repeat the above measurements for 243 MHz.

    (d) Modulation duty cycle.

    Step (1) Activate the EPIRB.
    Step (2) Trigger the oscilloscope and store at least one complete 
cycle of the audio waveform.
    Step (3) Measure the period (T) of the waveform. The period is the 
time difference between the half voltage points at the beginning and end 
of one cycle of the waveform. See Figure 2.
    Step (4) Measure the pulse width (tp) of the waveform. 
The pulse width is the time difference between the half voltage points 
on the rising and falling portions of the waveform. See Figure 2.
    Step (5) Calculate the duty cycle (D) as follows:
    [GRAPHIC] [TIFF OMITTED] TC03JN91.003
    
    Step (6) Repeat Steps 2 through 5 a sufficient number of times to 
determine the highest and lowest duty cycles.
    Step (7) Record instrument settings and the highest and lowest duty 
cycles in percent.
    Step (8) Repeat Steps 1-7 for 243 MHz.

    (e) Sweep repetition rate.

    Step (1) Connect a speaker to the detected output of the spectrum 
analyzer or receiver so the audio frequencies are audible. 
Alternatively, an FM radio tuned to 108 MHz placed in the vicinity of 
the EPIRB may be used.
    Step (2) Activate the EPIRB.
    Step (3) Time the number of audio sweeps (N) for a one minute 
interval.
    Step (4) Calculate the audio sweep rate (R) using R=N/60.
    Step (5) Record instrument settings and the sweep repetition rate in 
Hertz.