(4) Noise parameters Z1 through Z3 require determination of noise
characteristics in terms of signal and noise reference levels.
DCA gives
the ratio in decibels as (S+N)/N.
To obtain this decibel ratio, two
readings must be taken. To read S+N, transmit a test signal and measure the
received level on the VU-DB scale of the TTS-37B in dBm.
This measured
level contains the combination of signal and noise.
To read N, terminate
the send line (thus removing the test signal) and read the received noise
level on the DBRN scale in dBrn. Convert the noise reading in dBrn to the
equivalent value in dBm.
Now subtract the noise level in dBm from the
signal-plus-noise level in dBm. The difference between these two figures is
the ratio of (S+N)/N in decibels.
Example:
Assume that S+N measures -2 dBm and N measures +50 dBrn.
The
noise level of +50 dBrn is equivalent to -40 dBm (-90+50).
The
difference in decibels between -2 dBm and -40 dBm is 38 dB.
The
ratio in decibels of (S+N)/N is, therefore, 38 dB.
1-2. Impulse Noise Test.
Impulse noise is different from white noise in
that it is caused by abrupt changes in signal levels having very short
duration. Generally, impulse noise does not seriously degrade transmission
quality in a voice communications circuit unless the noise is of a
continually interfering nature. In data transmission, however, short high-
intensity bursts may alter the waveform of the digital pulses. These bursts
have little affect on the pulse amplitude since the pulses are normally
transmitted at constant level. The DCA parameters for AUTODIN and AUTOVON
specify impulse counts at particular levels for a given period of time.
a. Test arrangement. A block diagram of a typical test arrangement for
the impulse noise test is shown in figure 1-3. Test equipment includes a
terminating resistor at the sending end of the line and Impulse Noise
Counter Model TTS-58A (appendix A, section IV, figure 4) at the receiving
end of the line. Tests are made in each direction of transmission. Each
station records measurements at the receive side of the circuit under test.
Note the similarity to the block diagram for idle noise test; the difference
is in the test equipment employed and the length of time involved. A "hit'
counter is used in place of the analog noise measuring set TTS-37B to
measure the frequency of impulse noise peaks over a given time period.
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