Inovonics 531N, Инструкция по установке и эксплуатации, Страница: 27 / 60

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(26)
ALARM
INDICATORS
These LED alarm condition indicators (
26)
are du-
plicated with corresponding ‘tally’ closures to
ground on the Model 531N rear panel, as well as
with SNMP support through the Web interface,
which is covered in Section IV. See Page 16 for a
discussion of connecting remote indicators; an ex-
ample is shown in Figure 6 on that page.
(27)
NETWORK
SETTINGS
The
NETWORK SETTINGS
button
(27)
allows setup
of the various network parameters to secure com-
munication between the 531N and a Local Area
Network (LAN) or Internet.
Setting up a network connection is detailed in Sec-
tion IV.
MEASUREMENT PITFALLS AND LIMITATIONS
It is important for the user to recognize various inherent limita-
tions of making transmission performance measurements ‘off-
air.’ Although the Model 531N has a high level, ‘direct’ RF in-
put, the signal still passes through tuned RF and IF stages.
Thus even these directly-coupled measurements are subject to
some of the same limitations as an off-air signal.
Signal Strength
The term “FM Advantage” refers to the inherent freedom from
noise that a frequency-modulated signal enjoys over its ampli-
tude-modulated counterpart. Nevertheless, random noise does
result in a certain amount of ‘dither’ at carrier zero-crossings,
which translate to jitter in the time domain that can sabotage
accurate measurement of total carrier deviation.
The basis for FM receiver sensitivity specifications dates to the
early days of monaural FM broadcasting. Tuners from this era
boasted sensitivities in the low-microvolt range. Honest and
forthright manufacturers would qualify this specification with:
“signal required for 50dB of quieting,” which referred to a
monaural transmissions. This isn’t a particularly difficult spec
to meet in mono receivers, which can both tolerate and profit
from a much narrower IF bandwidth than what is required for
decent stereo performance.
Stereo reception adds all the noise present in the 23kHz–
53kHz ‘sub’ band that rides above the 30Hz–15kHz monaural
reception range. What’s worse, it’s the amplitude component
(AM) of the sub band that is converted down to audible noise
and added to the program signal. That’s why the noise perfor-
mance of FM-stereo is always about 20dB worse than that of a
monaural broadcast.
Reverting to the specification from days gone by, the Model
531N exhibits 50dB of mono quieting with only 10µV coming
in, actually pretty decent considering the “broad as a barn”
RF/IF passband that’s required for stereo signal parameter