Chapter 9 - Electromagnetic Interference Measurement
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9.5
Entering the limit
There are official regulations for observing the interference radiation of cable systems. They set
limits for the emission field strength at a distance of 3 m. The maximum field strength can be
entered into the instrument. The instrument uses it for certain warnings when the limit has been
exceeded. The maximum field strength in dBμV/m can be entered with the
LIMIT
button.
9.6
Analysis of identifier
The electromagnetic interference measurement is based on using the KFG 242 frequency
identification generator. This generator is used as a defined source of interference in a cable
system and should be integrated into the head end.
The signal of the interference transmitter is modulated with an identifier for the unique assignment
of the interference emission. This can be programmed in the frequency identification generator as
text with up to 13 characters.
The measuring instrument demodulates the identifier and shows it in the top row on the display. To
demonstrate that the identifier is being received continuously, the instrument clears the text and
shows it again.
9.7
Measuring the interference field strength
Figure 9-3 EMI measure field strength
When tuned to a frequency, the instrument measures the antenna voltage of the receiving antenna
and converts it into the equivalent field strength. The absolute field strength is displayed in dBµV/m
in a larger font. The measuring range is from 3 – 103 dBµV/m (EMI 241) or 5 – 105 dBµV/m (EMI
240) with a resolution of 0,1 dBµV/m.
At the same time, the instrument calculates, in connection with the current spacing, reference field
strength at a distance of 3 m to the building and displays it in a smaller font in the row above the
menu bar. If the reference field strength exceeds the set limit, a warning message will appear on
the display. A warning signal sounds at the same time over the loudspeaker.
