H Heuer Instruments UMDT51A, Руководство пользователя

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Making Measurements
LT51 User Manual 49
Near End X-Talk (NEXT) / Loss
The noise found on local loops is caused by many sources, including light
dimmers, radio signals, neon lights, electric trains and adjacent power lines, to
name a few. The most common and problematic types of noise are caused by
the electromagnetic coupling of signals from one local loop to another within the
same cable bundle; this effect is known as “cross-talk”. In fact, the reason that
the telephone cable is twisted is to reduce cross-talk.
This single-ended measurement displays the cross-talk measured at the near
end of the pair connected to the RTX terminals. The LT51 transmits either a
single tone or sweeps over the selected frequency span at 0 dBm on an
adjacent pair that is connected to the TX terminals. Simultaneously the LT51
measures the level at each sweep frequency at the RTX terminals and displays
the results.
This measurement mode is also used to measure the Loss through a network
connected between the TX and RTX terminals of the LT51. A 0 dBm tone (or
sweep) is transmitted from the TX terminals into the network and the received
signal is connected to the RTX terminals. The values / spectrum displayed
represent the Loss through the network.

press the key and then [f5] to select NEXT / Loss.