WIRE LINE REMOTE BASE
PHONE
LINE
LEVELS
D. MEASURING LEVELS ACROSS A BALANCED
LINE
I. Use a BALUN (balanced-to-unbalanced) device because
of
the unbalanced nature
of
the
DVM/Counter
In
jack.
The BALUN must be high impedance
(6K
ohm
or
more) to make bridging measurements without upsetting levels.
2. Connect the BALUN's balanced side (high impedance) across the phone line. Connect the BALUN's unbalanced
side to the
DVM/Counter
In jack, as shown in figure 4-18. Set the BALUN switch in the
OPEN
position.
3. Set DISPLAY to DVM
AC
mode. Measure the test tone level on the phone line by interpolating the AC VOLTS-
DBM scale
on
the display.
4.
Set DISPLAY to Scope AC. Analyze waveform
and
measure
AC
volts peak-to-peak by interpolating the volts per
division.
5.
With the BALUN switch in the
OPEN
position, the BALUN represents a nominal lOdb insertion loss from the
balanced side to the unbalanced side.
For
example, an actual line level
of
Odbm will indicate -10 dbm at the DVM
input. Characterize your unit before use.
4-32
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