Lesson 1/Learning Event 7
Telegraph MON jacks are designed to insert the DC
milliammeter in series with the circuit; the circuit is therefore never
broken by the meter, and traffic flow is unbroken.
(2) VU indicator.
The VU indicator is designed primarily to indicate
the amplitude levels in speech rather than to measure the level of a
constant tone. However, it can be used to indicate a constant tone level if
desired; with zero setting of the attenuator a constant tone of 1 milliwatt
(0 dBm) at 1,000 Hz will indicate 0 on the meter scale.
indicator can be used for this purpose, the recommended procedure is to use
a decibel meter for constant tone, and reserve the VU indicator for speech
(a) The numbers from 20 to 100 on the lower part of the scale show
approximate percentages of modulation when the VU indicator is used by an
The meter movement has been intentionally damped, making its movements
The purpose is to enable the operator to see the swing of the
needle on voice peaks.
(b) Crowding of scale numbers at small signal levels makes the VU
indicator less accurate than a decibel meter, which is designed to have a
more linear scale.
(c) The function of the attenuator is to keep the meter reading on
scale. That is, if a signal exceeds the meter scale, the operator adjusts
the attenuator to bring it on scale.
When the attenuator is used, its
setting must be taken into account in determining the true value of signal
level. For example, if the attenuator is at +8, and the meter reading is -2
VU, the true signal level is 8 dB above -2, or 6 VU.
The negative (-)
numbers on the attenuator scale indicate the amplifier gain being used to
raise the signal level to bring it on scale. For example, if the attenuator
is at -20 and the meter reads -5, the true level is 20 dB below -5, or -25
(d) The toggle switch below the attenuator provides the operator with a
choice of either terminating (ending) a line in 600 ohms within the meter
circuit or bridging the meter across the circuit.
When the line is
terminated in 600 ohms (upper position of the toggle switch), the meter
indicates true level, but the signal is dissipated in the attenuator and
ends there. When the line is bridged by the high-impedance circuit within
the meter (lower position of the toggle switch), a comparative level is
indicated, but the signal continues through the circuit unimpeded.
terminate a line, the VU indicator is patched into an OUT jack; to bridge a
line, the VU indicator is patched into a MON (monitoring) jack. It is never
patched to an IN jack.