c. Relative Power Levels.
Relative power levels are not measured, but
they are stated in terms of the power that exists, not a standard reference
power level. However, since the power that exists is referred back to 1 mw,
relative power levels must indicate that they refer back to some other selected
power level. The expression 0 dbm0 indicates that this is a level to which all
others will be referred in a given circuit. The expression does not indicate
the actual power, but that the power selected for reference does eventually
refer back to 1 milliwatt. Remember--relative power levels are calculated, not
measured; they are calculated with respect to a selected level which is
declared the reference. The expression 0 dbm0 has value only when the existing
power level selected for reference is stated. That point in the circuit where
the selected reference power level appears is thereafter called the
transmission level point (TLP).
d. Channel Signal Levels. In some situations the circuit conditioner must
determine the level of each channel signal within a composite signal from an
FDM terminal.
He can easily arrive at this determination by a simple
calculation.
Normally, he cannot measure the level of each channel signal
because it is enclosed in the "bundle" of signals that travel the line
together. However, before he makes the calculation he should know the measured
level of the composite signal, and he should know the number of channels in
use. Further, he has to assume that each of the various channel signals has
approximately the same level.
Example 1: Assume that the measured level of an eight-channel FDM composite
signal is +6 dbm. What is the level of each channel signal? The
scale in figure 2-1 shows that +6 dbm represents a power level of 4
mw. Dividing 4 mw by 8 channels we find the power in each channel
to be approximately 1/2 mw. Referring once again to the scale in
figure 2-1, it is evident that the level in dbm for 1/2 mw of power
is -3 dbm. To state this another way, the composite level of eight
-3-dbm channel signals is +6 dbm.
Example 2: If the level produced by each channel of an FDM terminal is -6 dbm,
what composite level can be expected for 16 channels? The scale in
figure 2-1 shows that the power equivalent for -6 dbm is 1/4 mw.
Multiply this by 16 to obtain 4 mw.
The scale shows 4 mw is
equivalent to +6 dbm.
2-3. CIRCUIT DESCRIPTION
Assume that a technical control center contains three bays through which
passes a line circuit carrying a signal to equipment, as shown in figure 2-2.
a. Signal Path. The input signal arrives at bay 1 and passes through a
variable-loss attenuator and a 35-db fixed-gain amplifier. The overall gain of
bay 1 depends only upon the attenuator setting. The signal then passes through
bay 2, which has jacks for equal-level patching.
The signal then passes
through bay 3, which contains pads to establish the output level needed by
station equipment.
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