Example 2: Assume that the input level to the circuit in Example 1 is
-3 dbm. What is the output level from the circuit?
Add +14-db gain to -3-dbm level to find the output level.
+14 + (-3) = +11 dbm.
e. Uniform Channel Levels. Equal levels are always desirable from each of
the several channels in a frequency-division-multiplexed (FDM) line signal. In
most cases, this line signal passes through modulators or amplifiers.
The
channel having the strongest of several signals will predominate at the expense
of the other channels. The result is an accentuated difference in level of the
various frequencies making up the signal, together with possible amplifier
overloading.
f. Maximum Undistorted Output Levels.
The maximum undistorted output
level of an amplifier must be observed to prevent distortion. In other words,
the amplifier must not be overloaded. For example, if the maximum undistorted
power output of an amplifier is +10 dbm, a technician should not decrease the
attenuator loss in an attempt to raise the output level above the maximum
permissible amount.
He must add an additional amplifier. Failure to follow
this precaution results in amplitude distortion in case of a single-tone or a
voice signal, and intermodulation distortion in case of an FDM signal.
2-2. ABSOLUTE VS RELATIVE LEVELS
The Defense Communications Agency specifies level parameters for the
Defense Communications System based on absolute and relative quantities.
a. Absolute Levels.
Absolute levels are the actual measured levels and
represent existing power levels.
Power levels are indicated as a number of
decibels above or below a given power level. Thus, dbm has a reference value
of 1 milliwatt (0 dbm). Likewise, dbw has a reference value of 1 watt (0 dbw).
The expression dbw is seldom used in communication because of the large power
involved, but it is widely used in connection with output power of radio
transmitters. Power levels can become very low in communication. For example,
the average human ear can hear noises as low as 1 x 10-6 of 1 milliwatt. Test
sets designed to measure noise are calibrated with respect to -90 dbm, which
represents an absolute level of 1 x 10-9 milliwatt. A graph relating power in
milliwatts to dbm is shown in figure 2-1. Note that every change of power in
the ratio of 1/2 or 2/1 results in a 3-db change of level in dbm. Likewise, a
change of 1/4 or 4/1 gives a 6 db change of level in dbm.
b. Voltage and Power. Power can be measured by test sets either in terms
of actual power or in terms of voltage. The relationship between voltage and
power is indicated in table II. Note that power development does not depend on
voltage alone. The most important consideration is the combination of measured
for determining power level is: 0 dbm = 0.775 volts in an impedance of 600
ohms. In 150 ohms impedance, approximately 0.39 volt produces 0 dbm (1 mw).
In 75 ohms, 0.274 provides 0 dbm. Working the other way in table II, a level
of -3 dbm produces a voltmeter reading of 0.548 in a 600-ohm circuit.
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