3-12
phase angle, a device that is proportional to both the power factor and the apparent power must be used.
Because power-level measurements are concerned with decibel units, a working knowledge of decibels is
required for proper interpretation of power tests. The decibel is used to determine the ratio of power
changes or to indicate the power level in a circuit with respect to either 0 or a standard reference level.
AF POWER
In the electrical transmission of speech or music, rapidly fluctuating amplitudes and frequencies are
involved. The average power-level measurement and its variation rate depend on the signal characteristics
and time interval over which this average is taken. Power measurements for af circuits are usually
indicated in terms of decibels (dB), decibels referenced to 1 milliwatt (dBm), or volume units (vu). For
example, the power gain of an amplifier can be expressed in dB; the power level of a sinusoidal signal
compared to a 1-milliwatt reference is indicated in dBm; and the power level of a complex signal, such as
voice, music, or multiplexed information, compared to a reference level of 1 milliwatt, is indicated in vu.
Q-8.
What are the three units of measure most commonly used when referring to af power
measurements?
DECIBEL METERS
A dB meter is a form of ac electronic voltmeter calibrated in dB's. These meters are useful for
making measurements where direct indication in decibels is desired. However, remember that these are
voltmeters, and power measurements are not meaningful unless the circuit impedance is known. When the
dB meter is calibrated, a reference point, based on a specific power or value of voltage across a specified
resistance, is selected to represent 0 dB. Many electronic voltmeters use a single dB scale based on 1
milliwatt into a 600-ohm load to represent 0 dBm. Based on this reference point, various voltage readings
could be made on the low ac-voltage scale. The +dB numbers corresponding to voltage ratios that exist
between successive ranges and the low ac range have been computed for each range. These numbers,
shown on the front panel of the instrument, are added algebraically to each successive range reading to
produce the correct value for the range. The term decibel does not, in itself, indicate power. It indicates a
ratio or comparison between two power levels that permits you to calculate the power. Often, it is more
desirable to express performance measurements in terms of decibels using a fixed power level as a
reference. The original standard reference level was 6 milliwatts, but to simplify calculations a standard
reference level of 1 milliwatt has been adopted.
Q-9.
In reference to dB meters, 0 dBm represents 1 milliwatt into what value of load?
VOLUME UNIT METERS
The volume unit (vu) meter is used in audio equipment to indicate input power to a transmitter or to
a transmission line. This type of meter has special characteristics, such as a standardized speed of pointer
movement, speed of return, and calibration. The measurement of the average power level and its rate of
variation with respect to time depends not only on the signal characteristics, but also on the time interval
over which the average is being taken. Accordingly, the speed of response of the instrument used to
measure average power is of particular concern. The unit of measurement is the volume unit (vu), which
is numerically equal to the number of dB above or below the reference level of 1 milliwatt into a 600-ohm
load (provided the standard instrument was calibrated under constant-amplitude, sine-wave conditions). A
change of one vu is the same as a change of one decibel. Therefore, the vu value obtained represents
averages of instantaneous power of speech or music obtained by an instrument having particular dynamic
characteristics. The vu readings are equivalent to the power level in decibels only if the sinusoidal
waveform is of constant amplitude.
Q-10.
What is the main difference between a vu and a dB meter?
