Db (Decibels And Audio Power) Modifier - Fluke 45 User Manual

45
Users Manual
modifier is selected, the relative base is shown in the secondary display. To turn off this
feature, press

dB (Decibels and Audio Power) Modifier

The decibels modifier takes a voltage measurement, converts it to dBm (measure of
decibels relative to one milliwatt), and displays the result on the primary display.
Press
selected, "dB" is shown on the primary display.
Decibels can be selected only when a voltage function is selected on the primary display
(volts ac, volts dc, or volts ac + dc). Decibels are always displayed in a single, fixed
range with 0.01 dB resolution. However, the basic measurement itself (e.g. volts ac)
autoranges.
A voltage measurement is converted to dBm using the following formula:
dBm = 10*log (1000*value
where "value" is the measurement value. The reference impedance can be set to any of 21
reference impedances listed in Table 3-8 by using the list editor as described in "Using
the List Editor" later in Chapter 3.
* Voltage annunciator lit
† Audio power readings possible
To access the reference impedance list, press
impedance currently selected is displayed, along with the db and
and return the primary display to the measurement function. Press any function (white) or
modifier (light grey) button to exit the reference impedance list without selecting a new
value.
Setting the dB reference resistance to 16, 8, 4, or 2 ohms allows you to use the meter to
calculate audio power. After the reference resistance has been set to 16, 8, 4, or 2 ohms,
press
secondary display.
The following equation is used to make a power calculation:
Audio power = Volts /reference resistance
where volts is the measurement value.
3-12
and, while holding it down, press
to toggle in and out of the decibels modifier. When the decibels modifier is
Table 3-8. Reference Impedances in Ohms
8000
1200
1000*
900
800
600
500
or to scroll to the desired value, then press
twice to select the audio power modifier. "POWER" will be shown on the
2
/reference impedance)
300
250
150
135
125
124
110
then press
.
93
75
50
16†
8†
4†
2†
(REF ). The reference
annunciators. Press
to select a reference impedance