Fluke 45 - Manual (Page 39)

45
Users Manual
3-12
modifier is selected, the relative base is shown in the secondary display. To turn off this
feature, press
S and, while holding it down, press K .
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
I to toggle in and out of the decibels modifier. When the decibels modifier is
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 2 /reference impedance)
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.
Table 3-8. Reference Impedances in Ohms
8000 300 93
1200 250 75
1000* 150 50
900 135 16†
800 125 8†
600 124 4†
500 110 2†
* Voltage annunciator lit
† Audio power readings possible
To access the reference impedance list, press
S then press I (REF e). The reference
impedance currently selected is displayed, along with the db and
e annunciators. Press
U or T to scroll to the desired value, then press E to select a reference impedance
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
I twice to select the audio power modifier. "POWER" will be shown on the
secondary display.
The following equation is used to make a power calculation:
Audio power = Volts /reference resistance
where volts is the measurement value.