Fluke 189 - Manual (Page 39)

Making Measurements
Measuring Voltage
3
3-3
dB Measurements in AC Volts Functions
The two ac volts functions allow you to display readings as
deviations in dB (decibels) above or below an established
level.
Set up dB measurements with the following procedure:
1. Make an ac volts measurement to be used as a
reference point.
2. Press % to select dB. The dBm (or dBV) value
appears in the primary display and the ac volts
reading appears in the secondary display. A typical
dB display appears in Figure 3-2.
3. Press % again to switch the ac volts and dB
readings. Press
%
a third time to turn dB off.
tc032f.eps
Figure 3-2. dBm Display
Normally, dB is measured as dBm, which is a measure of
decibels relative to 1 milliwatt. The meter assumes a
resistance of 600
Ω
in making this calculation. This
resistance can be set for any value from 1 to 1999
Ω
,
using the meter’s setup capabilities (see Chapter 5.)
When set to other than 600
Ω
the dBm reference
resistance appears in the Index Display. (See Figure 2-4,
item 17.)
Note
If dBm is displayed, check that the reference
resistance value closely matches the impedance
of the system being measured.