True RMS Digital Multimeters
Operating Basics
15
dB and dBm Voltage Measurements
Measurement Softkey Connect Leads
Main
Display
Upper
Display
dB (185 only)
1
AC dB
dBm (185
only)
2
4
dB
or dBm
(press to
toggle)
ahr08f.eps
AC dBm
1
dB readout = 20 X log (main display readout/ref), where ref = 1 V is the
default.
2
dBm readout = 10 X log (main display readout2/R), where R=600
e
.
Application: Using a Reference Value Other than Default
1.000 V RMS in dB Mode
To manually change the reference voltage in dB mode, press and hold the
yellow (shift) button to display the Setup menu. Adjust the
rEF (dB)
value to
the value you want and press softkey
4
for OK. Subsequent dB
measurements will use this stored value as the reference voltage until you turn
off the meter.
Application: Using Voltage in a Circuit as the Reference
Value in dB Mode
An example of using a voltage in a circuit as the reference value is measuring
the AC voltage gain of an amplifier. Set the meter to dB mode and measure the
voltage that you want to use for the reference (the input of the amplifier). Press
REL
t
to save the measured value as the reference value. Next, measure the
output of the amplifier. The voltage gain of the amplifier (in dB) shows in the
upper display.
In both of these applications, the
t
indicator shows in the upper display to
indicate the reference is a voltage other than the default value of 1.000 VRMS.
The reference value returns to 1.000 V when you exit dB mode.
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