Features and Functions 2
34405A User’s and Service Guide 33
Null
When making null measurements, also called relative, each
reading is the difference between a stored null value and the
input signal. For example, this feature can be used to make
more accurate resistance measurements by nulling the test
lead resistance.
After you enable the Null operation, the multimeter stores
the next reading into the Offset register and immediately
displays on the primary display:
Primary Display = Reading - Offset
You can view and edit the Offset value in the secondary
display as described in “Editing Values in the Secondary
Display” on page 45.
The multimeter allows Null settings for the following
measurement functions: DC Volts, AC Volts, DC Current, AC
Current, Resistance, Frequency, Capacitance and
Temperature.
dBm
The logarithmic dBm (decibels relative to one milliwatt)
scale is often used in RF signal measurements. The
multimeter’s dBm operation takes a measurement and
calculates the power delivered to a reference resistance
(typically 50, 75 or 600
W
). The formula used for conversion
from the voltage reading is:
dBm = 10 x Log
10
[ (Reading
2
/ R
REF
) / 0.001W ]
You can choose from several reference resistance values:
R
REF
= 2Ω, 4Ω, 8Ω, 16Ω, 50Ω, 75Ω, 93Ω, 110Ω, 124Ω, 125Ω,
135Ω, 150Ω, 250Ω, 300Ω, 500Ω, 600Ω, 800Ω, 900Ω, 1000Ω,
1200Ω, or 8000Ω.
Numeric results are in the range of ± 120.000 dBm with 0.01
dBm resolution shown, independent of the number of digits
setting.
Null
Null
Shift
dBm
MnMx
34405A users guide.book Page 33 Saturday, September 2, 2006 3:38 PM
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