Dbm Calculation - Keithley 2016 User Manual

2-30 Basic Measurements

dBm calculation

dBm is defined as decibels above or below a 1mW reference. With a user-programmable
reference impedance, the Model 2016 reads 0dBm when the voltage needed to dissipate 1mW
through the reference impedance is applied. The relationship between dBm, a reference
impedance, and the voltage is defined by the following equation:
Where: V
NOTE
If a relative value is in effect when dBm is selected, the value is converted to dBm then REL
is applied to dBm. If REL is applied after dBm has been selected, dBm math has REL applied
to it.
Configuration
To set the reference impedance, perform the following steps:
1.
2.
NOTES dBm is valid for positive and negative values of DC volts.
dBm = 10 log
is the DC or AC input signal.
IN
Z is the specified reference impedance.
REF
Do not confuse reference impedance with input impedance. The input impedance of
the instrument is not modified by the dBm parameter.
After selecting dBm, the present reference impedance is displayed (1-9999Ω):
REF: 0000
To change the reference impedance, use the
position. Then use the and
after changing the reference impedance.
The mX+b and percent math operations are applied after the dBm or dB math. For
example, if mX+b is selected with m=10 and b=0, the display will read 10.000 MXB
for a 1VDC signal. If dBm is selected with Z
 2 
V /Z
 
REF
IN
-------------------------------- -
1mW
and
keys to select the desired value. Be sure to press ENTER
= 50Ω, the display will read 130MXB.
REF
keys to select the numeric