Keithley 2000, User Manual, Page: 50 / 280

dBm calculation
dBm is defined as decibels above or below a 1mW reference. With a user-programmable ref-
erence impedance, the Model 2000 reads 0dBm when the voltage needed to dissipate 1mW
through the reference impedance is applied. The relationship between dBm, a reference imped-
ance, and the voltage is defined by the following equation:
Where: V
IN
is the DC or AC input signal.
Z
REF
is the specified reference impedance.
NOTE
Do not confuse reference impedance with input impedance. The input impedance of
the instrument is not modified by the dBm parameter.
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. After selecting dBm, the present reference impedance is displayed (1-9999
Ω
):
REF: 0000
2. To change the reference impedance, use the and keys to select the numeric po-
sition. Then use the
▲
and
▼
keys to select the desired value. Be sure to press ENTER
after changing the reference impedance.
NOTES
dBm is valid for positive and negative values of DC volts.
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
REF
= 50
Ω
, the display will read
130MXB.
dBm = 10 log
V
2
IN
/Z
REF
 
 
1mW
---------------------------------
2-32 Basic Measurements