Calculating Dbm Characteristics Accuracy - Keithley 2010 User Manual

Calculating dBm characteristics accuracy

As an example of how to calculate the actual reading limits for a 13dBm measurement with
a reference impedance of 50Ω, assume an applied signal 0.998815V. The relationship between
voltage and dBm is as follows:
dBm = 10 log
From the previous example on calculating DC characteristics accuracy, it can be shown that
0.998815V has an uncertainty of ±36.96445µV, or 0.998778V to 0.998852V, using one-year
specifications of the 1VDC range.
Expressing 0.998778V as dBm:
dBm = 10 log
and expressing 0.998852V as dBm:
dBm = 10 log
Thus, the actual reading range is 13dBm ± 0.00032dBm.
dBm and dB for other voltage inputs can be calculated in exactly the same manner using
pertinent specifications, ranges, and reference impedances.
2
/
V R
IN
REF
--------------------------- -
1mW
2
( )
0.998778V / 50Ω
------------------------------------------------ - = 13.00032dBm
1mW
2
( )
0.998852V / 50Ω
------------------------------------------------ - = 13.00032dBm
1mW
Specifications A-9