Calculating Dbm Characteristics Accuracy - Keithley 2015 Service Manual

A-10 Specifications

Calculating dBm characteristics accuracy

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