Keithley 2701, Service Manual

Page: 137 / 168

Model 2701 Service Manual Specifications A-
7
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
a measurement of 0.998815V on the 1V range has an uncertainty of ±36.9644mV or
0.998778V to 0.998852V, using one-year specifications.
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
pertinent specifications, ranges, and reference impedances.
dBm
10
V
2
IN
R
REF
⁄
1mW
---------------------------
log =
dBm
10
0.998778V
( )
2
50
Ω ⁄
1mW
-------------------------------------------------
log 12.99968dBm
= =
dBm
0.998852V
( )
2
50
Ω ⁄
1mW
------------------------------------------------- 13.00032dBm = =