Keithley 2015, Service Manual

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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
V
2
IN
/
R
REF
1mW
----------------------------
dBm = 10 log
0.998778V
( )
2
/ 50
Ω
1mW
------------------------------------------------- 13.00032dBm =
dBm = 10 log
0.998852V
( )
2
/ 50
Ω
1mW
------------------------------------------------- 13.00032dBm =