Keithley 2750, Service Manual

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Model 2750 Multimeter/Switch System Service Manual Accuracy and Optimization A-3
Sp
ecif
ica
tions
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

, 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.
Calculating dB characteristics accuracy
The relationship between voltage and dB is as follows:
As an example of how to calculate the actual readings limits for dB, with a user-defined
VREF of 10V, you must calculate the voltage accuracy and apply it to the above equation.
To calculate a -60dB measurement, assume 10mV RMS for a VREF of 10V. Using the
100mV range, one-year, 10Hz - 20kHz frequency band, and SLOW rate, the voltage limits
are as follows:
Accuracy = ±[(0.06% of reading) + (0.03% of range)]
±[(0.0006

10mV) + (0.0003

100mV)]
±[6µV + 30µV]
±36µV
dBm
10
V
2
IN
R
REF

1mW
---------------------------
log =
dBm
10
0.998778V
 
2
50
 
1mW
-------------------------------------------------
log 12.99968dBm = =
dBm
0.998852V
 
2
50
 
1mW
------------------------------------------------- 13.00032dBm = =
dB 20
V
IN
V
REF
------------
log =