Calculating Db Characteristics Accuracy; Additional Derating Factors - Keithley 2000 User Manual

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 V
of 10V, you must calculate the voltage accuracy and apply it to above equation.
To calculate a -60dB measurement, assume 10mVRMS for a V
range, one-year, 10Hz - 20kHz frequency band, and SLOW rate, the voltage limits are as fol-
lows:
Accuracy =
Thus, the actual reading accuracy is 10mV 36 V or 10.036mV to 9.964mV. Applying the
voltage reading accuracy into the dB equation yields:
Thus, the actual reading accuracy is -60dB + 0.031213dB to -60dB - 0.031326dB.
dBm and dB for other voltage inputs can be calculated in exactly the same manner using per-
tinent specifications, ranges, and other reference voltages.

Additional derating factors

In some cases, additional derating factors must be applied to calculate certain accuracy val-
ues. For example, an additional derating factor must be added for DC voltages over 500V. Before
calculating accuracy, study the associated specification notes carefully to see if any derating fac-
tors apply.
V
dB = 20 log -------------- -
V
REF
±[(0.06% of reading) + (0.03% of range)]
±[(0.006 10mV) + (0.0003 100mV)]
±[6µV + 30µV]
±36µV
10.036mV
dBm = 20 log ------------------------ - =
10V
9.964mV
dBm = 20 log --------------------- - =
10V
IN
REF
-59.96879dB
-60.03133dB
Specifications A-9
REF
of 10V. Using the 100mV