B&K Precision 5491B - Manual - Manual (Page 31)

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To set the reference voltage, perform the following steps:
1. Press + for dB math operation and the present reference value is displayed:
R.F：+0.00000
2. Use and keys to choose a numerical place and use and keys to
increment or decrement the digits. Enter a value and units prefix.
3. Press (ENTER) to confirm the reference voltage, the message “SAVED” will be displayed for
a moment. Then the meter will return back to the measurement status.
4. The display will now show result of the dB calculation.
If you want to change the parameter values when dB function is in effect, you can do the following:
1. Press → to enter the menu on the menu level,
“A: MATH MEU” will be displayed.
2. Press to move down to the command level within the MATH MEU,
“4: dB REF “will be
displayed.
3. Press to enter the parameter level, and the present reference value will be displayed:
R.F: +1.00000 .
4. Use and keys to choose a numerical place and use and keys to
increment or decrement the digits. Enter a value and units prefix.
5. Press (ENTER) to confirm the reference value, the message “SAVED” will be displayed for a
moment, and the meter will return to the command level. Press to cancel the reference value
input, and the meter will return back to the command level without changing the reference value.
6. Press → key to exit the menu and return to the dB math operation status.
Note: The dB calculation takes the absolute value of the ratio V
IN
/V
REF
. The largest negative
value of dB is -160 dB. This will accommodate a ratio of V
IN
= 1 µV, V
REF
= 1000 V.
3.9.3 dBm Calculation
dBm is defined as decibels above or below a 1 mW reference. With user-programmable reference
impedance, the meter reads 0 dBm when the voltage needed to dissipate 1 mW through the reference
impedance is applied. The relationship between dBm, reference impedance, and the voltage is defined
by the following equation:
mW
Z V
dBm REF IN
1
) (
log 10
2

Where:
IV
V is the DC or AC input signal.
REF
Z is the specified reference impedance.
If a relative value is in effect when dBm is selected, the value is converted to dBm then REL is applied to
dBm. If REL is applied after dBm has been selected, dBm has REL applied to it.