B&K Precision 5492BGPIB - Manual - Manual (Page 35)

Basic Measurements
35
changing the reference value.
5. Press → key to exit the menu and return to the dB math operation.
Notes: 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.
3.8.4 dBm Calculation
dBm is defined as decibels above or below a 1 mW reference. With a user-programmable reference
impedance, B&K 5492B reads 0 dBm when the voltage needed to dissipate 1mW through the reference
impedance is applied. The relationship between dBm, reference impedance, and the voltage is defined
by the following equation:
Where: V
IN
is the DC or AC input signal voltage value.
Z
REF
is the specified reference impedance.
If a relative value is in effect when dBm is selected, the relative value will be converted to dBm value
before REL is applied. If REL is applied after dBm has been selected, dBm calculation will have REL
applied to it directly.
To set the reference impedance, perform the following steps:
1. Press → , the voltage reference value for dB math function will be displayed.
2. Press (ENTER) to confirm the voltage reference value, now you have selected the dB math
function.
3. Press → again, and the impedance reference value for dBm math function will be
displayed:
REF: 0075 Ω
4. Use and keys to select cursor position and use and keys to increment
or decrement the digits respectively. Enter a value from 1 Ω to 9999 Ω.
5. Press (ENTER) to confirm the reference impedance, and the message “CHANGE SAVED”
will be displayed for a moment. The multimeter will then return back to the measurement status.
6. Now the meter will display the result of the dBm calculation.
If you want to change the impedance reference value after the dBm function is enabled, you can also do
the following:
1. Press → to enter the command level, “5: dBm REF” will be displayed.
2. Press to enter the parameter level, and the current impedance reference value will be
displayed:
REF: +1.0000
3. Use and keys to select cursor position and use and keys to increment
or decrement the digits respectively. Enter a value from 1 Ω to 9999 Ω.