BNC 1201 - 6 1/2 Digit Digital Multimeter - Manual (Page 87)

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calculation of dB is listed below:
or
dB = (Input signal in dBm) – (relative value in dBm)
※ NOTE: The
in
V is the input signal and the
ref
V is the relative reference.
The dB measurement is applied to DC and AC voltage only. The relative
value is adjustable and is stored in a volatile memory. The range for the
relative value is between 0 dBm and 200 dBm .
How to set the relative value
User can either manually select a number for the relative reference or let
the multimeter take the first reading (0 dB).
Front Panel Operation
Press CONFIG+ SHIFT+ NULL buttons, and then use ◁ and ▷ to move
through the digits and △ and ▽ buttons to increase or decrease the
numbers to your desired value. Press ENTER to set the value. The
locations of the buttons are shown with red rectangle frames in Figure
4-21.
Figure 4-21
How to make a dB measurement
Select the measurement function by pressing one of DCV and ACV
buttons.
Enable the dB operation by pressing SHIFT + NULL buttons.
After enabling the dB operation, user can set or alter the dB relative
value as mentioned above.
) log( 20
Vref
V
dB in  