Configuration
To set the reference impedance, perform the following steps:
1. After selecting dBm, the present reference impedance is displayed (1-9999Ω):
REF: 0075
2. To change the reference impedance, use the and keys to select the numeric
position. Then use the ▲ and ▼ keys to select the desired value. Be sure to press ENTER
after changing the reference impedance.
NOTES dBm is valid for positive and negative values of DC volts.
The mX+b and percent math operations are applied after the dBm or dB math. For
example, if mX+b is selected with m=10 and b=0, the display will read 10.000 MX
for a 1VDC signal. If dBm is selected with Z
REF
= 50
Ω
, the display will read 130MX.
dB calculation
Expressing DC or AC voltage in dB makes it possible to compress a large range of
measurements into a much smaller scope. The relationship between dB and voltage is defined
by the following equation:
where: V
IN
is the DC or AC input signal.
V
REF
is the specified voltage reference level.
The instrument will read 0dB when the reference voltage level is applied to the input.
If a relative value is in effect when dB is selected, the value is converted to dB then REL is
applied to dB. If REL is applied after dB has been selected, dB has REL applied to it.
Configuration
To set the reference voltage, perform the following steps:
1. After selecting dB, the voltage applied between HI and LO is acquired and presented as
the reference voltage. This level can then be adjusted.
REF: +1.000000^
2. To change the reference level, use the and keys to select the numeric position.
Then use the ▲ and ▼ keys to select the desired value. Move the cursor to the rightmost
position (^) and use the ▲ and ▼ keys to move the decimal point. Be sure to press
ENTER after changing the reference voltage.
NOTE The largest negative value of dB is -160dB. This will accommodate a ratio of
V
IN
= 10µV and V
REF
= 1000V.
dB= 20 log
V
IN
V
REF
------------------
Basic Measurements 2-35
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