GVT-427B User Manual
10
Operation Method
A.
Voltage measurement
1.
Turn off the power.
2.
Check the ZERO setting of the pointers. If there is offset,
you could use a screwdriver to adjust the zero adjustment
screw at the center of the meter front cover.
3.
Plug the AC plug into the AC line.
4.
Set the RANGE to 100 V and turn on the power.
5.
Connect leads to the INPUT terminal and the load is
under testing.
6.
Alter the RANGE selector switch until the pointer is at a
position which located at
≧
1/3 of the scale, therefore, the
reading can be taken easily.
B.
Use of decibel ranges
There are two dB scales provided on the dial which have been
calibrated as
0 dB = 1 V
0 dBm = 0.775 V (1 mW into 600
Ω
)
1.
dB:
“Bel” is a logarithmic unit which expresses the ratio of two
powers. One “decibel” (abbreviated dB) is one-tenth of a Bel.
The dB is defined as follows:
dB = 10 log P
2
/ P
1
If the impedance is at the place where P
1
and P
2
are equal to
each other, the ratio of power could be expressed as follows:
dB = 20 log E
2
/ E
1
= 20 log I
2
/ I
1
GVT-427B User Manual
11
Decibel is originally the ratio of power as explained above.
However, the logarithm of the ratio of other values (ratio of
voltage or current) can also been called “decibel”.
For example, If the input voltage of an amplifier is 10 mV and
its output voltage is 10 V, the degree of amplification could be
10 V / 10 mV = 1000 times. This is also expressed in dB as
follows:
Degree of amplification = 20 log 10 V / 10 mV = 60 dB
2.
dBm
“dBm” is the abbreviation of dB (mW). This decibel value
expressed the power ratio with respect to 1 mW. Normally,
“dBm” implies the condition where the power exists in an
impedance of 600
Ω.
Therefore, “0 dBm” can be signified as the following:
0 dBm = 1 mW or 0.775 V or 1.291 mA
3.
The power or voltage levels are determined by adding up
the scale readings and the selected RANGE settings.
Example:
Scale
RANGE
Level
(-1 dB) + (+20 dB) =
+19 dB
(+2 dBm) + (+10 dBm) = +12 dBm
