Table
2-5.
dB
Specifications for
AC
Volts
(600n
Ref)
dB
M
Range
200mV
ZV-75OV
de (Ref:
60001
InDut
1mV
to
2mV
(-58
to
-52dBm)
2mV
to
20mV
(-52
to
-32dBm)
20mV
to
200mV
(-32
to
-12dBm)
200mV
to
750V
1-12
to
+59.8dBm)
dBm
Measurements with
600a
Reference Impedance
dBm is defined as decibels above
or
below a 1mW reference.
The standard reference impedance of the Model 197 is 6000.
What that means is that the Model 197 is designed to read
OdBm when the voltage needed to dissipate 1mW through a
600a
impedance is applied to the Model 197. That calculated
voltage level is 0.7746V as derived from the basic power
equation.
~~
E=h’.R
-
E
=q0--3
W.6000
E
=
0.77456V
Thus with a 6000 reference impedance the Model 197 will
read OdBm whenever 0.7746v is applied.
NOTE
Do
not confuse reference impedance with input
impedance. The input impedance
of
the instru-
ment is not modified in the dB mode.
To
make dBm measurements referenced
to
6000, proceed as
follows:
1.
Connect the test leads to the INPUT
HI
and
LO
terminals
2. Select the ACV
or
DCV function.
3.
Select autorange for optimum resolution.
4.
Press the dB button.
5.
Connect the test leads to the voltage source.
6. Make the dBm reading from the display.
of the Model 197.
20Hz-
10kHz
2.00
0.85
0.18
0.18
Accuraci
10kHz-
2OkHz
3.00
1.10
0.18
0.18
(
k
dBw
50kHz
ZOkHz-
-
2.00
0.28
0.28
5OkHz-
100kHz
-
-
0.65
0.50
dBm Measurements with Other Reference Impedances
dBm measurements can be made with other reference im-
pedances. The most convenient method for using other
reference impedances is to algebraically subtract the
calculated dB offset for the desired reference impedance from
the reading on the display of the Model 197. Table 2-6 lists
common reference impedances and the corresponding offset
values. The following equation can be used to calculate the
offset the offset for impedances not listed in Table 2-6.
New ref
Z
Offset (for dBm)
=
10 log----
600Q
To
make dBm measurements referenced to another im-
pedance, proceed as follows:
1.
Choose the desired reference impedance.
2. Calculate
or
look up the offset value in Table 2-6 for the
3.
Determine dBm at the desired reference impedance as
dBm (at ref
Z)
=
197 reading
-
offset
desired reference impedance.
follows:
Example: Make dBm measurements references to a 1000
reference impedance.
A.
1000 not listed in Table 2-6
so
the offset must be
calculated as follows:
Offset
=
10 log
(z
)
Offset
=
-7.78dB
B.
Subtract -7.78 from all subsequent displayed readings
on the Model 197.
2-1 1
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