Fluke 8060A - Changing AC dB Reference Impedances with a DC Source

Applications
Changing AC dB Reference Impedances with a DC Source
3
3-9
Whenever you use Table 3-1, be sure you start with the 600
Ω
reference
impedance selected on the 8060A. Otherwise the values and formulas listed
in Table 3-1 will be incorrect because they use the 600
Ω
reference
impedance as a starting point.
After a reference impedance is stored with the REL button, the reference
impedance will remain stored until the instrument is turned off or until
another relative value is stored. You can cancel the stored reference by
pressing the REL button, in which case the REL indicator will disappear and
the reference impedance will revert to the power-on value, 600
Ω
. You can
use other functions without losing a stored reference impedance as long as
you do not store some other relative value.
3-8. Changing AC dB Reference Impedances
with a DC Source
The most straightforward method of changing the reference impedance is to
select the desired voltage dB function, apply the equivalent voltage, and
press the REL button. However, a precision ac voltage source is not always
as readily available as a dc voltage source. There is a method for using a
variable 0 to 200 mV dc voltage source to set up virtually any reference
impedance for ac voltage dB.
To use this method, place the AC/DC switch in the AC position and place
the other two function switches in the out position. Find the equivalent dB
level in Table 3-1 and select the appropriate range. Press the dB button.
Beginning with a 200 mV input signal, slowly decrease the input signal until
the proper dB level is displayed on the 8060A. Then press the REL button.
Now you can select the ac voltage dB function and subsequent
measurements will be referenced to the modified reference impedance.
Note that in this mode the input signal does not go through the voltage
divider or the ac rms converter, but is applied directly to the a/d converter.
Since the a/d converter inputs are between 0 and 200 mV for all ac ranges,
the voltage you apply will always be between 0 and 200 mV, regardless of
the range. For example, let’s assume you want to use this method to establish
a reference impedance of 90Ω. From Table 3-1 you can see this requires an
input of 0.3000V. So you select the 2V range, but you only apply 30 mV dc
of signal in the 2V range to make the reading appear to be 300 mV ac.
Similarly, 30 mV dc of signal in the 20 V range will appear to be 3V ac, and
in the 200V range will appear to be 30V ac.