Analog Generator Loading
When a voltage unit (any unit besides dBm) is used, the generator
amplitude shown on the panel is the open-circuit voltage. Unless there
is no external load, the actual voltage at the load will be less than the
open-circuit voltage, since the selected source impedance and the
impedance of the load will act as a voltage divider. The following
simplified schematic shows this relationship:
The voltage that will appear across the load can be calculated using the
following formula:
where Vl is the voltage across the load, Vs is the source voltage, Rl is
the load impedance, and Rs is the source impedance. For example, if
you output a 1 volt signal using the 50 Ω source impedance and load it
with the analyzer’s 100 kΩ load, the voltage across the load will be
0.9995 volts. As long as the load is 4 kΩ or higher, the load voltage
will differ from the open-circuit voltage by no more than 0.1 dB. For a
100 kΩ load, typical of many professional audio devices, the 50 Ω
source impedance will cause an error of about 0.05 dB.
The dBm unit is a unit of power, not voltage. However, the generator
is always set in open-circuit voltage. When you are using the dBm unit
for generator output, the generator will be set to a voltage that would
cause the specified power (in dBm) to be dissipated in a load of
600 Ω. If you are using a load with a different impedance, the power
& For more
information about
the dBm unit, see
page 4-135.
& For a more
detailed technical
diagram of the
analog generator, see
page 6-7.
Generator
Source
Impedance
DUT'S Load
Resistance
Vl
(Rs)
(Rl)
Vs
Generator
Source
Voltage
Generator
Output
Connector
Figure 3-11. Source and Load Impedances
Vl Vs
Rl
Rl Rs
=•
+
3 Operation
Operational Overview Controlling the Generators
ATS-1 Dual Domain User's Manual 3-19
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