Tektronix AFG3000 Series Function Generator Guide v1.0 Portland State University
14
9 – Output Voltage Amplitude
Suppose a load R
L
is connected to an output channel on the function generator. The signal source has
amplitude V
G
. The generator’s output impedance is R
O
. Let V
O
be the voltage present at the output
terminals of the generator.
Figure 10: Load connected to output terminals of the function generator
Let V
SET
be the amplitude value that the user sets on the front panel of the function generator. By
default, the instrument assumes an impedance match (i.e., R
L
is equal to R
O
), so it automatically
produces a V
G
that is twice the value of V
SET
, i.e., V
G
= 2V
SET
.
The value of V
O
can be calculated by realizing that R
O
and R
L
form a voltage divider:
General solution for any value of R
L
, when V
G
= 2V
SET
:
( )
SETSET
OL
L
G
OL
L
O
VFV
RR
R
V
RR
R
V ⋅=
+
=
+
= 2
, where
OL
L
RR
R
F
+
=
2
∴ The output voltage V
O
is a scaled version of V
SET
. The scale factor F is between 0 and 2, inclusive.
Special case #1: R
L
= R
O
(impedance is actually matched, i.e., R
L
= 50 Ω) → F = 1, so V
O
= V
SET
Implication: The voltage present at the generator’s output terminals is equal to the set voltage.
Special case #2: R
L
≈ ∞ (open circuit, i.e., no load attached) → F ≈ 2, so V
O
= 2V
SET
Implication: The voltage present at the generator’s output terminals is twice the set voltage.
Note: To make the instrument generate an output voltage that corresponds to the desired set voltage
when the load resistance is not 50 Ω, perform this procedure:
1. Press the Top Menu button on the front panel.
2. Select the Output Menu bezel button.
3. When the new menu appears, press the Select Load Impedance bezel button.
4. If R
L
> 10k Ω, pick the High Z option. For R
L
< 10k Ω, pick Load and enter the actual R
L
value.
Function
Generator
V
G
V
O
R
O
R
L
R
O
= 50 Ω
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