In this example, a diode (light emitting diode) is used as DUT (device under test). It nicely
demonstrates that a lockin amplifier practically detects the derivative of a signal vs. its modulation
source. The blue graph In(Udc) shows the voltage dropping over the resistance in series and thu
indirectly detects the current through the diode. The red graph R1(Udc), on the other hand,
exactly looks like the derivative of the blue graph.
In order to emphasize this, the results In(Udc) can be taken into a calculation software, the dI/dV
value can be calculated mathematically and plotted together with R(Udc) in one diagram:
One finds a nice accordance between these two signals.
AC/DC Coupling
Diagram 1 (In):
–
DC
–
Ampl = 0 V
Diagram 2 (R):
–
AC
–
f = 100 kHz
–
Ampl = 0.01 V
–
t
= 10 ms
Using this set-up to
investigate the threshold voltages of different light emitting diodes as shown in the two graphs
above, is an excellent teaching example for students to understand the way lockin amplifiers
Manual Anfatec PCI-Lockin Amplifier AMU2.4 – Rev. 1.10 dated 30/09/20
Page 56 (70)
Figure 49: Python Plot
Figure 48: qucs diagram
