5.2 D
EMONSTRATE
R
OLL
O
FF
In order to test the effect of the parameter RollOff on the shape of the pass band detected with a
lockin amplifier, an external function generator is connected to the lockin amplifier's input. (
25). Let's assume, the function generator provides 100 kHz with 200 µV rms-
amplitude. Then, the lockin amplifier will detect 200 µV if it is set to 100 kHz. When the centre
frequency is swept from 98 kHz to 102 kHz with a time constant of 5 ms (200 Hz detection
bandwidth), the lockin detects a peak shape with 200 µV height and ±200 Hz width (Fig. 9).
Now, this curve is taken with all available settings of RollOff (Fig. 7). One observes, that all three
curves meet in two points that represent the 0.7 times the amplitude of the signal. This shows,
that the detection bandwidth is kept independent on the RollOff. At the same time, the slope of
the lockin filter increases according to the RollOff settings. On the double-logarithmic scale it is
obvious, that 6 dB/oct equal one order of magnitude decay when the frequency is doubled. 12
dB/oct equal 2 orders of magnitude and 24 dB/oct four orders of magnitude, respectively.
Manual Anfatec PCI-Lockin Amplifier AMU2.4 – Rev. 1.10 dated 30/09/20
Page 24 (70)
Figure 17: Detection bandwidth shaoe in dependence on the
parameter RollOff.
frequency [kHz]
99
100
101
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
6 dB
12 dB
24 dB
t
= 5 ms
1/
t
Figure 16: Panel display and frequency sweep result with 200 µVrms and 100 kHz at the input. Time constant: 5 ms
