3.3 T
IME
CONSTANT
D
EFINITION
The "Time constant
t
" is a parameter of the low pass filter after demodulation. It defines the
detection bandwidth
f
Det
of the lockin amplifier.
In order to visualize this detection bandwidth, the input of the lockin amplifier is connected to a
constant input signal (here: 1 V
rms
amplitude, 100 kHz). The internal reference frequency is swept
over the input frequency.
A signal, which is
f
Det
off the centre frequency of the lockin amplifier, will be detected with 0.707
times it amplitude.
3.4 R
OLL
O
FF
D
EFINITION
The slope of the signal decay in Fig. 5 is defined by the Roll-Off without affecting the detection
bandwidth.
Manual Anfatec PCI-Lockin Amplifier AMU2.4 – Rev. 1.10 dated 30/09/20
Page 13 (70)
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
frequency [kHz]
85
90
95
100
105
110
115
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
6 dB
12 dB
24 dB
Figure 5: Visualization of the Detection Bandwidth for various time constant settings.
frequency [kHz]
99.5
100
100.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2 ms
5 ms
10 ms
0,707 V
1 V
1/
t
