The low-pass filter itself is a Butterworth with an effective noise bandwidth of
B
n
=
∫
0
∞
1
1
2n
d
.
The normalized Butterworth filter noise bandwidths are:
Filter order
Bandwidth
1
1,570796
2
1,110721
4
1,026172
R
OLL
O
FF
The meaning of the "RollOff" is shown in the curves in Figure 5. It equals the degree of the lowpass
filter. One can chose between 6 dB/oct (1
st
order), 12 dB/oct (2
nd
order) and 24 dB/oct (4
th
order).
D
YNAMIC
This switches the input amplification of the LockIn. With "high" dynamic, input amplification is 1.
The maximum signal amplitude is then +/-10 V. The "normal" input amplification is 10, which
equals maximum signal amplitudes of +/- 1 V. When the low dynamic is chosen, the resolution of
the LockIn is highest, but the signals cannot exceed 100 mV.
C
OUPLING
If the specification of the instrument allows it, this option switches between DC coupled input and
AC
coupled input. Note: The 3dB corner frequency of the input high pass is around 2Hz. Reference
frequencies around 2 Hz and below may cause misleading results.
T
HE
M
ETERS
The meters display the LockIn output channels, which are X and Y as well R and Phi, in physical
units.
Manual Anfatec PCI-Lockin Amplifier AMU5.0 – Rev. 1.01
Page 18 (34)
Figure 9: Effect of different time constants.
