Anfatec eLockIn 203, Manual

Page: 22 / 58

other.
In both entries
Tau
and
Tau1
, one can chose between a time ( 0.1 ms, 0.2 ms, ...
500 s , 1 ks
) and a kind of frequency related integration (
10/f
and
100/f
). The later
still uses the Butterworth filter, but chooses the closest time constant for the currently
selected frequency during spectra acquisition. This feature, however, is useful for noise
measurements only, because the switching between different filter settings causes peaks
in the acquired spectra.
For the acquisition of spectra with varying integration adapted to the currently selected
frequency, one has to enable the
Sync
filter. In this mode, the Butterworth filters are
disabled and the
Slope
has no meaning anymore, which is displayed as
Sync
in the
Slope
menu entry.
For instruments with the LoL-Mode option,
Tau1
is used for the 2 nd demodulation.
Time Constant [ LockIn / Input / Time ] → on a time base
Integration just on a timebase means that the time constant is not related to frequency
at all.
τ
, the time constant, defines the length of the time interval taken into account for
the low pass filtering. The
eLockn
employs a Butterworth-type low-pass filter.
Together with the
Slope
, which is nothing else than the order of the low pass filter, the
bandwidth and the raise time of the filters are given as follows:
Roll-Off Formula for
bandwidth
Raise time
/ settled
time to 90
%
Raise time
/ settled
time to 99
%
Raise time
/ settled
time to
99.9 %
Oversho
ot
6 dB/oct
π
2
τ
0.43 τ
0.74
τ
1.11
τ
0,00 %
12
dB/oct
π
2
√
2
τ
0.43
τ
0.52
τ
/
1.05
τ
0.53
τ
/
1.64
τ
4.32 %
24
dB/oct
π
8sin
(
π
8
)
τ
0.63 τ
/
0.96 τ
0.70
τ
/
1.66
τ
0.70
τ
/
2.72
τ
10.8 %
The time constant can be set in logarithmic steps (1-2-5) between 0.1 ms and 1 ks.
Syncron Filter
[ LockIn / Input / Time ] → synchronized integration
In synchronized low-pass filtering, the integration time is strictly connected to the
current center frequency. It is given in multiples of 1/f and can very between 1/f and
200/f.
The parameters “bandwidth” and “raise time” are not applicable for the synchronized
filtering, anymore. The result is provided the moment it is valid. Thus, the oscilloscope
might show staircase-like outputs for lower frequencies with accordingly high multiples
of the period.
During spectra acquisition, the integration time is constantly adapted to the current
frequency, so that the spectrum takes more time at low frequencies.
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