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Filter Response: Opt49LP
Figure 1: step response vs. time
120%
100%
80%
60%
40%
20%
0%
time, 2/Fc per division from 0 to 16/Fc
Figure 2: passband phase response
0
o
-90
o
-180
o
-270
o
-360
o
-450
o
-540
o
frequency, 0.1Fc per division from 0 to 1.1Fc
Figure 3: passband amplitude response
.5dB
0dB
0dB
-.5dB
-1dB
-20dB
-40dB
-60dB
-2dB
-80dB
-100dB
-3dB
frequency, 0.1Fc per division from 0 to 1.1Fc
UK:
tel
fax
Kemo ltd, 3 Brook Court, Blakeney Road
Beckenham, Kent, BR3 1HG
(+44) 181 658 3838
(+44) 181 658 4084
US:
tel
fax
-120dB
frequency, logarithmic scale from 0.1Fc to 10Fc
Kemo, Inc, 190 Raven Road
Landrum, SC 29356
864 895 8100
864 895 8900
Figure 4: overall frequency response
Description
The Option 49LP response is a modified elliptic filter; it has
a flat passband with ripple of less than 鹵0.1dB up to the
cutoff frequency, and a stopband of -77dB starting at 1.5
times cutoff (figures 3 and 4). The phase response of such
filters is quite non-linear (shown in figures 2,6,7 and 9). See
figures 1 and 5, and the table overleaf, for details of
overshoot and settling behaviour.
Applications
This response shape has found wide acceptance as an
alias protection filter for applications where analysis is
carried out in the frequency domain (e.g. FFT analysis), and
wide sampled bandwidth is more important than the time
history of the waveform. Minimum suggested sample rate is
2.5 times the filter cutoff frequency.
Availability
Option 49LP was designed for, and is primarily used on the
VBF10M laboratory filter instrument and the VBF35
multichannel system, though it can be supplied on some
other Kemo products. For a similar, industry-standard
response on most multi-channel Kemo products, see Option
01LP (document number KT70086).
filter order and type:
basic stopband:
document number:
this issue dated:
n=8 lowpass
-77dB at 1.5 times Fc
KT70180
23 October 1997
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