®
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
-.5dB
-1dB
-2dB
-3dB
frequency, 0.1Fc per division from 0 to 1.1Fc
Filter Response: Opt49LP
filter order and type:
n=8 lowpass
basic stopband:
-77dB at 1.5 times Fc
document number:
KT70180
this issue dated:
23 October 1997
Description
The Option 49LP response is a modified elliptic filter; it has
a flat passband with ripple of less than ±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).
Figure 4: overall frequency response
0dB
-20dB
-40dB
-60dB
-80dB
-100dB
-120dB
frequency, logarithmic scale from 0.1Fc to 10Fc
UK:
Kemo ltd, 3 Brook Court, Blakeney Road
US:
Kemo, Inc, 190 Raven Road
Beckenham, Kent, BR3 1HG
Landrum, SC 29356
tel
(+44) 181 658 3838
tel
864 895 8100
fax
(+44) 181 658 4084
fax
864 895 8900
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