® Filter Response: Opt49LP Figure 1: step response vs. time 120% filter order and type: basic stopband: document number: this issue dated: n=8 lowpass -77dB at 1.5 times Fc KT70180 23 October 1997 100% 80% Description 60% 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. 40% 20% 0% time, 2/Fc per division from 0 to 16/Fc Figure 2: passband phase response 0 o -90o 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. -180o Availability -270o 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). -360o -450o -540o frequency, 0.1Fc per division from 0 to 1.1Fc Figure 4: overall frequency response Figure 3: passband amplitude response .5dB 0dB 0dB -20dB -.5dB -40dB -1dB -60dB -80dB -2dB -100dB -120dB -3dB frequency, logarithmic scale from 0.1Fc to 10Fc 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 Kemo, Inc, 190 Raven Road Landrum, SC 29356 864 895 8100 864 895 8900 Filter Response Figure 5: accuracy vs. time Figure 8: vector error 0dB 16bit -10dB 15bit -20dB 14bit -30dB 13bit -40dB 12bit -50dB 11bit -60dB 10bit -70dB frequency, 0.1Fc per division from 0 to 1.1Fc time, 2/Fc per division from 0 to 16/Fc 100o Figure 6: passband phase deviation Figure 9: passband phase linearity 10o 80o 8o 60o 6o 40o 4o 20o 2o 0o 0o -20o -2o -40o -4o -60o -6o -80o -8o -100o -10o frequency, 0.1Fc per division from 0 to 1.1Fc frequency, 0.1Fc per division from 0 to 1.1Fc Figure 7: passband group delay 4/Fc 3/Fc 2/Fc 1/Fc 0 frequency, 0.1Fc per division from 0 to 1.1Fc Response information for: OPT49 stopband response -77.23dB at 1.525Fc equivalent attenuation slope 126.84 dB per octave zero frequency delay 0.7831/Fc z.f. phase line (used in Figure 6) -281.92deg x f/Fc mean phase line (used in Figure 9) -326.32deg x f/Fc best phase line (used in Figure 8) -335.64deg x f/Fc attenuation: 0.1dB 0.989Fc 0.25dB 0.996Fc 0.5dB 1.005Fc 1dB 1.016Fc 3dB 1.042Fc 6dB 1.068Fc 12dB 1.11Fc 18dB 1.151Fc 24dB 1.194Fc 36dB 1.282Fc 48dB 1.366Fc 60dB 1.438Fc 72dB 1.487Fc 84dB [2.367Fc] 96dB [2.544Fc] overshoot 19.81% at 1.4/Fc risetime to 0.996Vin 1.142/Fc approximate settling time to 9 bits 9.9/Fc add on for each subsequent bit: 1.427/Fc (page 2)