App-34
IM 701210-06E
• Band Pass
• Pass band is flat.
• The attenuation falls to –3 dB at each end of the pass band.
• 4th order Butterworth. No ripple in the stop band. For the cutoff characteristics,
see the characteristics examples.
• Has non-linear phase characteristics.
• Lower frequencies can be specified as compared to SHARP filters.
-80
-60
-40
-20
0
Center
frequency
-3 dB
IIR (Butterworth) Band-pass Frequency Characteristics Example
Pass band
0 f
0.1 f
0.2 f
0.3 f
0.4 f
0.5 f
dB
(f: sampling frequency [Hz])
For IIR (Butterworth) band-pass filters, the center frequency that can be specified is
limited by the pass-band width.
Selectable Range of IIR (Butterworth) Bandpass Filter Frequency
Passband Width Setting [%]
Lower Limit of Center
Upper Limit of Center
Frequency [%]
Frequency [%]
(Passband Region)
(Passband Region)
1
0.6
30
(0.1 to 1.1)
(29.5 to 30.5)
2
1.2
30
(0.2 to 2.2)
(29 to 31)
5
2.6
30
(0.1 to 5.1)
(27.5 to 32.5)
10
5.2
30
(0.2 to 10.2)
(25 to 35)
15
7.6
30
(0.1 to 15.1)
(22.5 to 37.5)
20
10.2
30
(0.2 to 20.2)
(20 to 40)
Computation Delay
In the case of IIR filters, the computation delay cannot be determined uniquely (unlike
FIR filters). Because IIR filters do not have linear phase characteristics, the delay varies
depending on the input frequency.
The group delay characteristics express the relationship between the input signal
frequency and the delay. The computation delay is a value obtained by adding 4
sampling clock cycles to the delay time indicated by the group delay characteristics.
The computation delay can be derived from the following equation.
Computation delay = (4 + group delay)
×
sampling period
However, if the sampling frequency exceeds 100 kHz, it is fixed to 100 kHz (10
µ
s).
It is also fixed to 100 kHz (10
µ
s) when in envelope mode.
Appendix 6 DSP Channel Computation (Optional)