YOKOGAWA DL750 - Appendix 6 DSP Channel Computation (Optional)

App-23
IM 701210-06E
Appendix
App
Appendix 6 DSP Channel Computation (Optional)
Digital Filter Computation of DSP Channels
Filter Types
On DSP channels, the following two types of digital filter computation can be performed.
• FIR
• IIR
• FIR
FIR digital filter. The signal block diagram of the computation is shown below. The
characteristics of FIR filters are as follows.
1. Achieves steep high-order filter within the allowed computation time.
However, computation delay increases as the order increases.
2. Group delay is constant due to its linear phase characteristics. Therefore,
phase distortion is small.
DSP channels can use the following FIR filters.
• SHARP
• GAUSS
• MEAN (moving average)
Signal Block Diagram of an FIR Filter
Z
-1
Z
-1
Z
-1
Z
-1
Z
-1
a
1
a
0
a
2
a
3
a
n-2
a
n-1
+
Z
-1
a
k
Input
Output
nth order FIR (number of taps n)
• IIR
IIR digital filter. The signal block diagram of the computation is shown below. The
characteristics of IIR filters are as follows.
1. Sufficient cutoff characteristics are obtained even at a relatively low order.
Therefore, the computation delay and group delay are smaller than those of
FIRs.
2. The frequency can be set lower than FIRs.
3. Phase distortion is greater than FIR filters due to its non-linear phase
characteristics.
DSP channels can use IIR (Butterworth) as an IIR filter.
Signal Block Diagram of an IIR Filter
Output
Z
-1
+
a
11
b
11
+
Z
-1
a
21
b
21
+
+
Z
-1
+
a
12
b
12
+
Z
-1
a
22
b
22
+
+
b
01
b
02
Input 4th order IIR