*Create a digital lowpass filter with SPTool.
The sampling frequency is 22050 Hz, the passband edge
is at 1800 Hz ( F_{p}), the stopband edge at 2400 Hz (F_{s}),
the passband ripple 1 dB (R_{p}),
the minimum stopband attenuation 40 dB (R_{s}).*

**Solution**

- 1.
- Open SPTool and the Filter Designer. Open Filter Viewer
for observations.
- 2.
- Choose Butterworth IIR from the popup-menu Algorithm.
Set the sampling frequency and type specifications into
the edit boxes in the left.
Press
*enter*in the keyboard after each value. After giving all values, press Apply button. The order of the filter should be 18, actual (measured) ripple 0.6471 dB and the 3dB point at 1887 Hz. - 3.
- Choose Elliptic IIR from the popup-menu Algorithm.
Check that specifications stayed the same. Now the
order should be 5. See the figures 3
(Designer) and 4 (Viewer).
- 4.
- Choose Equiripple FIR from the popup-menu Algorithm.
Now the order is as high as 52. The specifications
are actually not met. Notice from the Filter
Viewer that the phase is linear which implies that
group delay is constant (52/2=26 time units), see
the figures 5 and 6.