Practical Filter Design (5)

30 PRACTICAL FILTER DESIGN (5) by H. Baggott Last month we discussed the design of wide band-pass filters. This month"s instalment in the series deals with narrow band-pass network Not only the gradient of the skirts of a band-pass filter, but also its relative band- width, B, is important - see Fig. 26. Steep slopes and a narrow pass band are two conflicting requirements, so that the filter design is really a compromise. pass filter with a centre frequency of 1000 Hz, a bandwidth of 250 Hz and an attenua- tion of not less than 40 dB at 400 Hz and 3000 Hz. The frequency corresponding to 400 Hz at the other side of fe is calculated as follows: 400f2 = 10002, so that f2 = 10002/ 400 = 2500 Hz, that is, an attenuation of 40 dB at 400 Hz is already achieved at 2500 Hz. If we choose an attenuation of 40 dB at 3000 Hz, the corresponding frequency at the low-frequency side of the curve will be: f1 = 10002/ 3000 = 333.3 Hz. This does not give the required attenuation and, therefore, we ...