Calculator program evaluates elliptic filters

Many designers consider the elliptic-transfer function to be the most useful of all analog-filtering functions, because of its steep roll-off at the band edges. You can use a Texas Instruments model V200 Voyage programmable calculator and the program in Listing 1 to evaluate a lowpass elliptic filter by finding its characteristic's poles and zeros. To do so, this program implements Darlington's algorithm (Reference 1). The program accepts as input the filter's maximum passband-attenuation ripple in decibels, its stopband and passband frequencies in radians per second, and its order, or number of poles (Figure 1).

As an example, calculate the zeros, poles, and stopband attenuation of an elliptic, fifth-order, analog lowpass filter with maximum gain of 0.1 dB and stopband frequency of 1.05 radians/sec. Figure 2 illustrates the calculator's display screens during program execution.