This program for the Timex-Sinclair 1000 computer was written for completely automated design of Butterworth low-pass, high-pass, band-pass or band-stop filters. Modern solid-state transmitters are more prone to harmonic and parasitic frequency radiation, and therefore it is desirable for the radio amateur to have access to modern filter design in order to comply with present FCC rules for purity of emissions. Butterworth filters are superior to image-parameter filters (constant-k and m-derived) in their efficiency and response characteristics. For further details on the theory of modern filter design, the reader is referred to recent editions of the Radio Amateur’s Handbook and to the text Electronic Filter Design Handbook (Mc-Graw-Hill, 1981) by Arthur B. Williams, from which the formulas used in this program were derived.