Before revision: Zobel, O J, Electrical wave filters, US patent 1 850 146, filed 25 Nov 1930, issued 22 Mar 1932 Like the m-type filter from which it is derived, the mm'-type filter type was intended to provide an improved impedance match into the filter termination impedances and originally arose in connection with telephone frequency division multiplexing.
After revision: Like the m-type filter from which it is derived, the mm'-type filter type was intended to provide an improved impedance match into the filter termination impedances and originally arose in connection with telephone frequency division multiplexing.