Precision Methods Used in Constructing Electric Wave Filters for Carrier Systems

NE of the most important fields of usefulness of the electric wave filter in the Bell System is found in carrier current telephone and telegraph systems. A carrier telephone system transmits several messages over the same line by employing several carrier currents of frequencies higher than those in the ordinary voice band, and modulating these carriers with the messages to be transmitted. The messages are then transmitted over the line as side bands of their respective carriers, and are demodulated to their original voice frequencies at the receiving end. Usually only one side band of a carrier is transmitted in order to reduce the frequency space required for each channel. Each message as it appears on the line occupies its own portion of the frequency spectrum, distinct from that occupied by any other message, and is transmitted without interference from other messages; but in order that the operations of modulation and demodulation may be carried on without interference and in order that unwanted side bands may be suppressed, it is necessary that each modulator and demodulator be equipped with some apparatus which will pass all the side band frequencies making up one message and reject all others. Electric wave filters are the instruments used for this purpose, since they possess the property of passing currents of certain chosen frequencies with very small loss, and of offering high attenuation to other chosen frequencies. The theory of electric wave filters as used in carrier systems has been discussed in previous articles in this Journal, notably "Physical 264 ELECTRIC WAVE FILTERS 265 Theory of the Electric Wave Filter" by G.