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United States Patent im
3,794,920  Feb. 26, 1974
 FAIL-SAFE CODE KEYING TRANSMITTER  Inventor: John O. G. Darrow, Murrysville, Pa.
 Assignee: Westinghouse Air Brake Company,
 Filed: Sept. 15, 1971  Appl. No.: 180,644
 U.S. CI 325/105, 325/123, 325/161
 Int.CI H04b 1/04
 Field of Search... 325/105, 123, 161, 170, 164;
307/260, 261; 328/21, 165, 167; 332/37 R, 37 D; 178/66 A, 68
 References Cited
UNITED STATES PATENTS
2,401,619 6/1946 Trevor... 325/164 X
3,644,832 2/1972 Sherman, Jr 325/186
3,202,939 8/1965 Reiling 332/43
3,626,417 12/1971 Gilbert 325/105 UX
3,667,049 5/1972 Ostroffetal 325/170 X
Primary Examiner—Benedict V. Safourek
Attorney, Agent, or Firm—H: A. Williamson et al.; J.
This disclosure relates to a fail-safe code keying transmitter wherein a modulating signal is reshaped to remove the harmonic producing portions therefrom prior to the modulation of a carrier wave. The code keying transmitter includes an electronic switching circuit and a series resonant L-C circuit for transforming the leading edge of the square-wave input signal into one half cycle of a cosine wave and for transforming the trailing edge of the square wave input signal into another half cycle of a cosine wave and employs a modulating circuit for varying the carrier wave in accordance with the transformed cosine wave.
6 Claims, 2 Drawing Figures
PATENTED FEB 2 61974 3,794,920
SHEET 1 flf 2
PATENTED FEB 2 61974 3.794,920
SHtET 2 OF 2
FAIL-SAFE CODE KEYING TRANSMITTER
My invention relates to a coded type of fail-safe keying transmitter and more particularly to an electronic transmission circuit arrangement for removing sharp harmonic producing portions from a square wave signal by transforming the leading and trailing edges of the square wave signal into appropriate sections of a raised cosine wave and for modulating a carrier frequency signal in accordance with the raised cosine wave.
In certain types of signal and communication systems, such as railroad and mass and/or rapid transit operations, information and commands are transmitted from one location to another in coded form. Usually the coded format takes the form of a series of marks and spaces, such as a train of rectangular or square wave pulses. It will be appreciated that rectangular or square waves have sharp demarcation portions which are troublesome to various circuits, particularly to carrier transmission systems which employ tuned receiver circuits. The problem arises from the fact that a rectangular wave form contains an infinite number of harmonics of the fundamental frequency. It has been found that these harmonics are capable of interfering with the normal operation of other receiver circuits in the system. For example, a harmonic of sufficient amplitude will pass through an unrelated receiver circuit tuned to the frequency of the harmonic and will cause the receiver to perform its function, such as, picking up or energizing a relay or the like. In a vital transmission system, this is wholly unacceptable in that a falsely operated circuit could establish a condition which could cause damage to the equipment or could result in injury or death to attending personnel. Thus, the harmonics should be removed from the coded signals prior to usage in a vital type of transmission system. In an ordinary or nonvital system, it is desirable to eliminate the harmonics simply in order to prevent cross-talk and noise signals from interfering with other circuits. Another requirement of a vital operation is that each portion or circuit of the transmission system itself must be capable of functioning in fail-safe fashion. Thus, under no circumstance should a critical circuit or component failure be permitted to allow a transmitter to produce a modulating signal having a square wave envelope, except at greatly reduced levels.
Accordingly, it is an object of my invention to provide a circuit arrangement for shaping the wave form of a modulating signal in order to remove the presence of harmonics prior to the modulation of a carrier wave.
Another object of my invention is to provide a failsafe code keying transmitter circuit for removing sharp harmonic producing portions from a modulated carrier wave form.
A further object of my invention is to provide a failsafe circuit arrangement for transforming the leading and trailing edges of a square wave signal into the respective half cycles of a raised cosine wave and thereafter modulating a carrier signal for transmission over a communication channel.
Yet another object of my invention is to provide a transmitter circuit which eliminates harmonics in a modulated carrier signal and which operates in a failsafe manner.
Yet a further object of my invention is to provide a code keying carrier transmitter for producing a modulated carrier signal having an envelope in the form of
a raised cosine wave so that harmonics are not present.
Still another object of my invention is to provide a new and improved transmitter for transforming harmonic frequency signals from a substantially square
5 wave form by transforming the leading edge of the wave form into one half cycle of a raised cosine wave and by transforming the trailing edge of the wave form into the other half cycle of a raised cosine wave and thereafter modulating a carrier signal with the trans
10 formed wave.
Still a further object of my invention is to provide a wave shaping circuit which eliminates sharp harmonic producing portions of a modulated carrier signal and which does it in a fail-safe fashion.
15 Still yet another object of my invention is to provide a fail-safe electronic circuit arrangement including a switching circuit and a series L-C network which removes harmonic producing portions of an input signal by transforming each harmonic producing portion into
20 a cosine function and a modulating circuit having a carrier input which is modulated by the transformed signal.
Still yet a further object of my invention is to provide a code keying transmitting circuit including an elec25 tronic switching circuit, an inductance-capacitance circuit and a clipping modulating circuit which are economical in cost, simple in construction, reliable, and efficient in operation. Briefly, the present invention relates to a fail-safe 30 code keying transmitter including an electronic switching circuit, a series resonant L-C circuit, and a clipping modulating circuit. The electronic switching circuit employs a pair of driving transistors and a pair of seriesconnected driven transistors. The driving transistors 3-i are connected in cascade so that both transistors are simultaneously rendered conductive and nonconductive by the square wave input signals. The conduction of the driving transistors causes one of the driven transistors to be conductive and causes the other driven transistor to be cut off. The conduction of the one driven transistor establishes a charging circuit path for the series resonant L-C circuit so that the leading edges of the square wave input signals are transformed into one half cycle of a raised cosine wave. Conversely, the nonconduction of the driving transistors causes the one driven transistor to cut off and causes the other driven transistor to conduct. The conduction of the other driven transistor establishes a discharge circuit path for the series resonant L-C circuit so that the trailing edges of the square wave input signals are transformed into another half cycle of a raised cosine wave. The transformed waves are applied to the input of the clipping modulating circuit upon which is impressed a carrier 55 signal. A modulated carrier signal is derived from the output of the modulating circuit and the modulations will have an envelope in the form of a raised cosine wave so that no accompanying harmonics are transmitted over the communication channel. 6Q The foregoing objects and other additional features and advantages of my invention will become more fully evident from the following detailed description when considered in connection with the accompanying drawings wherein:
65 FIG. 1 is a schematic circuit diagram illustrating a preferred embodiment of the present invention.
FIG. 2 is a diagrammatic illustration of a series of wave forms which will be helpful in understanding the