Cab signal pickup system with motor noise reduction

In a cab signal system having a receiver on board a locomotive and in which control information transmitted through the rails of a railroad track to the locomotive utilizes a carrier having a frequency in a range from 40 Hz or less to approximately 250 Hz, wherein the locomotive employs an alternating current drive motor connected to a pair of rail-engaging wheels and the drive motor emits high level noise in said frequency range, the improvement comprising:

• a pair of pickup coils for sensing magnetic fields around the rails produced by said control information, each of said coils having a core of sufficiently short length to cause the sensitivity of the coil to have a pronounced roll-off characteristic with increasing distance from a current-carrying conductor,
• means for mounting said pickup coils on said locomotive in an environment in which said noise is present but in operative positions closely spaced from corresponding underlying rails, whereby said roll-off characteristic and the proximity of the coils to the rails increases the signal strength of the sensed information relative to said noise,
• a cancelling coil responsive to said noise,
• means for mounting said cancelling coil on said locomotive in said environment and in a position intermediate the respective vertical planes of the rails rendering the cancelling coil substantially insensitive to the control information flowing in the rails, and
• input circuit means for said receiver interconnecting said pickup coils and said cancelling coil with said pickup coils in phase and said cancelling coil out of phase therewith, whereby to enhance the signal-to-noise ratio and deliver the sensed information to the receiver with reduced interference from the drive motor.