Abstract A continuous rail alternating current track circuit has been provided having shunts at extreme ends for terminating the circuit. A track transmitter is coupled to the rails for center feeding the circuit with a first signal having an associated effective range less than the distance to the shunt. The improvement for providing accurate detection of a vehicle shunt between the termination shunt and the end of the effective range of the signal includes a transmitter located at each end of the track circuit coupled to the rails for propagating a second signal at least to the range end of the first signal. Means at the range end of the second signal modulates the first and second signals in the rails to high and low side-band frequencies and a detector means at each end of the track circuit responds to at least one side-band frequency for detecting the modulated frequency and providing indication of vehicle presence when the side-band frequency is interrupted.
A continuous rail alternating current track circuit having shunts at extreme ends for terminating the circuit and a track transmitter coupled to the rails for center feeding the circuit with a first signal having an associated effective range less than the distance to the shunt wherein the improvement for providing accurate detection of a vehicle shunt between the shunt and the end of the effective range of the first signal comprises: a. a second transmitter located at each end of the track circuit coupled to the rails for propagating a second signal at least to the effective range end of the first signal; b. means at the range end of the second signal for modulating the first and second signals in the rails to high and low sideband frequencies; and c. detector means located at each end of the track circuit coupled to the rails and responsive to at least one side-band frequency for detecting the modulated frequency and providing indication of vehicle presence when said side-band frequency is interrupted.
Claim 2
Dependent Claim
170 words
A continuous rail alternating current track circuit having shunts at extreme ends for terminating the circuit and a track transmitter coupled to the rails for center feeding the circuit with a first signal having an associated effective range less than the distance to the shunt wherein the improvement for providing accurate detection of a vehicle shunt between the shunt and the end of the effective range of the first signal comprises: a. a second transmitter located at each end of the track circuit coupled to the rails for propagating a second signal at least to the effective range end of the first signal; b. means at the range end of the second signal for modulating the first and second signals in the rails to high and low sideband frequencies; and c. detector means located at each end of the track circuit coupled to the rails and responsive to at least one side-band frequency for detecting the modulated frequency and providing indication of vehicle presence when said side-band frequency is interrupted.
Claim 3
Dependent Claim
19 words
The apparatus of claim 1 wherein said modulating means is a rectifier coupled from one rail to the other.
Claim 4
Dependent Claim
26 words
The apparatus of claim 2 wherein said modulating means further includes a series connected choke coil for attenuating harmonic modulation products above the high side-band frequency.
Claim 5
Dependent Claim
11 words
The apparatus of claim 2 wherein said rectifier is a diode.
Claim 6
Dependent Claim
20 words
The apparatus of claim 1 wherein said detector means is responsive to the low side-band for receiving maximum signal strength.
Claim 7
Dependent Claim
39 words
The apparatus of claim 1 wherein said first and second signals are relatively of low and high frequency respectively for utilizing long effective range capability of the low frequency signals and short ambiguous region capability of high frequency signals.