LOW FREQUENCY SIGNAL TRANSMISSION AND INDICATING SYSTEM

• US 3,818,345 A
• Filed: 12/29/1972
• Issued: 06/18/1974
• Est. Priority Date: 12/29/1972
• Status: Expired due to Term

• Current Assignee: Hidetoshi Yarita, Hirotaka Atsuzaka, Masami Miya
• Original Assignee: Hidetoshi Yarita, Hirotaka Atsuzaka, Masami Miya
• Inventors: Miya, Masami, Yarita, Hidetoshi, Atsuzaka, Hirotaka
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Bookbinder, Marc E.
• Application Number:
  US05/319,827
• Time in Patent Office: 536 Days
• Field of Search: 325/29,31,51,53,54,55,64,111,301,305,430 340/311,312,147C,147CV,147CN,152T 343/200
• US Class Current: 455/517
• CPC Class Codes: G08B 3/1083 Pager locating systems

Abstract

Low frequency signals fl through fn are respectively assigned to respective portable radio transmitters and each transmitter transmits a wave modulated by this assigned low frequency signal. On the other hand, a set of a radio receiver, local receiver and a local transmitter is provided in each divided area to which one of low frequency signals fL1 through fLn is assigned. A center transmitter selects a signal designating a portable radio transmitter to be detected and transmits this signal to the local receivers in these divided areas. When the object radio transmitter exists in one of these areas, the local transmitter in this area transmits the low frequency signal assigned to this area to a center receiver. The center receiver operates indication means corresponding to the signal from the local transmitter, i.e., the signal corresponding to the area, thereby detecting the area in which the object radio transmitter designated by the center transmitter is located.

Claims

1. A low frequency signal transmission and indication system comprising one or more portable radio transmitters each transmitting a modulated wave signal having the same carrier and obtained by modulating this carrier by a respectively different low frequency signal which is assigned to each transmitter, one or more radio receivers each removing said low frequency signal from said modulated wave when it receives said modulated wave, one or more control center transmitters each transmitting a modulated wave signal having the same carrier and obtained by modulating this carrier by one of a plurality of signals which respectively differ from said low frequency signals assigned to each transmitter by a constant frequency fO, one or more receivers assigned to respective geographical areas removing the low frequency signal from said modulated wave signal transmitted from said center transmitter, means for detecting the difference between the low frequency signal from said radio receiver and the low frequency signal from said geographically assigned receiver, one or more transmitters assigned to respective geographical areas each transmitting a modulated wave signal having the same carrier and obtained by modulating said carrier by another respectively different low frequency signal only when said detected difference in frequency is identical with the constant frequency fO, and a control center receiver receiving the signal from said geographically assigned transmitter, removing the low frequency signal contained in this signal and activating an indication circuit corresponding to this low frequency signal, whereby, when a carrier modulated by a selected specific low frequency signal is transmitted from said center transmitter, only a geographically assigned transmitter connected to a radio receiver which has received from one of said portable radio transmitters a wave modulated by the low frequency signal which is different from this selected specific low frequency signal by the frequency fO is activated to inform that this particular portable radio transmitter is in the vicinity of said particular geographically assigned receiver.

2. A low frequency signal transmission and indication system as defined in claim 1 in which said portable radio transmitter is of a type which intermittently transmits the modulated wave.

3. A low frequency signal transmission and indication system as defined in claim 1 in which said portable radio transmitter comprises an Oscillation circuit for oscillating the assigned low frequency, a high frequency oscillation circuit for oscillating the carrier, a switching circuit for intermittently actuating said oscillator and modulation circuit for modulating said carrier by said assigned low frequency.

4. A low frequency signal transmission and indication system as defined in claim 1 in which said geographically assigned transmitter comprises a frequency conversion circuit receiving the signal from the local receiver and the signal from the radio receiver and producing a signal which represents difference between the two signals, a narrow-band filter for filtering only the frequency fO from the signal of said frequency conversion circuit, a switching circuit for activating said high frequency oscillation circuit upon receipt of the output of said filter and means for modulating the carrier output from said high frequency oscillation circuit by a low frequency signal.

5. A low frequency signal transmission and indication system as defined in claim 1 in which said center receiver comprises a detection circuit which detects the input signals, one or more filters which respectively filter the output of the detection circuit, said indication circuit including indicators coupled to each filter.