Communications: electrical – Digital comparator systems
Patent
1978-11-09
1980-04-08
Yusko, Donald J.
Communications: electrical
Digital comparator systems
324 78D, 324 82, 328134, G01K 2302
Patent
active
041975251
ABSTRACT:
An improved decoder for providing a decoder output signal when a tone signal contains frequencies that are sequentially within a predetermined passband surrounding first and second detection frequencies f.sub.1, f.sub.2. A tone signal is mixed with a reference signal which alternates, at a multiplex frequency, between sine and cosine signals, each being at a predetermined reference frequency which is less than the multiplex frequency. A tone switching logic circuit normally maintains the reference frequency at a value substantially equal to the frequency f.sub.1. The mixed signal is coupled through a demultiplexer to respective inputs of first, second, third and fourth low pass filters, each of which has a cut-off frequency which is one-half of the decoder passband. The respective outputs of the filters are coupled through a multiplexer to a squarer. Normally, the demultiplexer and the multiplexer are controlled by the tone switching logic ciruit so as to alternately couple, at the multiplex frequency, the mixed signal to the inputs of the first and second filters, and the outputs thereof to the squarer. The squarer output is applied to a summing amplifier, which removes the multiplex frequency, and which provides a dc signal which is applied to a comparator. When the value of the dc signal exceeds a threshold value, the comparator provides a first output signal which causes the tone switching logic circuit to change the reference frequency to the frequency f.sub.2, and to control the demultiplexer and multiplexer so that the mixed signal is thereafter alternately coupled, at the multiplex frequency, to the inputs of the third and fourth low pass filters, and so that the outputs thereof are alternately coupled, at the multiplex frequency, to the squarer. A decoder output signal is provided, however, only if a second output signal from the comparator is provided within a predetermined period of time following the provision of the first output signal therefrom. The reference signal is preferably provided by a sine/cosine generator from pulses provided by a divide-by-N counter occurring at the multiplex frequency, with the divisor N being controlled by the tone switching logic circuit.
REFERENCES:
patent: Re27044 (1971-02-01), Rothenbuhler, et al.
patent: 3395346 (1968-07-01), Kincheloe et al.
patent: 3806664 (1974-04-01), Bowen et al.
patent: 3984770 (1976-10-01), Chu
patent: 4021653 (1977-05-01), Sharp et al.
Biery, Jr. Galen A.
Rothenbuhler Dan E.
Rothenbuhler Engineering Co.
Yusko Donald J.
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