Threshold correction system in FSK transmissions

Details Threshold correction system in FSK transmissions Threshold correction system in FSK transmissions

1974-10-23

1976-03-30

Griffin, Robert L.

Telegraphy

Systems

Printing

178 88, H04B 112, H04L 2714

Patent

active

039477690

ABSTRACT:
A demodulator produces mark and space signals in response to mark and tone signals received from a radio receiver. The signals after detection are in the form of characteristic tones and are separated by corresponding band pass filters. After separation, AM detection of the mark and space signals is accomplished in envelope detectors whose output voltages are D.C. voltages. In the absence of noise, the detector voltages are essentially a rounded off version of the binary signal (mark) and a rounded off version of the complement (space) of the binary signal. These voltages are compared and their difference, a binary signal, after filtering is applied to a system which automatically computes a mark-space decision threshold for a comparator or slicer such that the slicer produces a mark output when the filtered output is above the threshold and a space output when the signal is below the threshold to achieve optimum detection of the mark-space data during fading of either one or both of the carrier waves on which such data is transmitted to the radio receiver. The optimum decision threshold is computed from the signal peaks which are stored in two peak detectors, one for the positive peak and one for the negative peak. The peak detectors are periodically discharged to allow a fresh peak to be stored and the discharge circuits obtain their cues from state changes observed at the slicer output. Hence the decisions themselves signal the timing of the discharge of the peak detectors. A delay network ensures that the charge stored by each peak detector is not caused by noise; and also a sequencing logic circuit prevents the same detector from being discharged twice in a row thereby providing additional immunity to noise.

REFERENCES:
patent: 2476990 (1949-07-01), Mason
patent: 2999925 (1961-09-01), Thomas
patent: 3348153 (1967-10-01), Featherston
patent: 3372234 (1968-03-01), Bowsher et al.
patent: 3473131 (1969-10-01), Perkins, Jr.

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