Device for disconnecting the receiver in case of a small signal-to-noise ratio for a digital-modulated radio system

Claim 1

Apparatus for disconnecting a receiver in response to a small signal-to-noise ratio in a digital modulated radio system, comprising: a receiving mixer operable to receive a transmitted radio signal and convert the same to an intermediate frequency; a frequency shift keyed demodulator comprising an input coupled to said mixer, two channels each including in series a band-pass filter connected to said input, an amplifier and a detector, and a voltage comparator connected to said detectors and operable to be switched at a frequency which increases with decreasing signal-to-noise ratio; an AND gate; connecting means connecting said voltage comparator to said AND gate; and frequency-to-voltage conversion means for obtaining a voltage which is inversely proportional to the signal-to-noise ratio connected between said voltage comparator and said AND gate and including in series a high-pass filter, a detector and a further comparator having a first input connected to said last-mentioned detector and a second input, and a variable voltage divider connected to said second input for setting a minimum acceptable signal-to-noise ratio, said AND gate operated to disconnect the receiver in response to signal-to-noise ratios less than the set minimum.