Circuit for attenuating disturbing noise signals
First Claim
1. A circuit for attenuating noise signals in an FM receiver, having an FM-demodulator coupled to a variable attenuator having a control input, wherein the output signal from said FM-demodulator is also applied to an absolute value function signal generator having an output signal from which a control signal for said variable attenuator is derived, which controls signal increases the attenuation of said variable attenuator when the absolute value of the demodulator output signal rises above a threshold level, characterized in that said circuit further comprises means coupled to the control input of said variable attenuator for generating a variable control signal dependent on the absolute value of said demodulator output signal when said absolute value exceeds said threshold level, said control signal having a first and a second time constant associated respectively with an increase or a decrease in the the absolute value output signal, said second time constant being considerably larger than said first time constant.
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Abstract
In the circuit according to the invention the entire FM-demodulator output signal is evaluated, that is to say the sum of the low-frequency signal and the d.c. voltage (AFC) component. The absolute value is produced therefrom and an attenuator circuit becomes operative in the low-frequency channel when the absolute value of the demodulator output signal exceeds a predetermined threshold value. This not only enables the suppression of spurious receptions, but also the suppression of intermediate receptions (the reception of two transmitters on the filter edges) and surging of noise. The circuit can be easily implemented in integrated circuit technique.
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Citations
6 Claims
- 1. A circuit for attenuating noise signals in an FM receiver, having an FM-demodulator coupled to a variable attenuator having a control input, wherein the output signal from said FM-demodulator is also applied to an absolute value function signal generator having an output signal from which a control signal for said variable attenuator is derived, which controls signal increases the attenuation of said variable attenuator when the absolute value of the demodulator output signal rises above a threshold level, characterized in that said circuit further comprises means coupled to the control input of said variable attenuator for generating a variable control signal dependent on the absolute value of said demodulator output signal when said absolute value exceeds said threshold level, said control signal having a first and a second time constant associated respectively with an increase or a decrease in the the absolute value output signal, said second time constant being considerably larger than said first time constant.
Specification