Arrangement for suppressing noise in an FM receiver

Claim 2

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present, in said intermediate frequency stage; and switching means connected to said first circuit means and said noise measuring means, for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field strength signal and absence of said noise signal.

Claim 3

An arrangement as set forth in claim 1, wherein said first circuit means comprise first threshold means for furnishing a first threshold output signal only when said field strength signal exceeds a first reference signal; wherein said noise measuring means comprise a second threshold stage for furnishing a second threshold output signal only when said noise signal exceeds a second reference signal; and wherein said switching circuit means comprise an inverter connected to said second threshold stage for furnishing an inverted second threshold output signal and a NAND-gate having a first input connected to the output of said first threshold stage, a second input connected to the output of said inverter and a NAND-gate output for furnishing said enabling signal.

Claim 4

An arrangement as set forth in claim 1, wherein said intermediate frequency stage limits said field strength signal to a predetermined maximum field strength signal; wherein said switching circuit means comprise a subtraction circuit for subtracting said noise signal from said field strength signal and furnishing a subtraction output signal as a function of the difference therebetween; and threshold circuit means having a threshold corresponding to a reference value slightly less than said predetermined maximum field strength signal, for furnishing said enabling signal when said subtraction output signal is equal to said reference value.

Claim 5

An arrangement as set forth in claim 1, wherein said means for furNishing a field strength signal comprise an envelope demodulator circuit connected to said intermediate frequency stage, and a DC amplifier connected to the output of said envelope demodulator circuit, said DC amplifier having an output resistor for furnishing said field strength signal; and wherein said noise measuring means comprise a capacitor connected to the output of said DC amplifier circuit for furnishing an AC signal, said AC signal constituting said noise signal.

Claim 6

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said first circuit means comprising first threshold means for furnishing a first threshold output signal only when said field strength signal exceeds a first reference signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present, in said intermediate frequency stage, said noise measuring means comprising a second threshold stage for furnishing a second threshold output signal only when said noise signal exceeds a second reference signal; and switching means connected to said first circuit means and said noise measuring means for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field-strength signal and absence of said noise signal, said switching circuit means comprising an inverter connected to said second threshold stage for furnishing an inverted second threshold output signal, and a NAND-gate having a first input connected to the output of said first threshold stage, a second input connected to the output of said inverter and a NAND-gate output for furnishing said enabling signal.

Claim 7

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said first circuit means comprising first threshold means for furnishing a first threshold output signal only when said field strength signal exceeds a first reference signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present, in said intermediate frequency stage, said noise measuring means comprising a second threshold stage for furnishing a second threshold output signal only when said noise signal exceeds a second reference signal; and switching means connected to said first circuit means and said noise measuring means for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field-strength signal and absence of said noise signal, said switching circuit means comprising an inverter connected to said second threshold stage for furnishing an inverted second threshold output signal, and a NAND-gate having a first input connected to the output of said first threshold stage, a second input connected to the output of said inverter and a NAND-gate output for furnishing said enabling signal.

Claim 8

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said intermediate frequency stage limiting said field strength signal to a predetermined maximum field strength signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present in said intermediate frequency stage; and switching means connected to said first circuit means and said noise measuring means, for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field strength signal and absence of said noise signal, said switching means comprising a subtraction circuit for subtracting said noise signal from said field strength signal and furnishing a subtraction output signal as the function of the difference therebetween, and threshold circuit means having a threshold corresponding to a reference value slightly less than said predetermined maximum field strength signal, for furnishing said enabling signal when said subtraction output signal is equal to said reference value.

Claim 9

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said intermediate frequency stage limiting said field strength signal to a predetermined maximum field strength signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present in said intermediate frequency stage; and switching means connected to said first circuit means and said noise measuring means, for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field strength signal and absence of said noise signal, said switching means comprising a subtraction circuit for subtracting said noise signal from said field strength signal and furnishing a subtraction output signal as the function of the difference therebetween, and threshold circuit means having a threshold corresponding to a reference value slightly less than said predetermined maximum field strength signal, for furnishing said enabling signal when said subtraction output signal is equal to said reference value.

Claim 10

An arrangement as set forth in claim 6, wherein said means for furnishing a field strength signal comprise a first voltage doubler circuit having a first resistor connected in parallel therewith; wherein said noise measuring means comprise a second voltage doubler circuit having a second resistor connected in parallel therewith; and wherein said subtraction circuit means comprise means for connecting said first and second resistors in series in such a manner that the voltage across the series connected resistors is equal to said subtraction output signal.

Claim 11

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said first cirCuit means comprising an envelope demodulator circuit connected to said intermediate frequency stage, and a DC amplifier connected to the output of said envelope demodulator circuit, said DC amplifier having an output resistor for furnishing said field strength signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present, in said intermediate frequency stage, said noise measuring means comprising a capacitor connected to the output of said DC amplifier circuit for furnishing an AC signal constituting said noise signal; and switching means connected to said first circuit means and said noise measuring means, for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field strength signal and absence of said noise signal.

Claim 12

In an FM receiver having an intermediate frequency stage and a low frequency stage, a noise suppression arrangement, comprising, in combination, first circuit means connected to said intermediate frequency stage for furnishing a field strength signal, said first cirCuit means comprising an envelope demodulator circuit connected to said intermediate frequency stage, and a DC amplifier connected to the output of said envelope demodulator circuit, said DC amplifier having an output resistor for furnishing said field strength signal; noise measuring means connected to said intermediate frequency stage for furnishing a noise signal corresponding to AM signals, if present, in said intermediate frequency stage, said noise measuring means comprising a capacitor connected to the output of said DC amplifier circuit for furnishing an AC signal constituting said noise signal; and switching means connected to said first circuit means and said noise measuring means, for furnishing an enabling signal for enabling said low frequency stage only in the simultaneous presence of said field strength signal and absence of said noise signal.