Method and arrangement for furnishing an indication of multipath reception in an FM receiver

In a frequency modulation receiver having discriminator means furnishing a discriminator output signal having a plurality of desired frequency signals in the absence of multipath reception and at least one harmonic of one of said desired frequency signals in the presence of multipath reception, the frequency of said harmonic differing from the frequencies of said desired frequency signals, an arrangement for furnishing an indicator signal indicative of said multipath reception, comprising, in combination, narrow bandwidth filtering means connected to said discriminator means for passing said harmonic when present, while rejecting signals of frequencies both higher and lower than said harmonic, thereby furnishing filtered harmonic signals occuring only during multipath reception;

• indicator means connected to said narrow bandwidth filtering means, for furnishing said indicator signals in response to said filtered harmonic signals;
• further comprising rectifier means connected to said narrow bandwidth filtering means, for furnishing a pulse sequence in response to said filtering harmonic signals;
• and converter means connected between said rectifier means and said indicator means for converting said pulse sequence into a substantially constant voltage for activating said indicator means, said converter means comprising pulse stretch means including monostable multivibrator means having a stable and an unstable state, and time constant means determining the time interval said monostable multivibrator means remain in said unstable state, and additional circuit means for restarting said time constant means in response to subsequent pulses in said pulse sequence thereby causing said monostable multivibrator means to remain in said unstable state for a time corresponding to the duration of said pulse sequence.

In a frequency modulation receiver having discriminator means furnishing a discriminator output signal having a plurality of desired frequency signals in the absence of multipath reception and at least one harmonic of one of said desired frequency signals in the presence of multipath reception, the frequency of said harmonic differing from the frequencies of said desired frequency signals, an arrangement for furnishing an indicator signal indicative of said multipath reception, comprising, in combination, narrow bandwidth filtering means connected to said discriminator means for passing said harmonic when present, while rejecting signals of frequencies both higher and lower than said harmonic, thereby furnishing filtered harmonic signals occurring only during multipath reception;

• indicator means connected to said narrow bandwidth filtering means, for furnishing said indicator signals in response to said filtered harmonic signals;
• further comprising rectifier means connected to said narrow bandwidth filtering means for furnishing a pulse sequence in response to said filtered harmonic signals, said converter means comprising pulse stretch means including a charging capacitor, and further circuit means for discharging said capacitor rapidly in response to each pulse in said pulse sequence and charging said capacitor slowly in the absence of pulses in said pulse sequence;
• and converter means connected between said rectifier means and said indicator means for converting said pulse sequence into a substantially constant voltage for activating said indicator means.