Signal-to-noise ratio indicating circuit for fm receivers

Claim 2

Apparatus for detecting the reception of an FM signal having a predetermined signal quality, said apparatus comprising: demodulator means for recovering signal information from said FM signal; out-of-band filter means coupled to said demodulator means for isolating a noise signal in a frequency range substantially unaffected by said signal information; field intensity means for generating a field intensity signal having a magnitude proportional to the field intensity of said FM signal being received; signal gain means coupled to said out-of-band filter means and to said field intensity means for amplifying said isolated noise signal by a gain which is a function of said intensity signal to produce an amplified noise signal; and peak detector means coupled to said signal gain means for comparing the peak value of said amplified noise signal to a predetermined value and for producing a signal to indicate whether an FM signal is being received having said predetermined signal quality.

Claim 3

The apparatus of claim 1 wherein said out-of-band filter means is comprised of a filter having a lower cutoff frequency in the range of about 150 kilohertz and higher.

Claim 4

The apparatus of claim 2 wherein said lower cutoff frequency is about 200 kilohertz.

Claim 5

The apparatus of claim 1 wherein said out-of-band filter means is comprised of a passband filter having a lower cutoff frequency in the range of about 150 kilohertz and higher.

Claim 6

The apparatus of claim 4 wherein said passband filter has a passband of about 2 kilohertz and wherein said lower cutoff frequency is about 200 kilohertz.

Claim 7

The apparatus of claim 1 wherein said signal gain means includes a gain control such that said gain decreases in response to an increase in said field intensity signal above a predetermined intensity.

Claim 8

The apparatus of claim 6 wherein said gain control includes a trigger switch for setting said predetermined intensity and switching said signal gain means between a high gain and a low gain.

Claim 9

An FM radio receiver for receiving broadcast signals in the FM band, said receiver comprising: tuner means for tuning to a portion of said FM band; scan tuning means coupled to said tuner means for scan tuning said receiver in response to a scan signal and for stopping said scan tuning in response to a stop signal; demodulator means coupled to said tuner means for recovering audio signal information from said FM signal; out-of-band filter means coupled to said demodulator means for isolating a noise signal in a frequency range substantially unaffected by said audio signal information; field intensity means coupled to said tuner means for generating a field intensity signal having a magnitude proportional to the field intensity at which said FM signal is being received; signal gain means coupled to said out-of-band filter means and to said field intensity means for amplifying said noise signal by a gain which is a function of said field intensity signal to produce an amplified noise signal; and peak detector means coupled to said signal gain means and to said scan tuning means for comparing the peak value of said amplified noise signal to a predetermined value and for producing said stop signal when said peak value is below said predetermined value to indicate that an FM signal is being received having a predetermined signal quality.

Claim 10

An FM radio receiver for receiving broadcast signals in the FM band, said receiver comprising: tuner means for tuning to a portion of said FM band; scan tuning means coupled to said tuner means for scan tuning said receiver in response to a scan signal and for stopping said scan tuning in response to a stop signal; demodulator means coupled to said tuner means for recovering audio signal information from said FM signal; out-of-band filter means coupled to said demodulator means for isolating a noise signal in a frequency range substantially unaffected by said audio signal information; field intensity means coupled to said tuner means for generating a field intensity signal having a magnitude proportional to the field intensity at which said FM signal is being received; signal gain means coupled to said out-of-band filter means and to said field intensity means for amplifying said noise signal by a gain which is a function of said field intensity signal to produce an amplified noise signal; and peak detector means coupled to said signal gain means and to said scan tuning means for comparing the peak value of said amplified noise signal to a predetermined value and for producing said stop signal when said peak value is below said predetermined value to indicate that an FM signal is being received having a predetermined signal quality.

Claim 11

The receiver of claim 8 wherein said out-of-band filter means is comprised of a filter having a lower cutoff frequency in the range of about 150 kilohertz and higher.

Claim 12

The receiver of claim 9 wherein said lower cutoff frequency is about 200 kilohertz.

Claim 13

The receiver of claim 8 wherein said out-of-band filter means is comprised of a passband filter having a lower cutoff frequency in the range of about 150 kilohertz and higher.

Claim 14

The receiver of claim 11 wherein said passband filter has a passband of about 2 kilohertz and wherein said lower cutoff frequency is about 200 kilohertz.

Claim 15

The receiver of claim 8 wherein said signal gain means includes a gain control such that said gain decreases in response to an increase in said intensity signal above a predetermined intensity.

Claim 16

The receiver of claim 13 wherein said gain control includes a trigger switch for setting said predetermined intensity and switching said signal gain means between a high gain and a low gain.

Claim 19

The method of claim 15 wherein said gain has a high gain level and a low gain level, said method further comprising the step of: comparing said field intensity signal to a predetermined intensity signal, said high gain level being used in said amplifying step when said field intensity signal is below said predetermined intensity signal, and said low gain level being used in said amplifying step when said field intensity signal is above said predetermined intensity signal.

Claim 20

The method of claim 16 further comprising the step of: setting the magnitude of said predetermined intensity signal to control the criterion for said predetermined signal quality.

Claim 23

The method of claim 18 wherein said gain has a high gain level and a low gain level, said method further comprising the step of: comparing said field intensity signal to a predetermined intensity signal, said high gain level being used in said amplifying step when said field intensity signal is below said predetermined intensity signal, and said low gain level being used in said amplifying step when said field intensity signal is above said predetermined intensity signal.

Claim 24

The method of claim 19 further comprising the step of: setting the magnitude of said predetermined intensity signal to control the criterion for said predetermined signal quality.