TUNE DETECTING CIRCUIT

Abstract: A tune detecting circuit that is used, for example, for an FM radio receiver and that has the function of detecting the mute band. The tune detecting circuit helps solve the problem related to setting a band ratio of the tune detecting band to the non-mute band. For this purpose, a resistor for setting a band to be detected is provided between an end of a capacitor that removes AC signals included in the FM detect signals and an end of the power source that supplies reference power source. An input terminal of a first buffer circuit is connected to a point where the resistor and the capacitor are connected together, and an input terminal of a second buffer circuit is connected to a point where the resistor and the power source are connected together. A tuning band detector that detects the tuning band is connected to the output terminals of the first and second buffer circuits. A detuned mute band detector that detects the detuned mute band is provided between any divided point of the resistor and an end of the power source. In response to an output signal from the detuned mute band detector, the mute circuit controls the gain of an amplifier that amplifies FM detect signals.

Description: SPECIFICATION Technical Field The present invention relates to a tuning detector used for, e.g., an FM receiver, and having a mute band detecting function for detecting a mute band when a signal is detuned. Background Art Fig. 4 shows a conventional tuner. An input terminal 11 for receiving an FM intermediate-frequency signal is connected to the input terminal of an FM detector 12. The output terminal of the FM detector 12 is connected to amplifiers 13 and 14, and the output of the amplifier 14 is connected to an output terminal 15. The output terminal of the amplifier 13 is grounded through a capacitor C1 for removing an AC signal and grounded through a resistor R11 and a reference voltage Vref. Transistors Q11, Q12, Q13 for constituting a detuning mute band detecting section is connected between the two terminals of the resistor R11. The collector of the transistor Q12 is connected to a mute circuit 16 for controlling the gain of the amplifier 14 through transistors Q16, Q15, and Q14 and a resistor R12. In addition, the collector of the transistor Q12 is connected through a resistor R13 to a transistor Q17 for generating a tuning detection signal. The collector of the transistor Q17 is connected to a power source Vcc through a resistor R14 and to an output terminal 17 for the tuning detection signal. With the above arrangement, an FM intermediate-frequency signal supplied to the input terminal 11 is detected by the FM detector 12. This detection output signal is amplified by the amplifier 13 and output to the output terminal 15 through the amplifier 14. The AC component of the output signal from the amplifier 13 is removed by the capacitor C1, and the DC part of the output signal is supplied to the resistor R11. When a voltage generated across two terminals A and B of the resistor R11 is lower than the voltages of the transistors Q11 and Q12, i.e., ±V BE , the transistors Q11 and Q12 are set in an OFF state. Therefore, the transistors Q17 and Q14 driven by the transistors Q11 and Q12 are set in an OFF state. For this reason, a high-level tuning detection signal is supplied to the output terminal 17, and the mute circuit 16 is not driven. When the voltage between the terminals A and B of the resistor 11 is higher than the voltage ±V BE of the transistor Q11 or Q12, the transistor Q11 or Q12 is turned on. Therefore, the transistor Q17 is turned on, and a low-level detuning detection signal is supplied to the output terminal 17. The transistor 14 is turned on to drive the mute circuit 16, and the detection output is attenuated. In the above conventional tuning detecting circuit, the transistors Q17 and Q14 are driven by currents obtained by shunting the collector output current of the transistor Q11 or Q12 through the resistors R12 and R13. That is, the currents shunted by the resistors R12 and R13 are used to detect tuning and detuning and to drive the mute circuit 16. For this reason, when a ratio of a tuning detection band to a non-mute band is set to be a predetermined ratio, if, e.g., the resistance of the resistor R13 is decreased, the tuning detection band is narrowed, and the non-mute band is widened. Therefore, the band ratio cannot be easily set. In addition, since an impedance from the resistor R13 to the transistor Q17 is different from an impedance from the resistor R12 to the transistor Q15, the band ratio is not easily set by a ratio of the resistor R13 to the resistor R12. The band ratio is influenced by a current amplification factor β,i.e., a base current value of the transistor Q17. For example, when the transistor Q17 is set in an ON state, the base current value is changed by the value of the resistor R14 for controlling the collector current of the transistor Q17. Therefore, the band width is influenced, and the band ratio is changed. JP-A-59 24 200 teaches employing a circuit for generating both a muting signal having wide-band characteristics and a stop-trigger signal having narrow-band characteristics for stopping tuning operation of an auto tuning circuit. Document JP-A-61 141 224 relates to a muting circuit for preventing the center of muting from being biased by a current flowing to a resistor inserted to a point giving a reference voltage and an FM detection S-curve output voltage to a muting switching circuit. Besides disclosing the features contained in the preamble of appended claim 1, this document also discloses that the width of a muting signal can be decided by a resistor. JP-A-58 164 329 discloses an FM muting circuit with the object of eliminating distortion by making the extent of attenuation only in a range where an S-curve is present, the outside of a muting band greater than the extent of the muting attenuation when no electric field is normally established during detuning. This leads to an increase in the degree of attenuation in the vicinity of the tuning band. Document JP-A-57 041 043 shows a frequency modulation radio receiver where the output of a band detecting circuit is supplied to two voltage comparing circuits whose comparison voltage is different respectively, and taking out a muting signal and a tuning instruction signal from respective voltage comparing circuits. Disclosure of Invention It is an object of the present invention to provide a tuning detecting circuit in which a ratio of a tuning detection band to a non-mute band can be easily set, and the band ratio is not influenced even when characteristics of transistors are not identical to each other. The present invention is achieved by the features described in the characterizing portion of appended claim 1. That is, according to the present invention, the tuning detecting section for detecting tuning is arranged between the two terminals of the resistor for setting the band to be detected, and the detecting section for detecting the detuning mute band in accordance with the arbitrary voltage divided by the resistor is arranged. A ratio of a turning detection band to a non-mute band can be accurately set by a ratio of resistances. In addition, since an input voltage to each detecting section can be determined by a ratio of resistances, an influence of an error of a circuit element can be eliminated. Brief Description of Drawings Fig. 1 is a circuit diagram showing a tuning detector according to an embodiment of the present invention, Figs. 2 and 3 are graphs for explaining an operation of the tuning detector in Fig. 1, and Fig. 4 is a circuit diagram showing a conventional tuning detector. Best Mode of Carrying Out the Invention An embodiment of the present invention will be described below with reference to the accompanying drawings. In Fig. 1, an input terminal 1 for receiving an FM intermediate-frequency signal Fin is connected to an FM detector 2. The output terminal of the FM detector 2 is connected to amplifiers 3 and 4. The output terminal of the amplifier 4 is connected to an output terminal 5. The output terminal of the amplifier 3 is grounded through a capacitor C for removing an AC signal and grounded sequentially through resistors R1 and R2 for setting a band to be detected and a reference voltage Vref. In addition, transistors Q8 and Q9, each of which has a base and a collector connected to each other to serve as a diode, and which constitute a limiter, are connected between two terminals A and B of the resistor R1. Transistors Q7, Q5, and Q6 constituting a detuning mute band detecting section MD are connected between two terminals B and C of the resistor R2. That is, the bases of the transistors Q7 and Q5 and the emitter of the transistor Q6 are connected to a connection point B of the resistors R1 and R2. The collector of the transistor Q7 is connected to its base, the emitters of the transistors Q7 and Q5 are connected to the base of the transistor Q6 and to a connection point C of the resistor R2 and the reference voltage Vref. The collector of the transistor Q6 is connected to the collector of the transistor Q5. Note that the detuning mute band detecting section MD may be connected between the two terminals A and B of the resistor R1, the transistors Q8 and Q9 constituting the limiter circuit may be connected between the two terminals B and C of the resistor R2. The base of a transistor Q1 having a high-input impedance is connected to the connection point A, the base of the the transistor Q2 is connected to the connection point C of the resistor R2 and the reference power source Vref, and the emitters of the transistors Q2 and Q1 are grounded through a constant current source I respectively. The emitter of the transistor Q1 is connected to the emitter of a transistor Q4 through a resistor R3. The base of the transistor Q4 is connected to the emitter of the transistor Q2 and to the emitter of a transistor Q3 through a resistor R4. The base of the transistor Q3 is connected to the emitter of the transistor Q1, and the collector of the transistor Q3 is grounded with the collector of the transistor Q4 through a diode-connected transistor Q10 and a resistor R5. These transistors Q3 and Q4 and the resistors R3 and R4 constitute a tuning band detecting section TD, and the resistor R5 and the transistor Q10 are arranged to reliably operate a transistor Q11 to be describe later. In addition, the collectors of the transistors Q3 and Q4 are connected to the base of the transistor Q11 through a resistor R8. The emitter of the transistor Q11 is grounded, and the collector of the transistor Q11 is connected to the power source Vcc through a resistor R9 and to a tuning/detuning output terminal 6. The resistor R8 and the transistor Q11 constitute a tuning detection signal output section TO. The collectors of the transistors Q5 and Q6 are grounded through a resistor R6 and a diode-connected transistor Q12 and to the base of a transistor Q13 through a resistor R7. The emitter of the transistor Q13 is grounded, and the collector of the transistor Q13 is connected to a mute circuit 7. The resistor R7 and the transistor Q13 constitute a detuning mute signal generator MG. The mute circuit 7 is connected to the amplifier 4, and the gain of the amplifier 4 is decreased to attenuate the detection output by the mute circuit 7. An operation of the tuning detector with the above arrangement will be described below with reference to Figs. 2 and 3. When a signal having a frequency having a range of ±Δfo from a center frequency fo (10.7 MHz) is input to the FM detector 2 through the input terminal 1, a potential difference of ±ΔV is generated across the two terminals A and C of the resistors R1 and R2 proportionally to the output signals from the FM detector 2 and the amplifier 3. The potential difference ±ΔV is transferred to the transistors Q3 and Q4 through the transistors Q1 and Q2. In this case, when a relationship between the potential difference ±ΔV and voltages V BE of transistors Q3 and Q4 is given by relation (1), ±ΔV ≥ V BE Q3(on), V BE Q4(on) the transistor Q3 or Q4 is turned on, the transistor Q11 is driven by the collector current of the transistor Q3 or Q4, and a detuning detection signal of low level is output from the tuning/detuning output terminal 6. When the above condition is given by relation (2), ±ΔV < V BE Q3(on), V BE Q4(on) the transistors Q3 and Q4 are set in an OFF state. Therefore, the transistor Q11 is set in an OFF state, and a tuning detection signal of high level is output from the tuning/detuning output terminal 6. When the detuning detection signal is output, a potential between the emitter of the transistor Q1 (Q2) and the base of the transistor Q4 (Q3) is substantially equal to a potential V BE Q4(on) (V BE Q3(on)), and the potential is equal to a potential |±▵V| across two terminals A and C of the resistor R1 and R2. In this state, since a potential between the two terminals C and B of the resistor R2 is obtained by dividing a potential |±▵V| (V BE Q4(on), V BE Q3(on)) by the resistors R1 and R2, it is smaller than the potential V BE (on). Therefore, the transistors Q5 to Q7 constituting the detuning band detecting section MD are still set in an OFF state, and the mute circuit 7 is not driven. The detuning is advanced from the above state to increase the frequency ±▵f, and the potential between the terminals C and B of the resistor R2 exceeds a potential for turning on the transistor Q5, i.e., a potential V BE Q5(on), or a potential for turning on the transistor Q6, i.e., a potential V BE Q6(on). At this time, the transistor Q5 or Q6 is turned on. For this reason, the transistor Q13 is turned on by a collector current of the transistor Q5 or Q6. Therefore, the mute circuit 7 is driven to decrease the gain of the amplifier 4, thereby attenuating the detected output. A ratio of a tuning detection band to a non-mute band will be described below. When a potential difference ±▵V is generated across the two terminals A and C of the resistors R1 and R2 in response to an output signal from the amplifier 3, a tuning state is given by relation (3): ±ΔV < V BE Q3(on), V BE Q4(on) and a detuning state is given by relation (4): ±ΔV ≧ V BE Q3(on), V BEQ 4(on) On the other hand, the potential difference between the two terminals A and C of the resistors R1 and R2 when a signal is detuned to be muted is represented by ±ΔV 1 , ±ΔV 1 ·R2/(R1 + R2) ≥ V BE Q5(on), V BE Q6(on) When relation (5) is satisfied, the transistor Q5 or Q6 is turned on, and the mute circuit 7 is driven through the detuning mute signal generator MG. Since a ratio of the tuning detection band to a non-mute band is equal to a ratio of the potential difference |±ΔV| between the two terminals A and C for turning on the transistors Q3 and Q4 to the potential difference |±ΔV 1 | between the two terminals A and C for turning on the transistors Q5 and Q6, when V BE Q3(on) = V BE Q4(on) = V BE Q5 = V BE Q6(on), non-mute band/tuning detection band = ±ΔV 1 / ±ΔV = 1 + R1/R2 and the ratio of the tuning detection band to the non-mute band is determined by the ratio of the resistor R1 to the resistor R2. Therefore, a detection band ratio can be accurately determined. In addition, since the detection band ratio is determined by a resistance ratio, it is easily influenced by an error of a current amplification factor β of a transistor. Fig. 3 shows the characteristics of the tuning detection band, the detuning mute band, and non-mute band when R1/R2 = 1, i.e., the band ratio is set to be "2". With the above embodiment, the tuning band detecting section TD for detecting the tuning band is arranged between the resistors R1 and R2, and the detuning mute band detecting section MD for detecting the detuning mute band is arranged between the two terminals of the resistor R2. Therefore, the ratio of the non-mute band to the tuning detection band of the detecting section MD can be accurately set by the ratio of the resistance of the resistor R1 to the resistance of the resistor R2. In addition, since the input voltage of the detuning mute band detecting section MD can be determined by the ratio of the resistor R1 to the resistor R2, an error which occurs in the band ratio due to a variation in the current amplification factor β can be prevented. Industrial Applicability As is described above, according to the present invention, a ratio of a turning detection band to a non-mute band can be accurately set by a ratio of resistances. In addition, since an input voltage of a detecting section in a detuning mute band can be determined by a ratio of resistances, an error which occurs in the band ratio due to an error of a circuit element can be prevented. Therefore, the tuning detector of the present invention is effectively utilized as an FM tuning detector, and its industrial applicability is high.

Claims: A tuning detector comprising a capacitor (C) for removing an AC signal included in an FM detection signal; a power source for applying a reference voltage (V ref ); and a detuning mute band detecting section (MD) for detecting a detuning mute band; characterized by a tuning detecting section (TD) for detecting a tuning band; two resistors (R1, R2), connected sequentially between one terminal of said power source and one terminal of said capacitor, for setting a band to be detected and dividing a voltage between said terminals; said tuning detecting section (TD) being connected to those terminals of said resistors (R1, R2) connected to said one terminal of said power source and said one terminal of said capacitor; and said detuning mute band detecting section (MD) being connected to one of said resistors (R1, R2) for detecting a detuning mute band in accordance with an arbitrary voltage divided by said resistors (R1, R2). A tuning detector according to claim 1, characterized by an amplifier (4) for amplifying the FM detection signal and a mute circuit (7) for controlling a gain of said amplifier (4) in response to an output signal from said detuning mute band detecting section (MD). A tuning detector according to claim 1 or 2, characterized by a first buffer circuit (BF1) having an input terminal connected to a connection point of said resistors (R1, R2) and said capacitor (C); a second buffer circuit (BF2) having an input terminal connected to a connection point of the resistors (R1, R2) and said power source; said tuning band detecting section (TD) being connected to output terminals of said first (BF1) and second (BF2) buffer circuits; and said detuning mute band detecting section (MD) being connected between a connecting point between said two resistors (R1, R2) and said one terminal of said power source. A tuning detector according to claim 3, characterized in that said first buffer circuit (BF1) is constituted by a first transistor (Q1) having a base connected to the connection point of said capacitor (C) and said resistors (R1, R2). A tuning detector according to claim 3, characterized in that said second buffer circuit (BF2) is constituted by a second transistor (Q2) having a base connected to the connection point of said power source and said resistors (R1, R2). A tuning detector according to claims 4 and 5, characterized in that said tuning band detecting section (TD) is constituted by third (Q3) and fourth (Q4) transistors having bases connected to emitters of said first (Q1) and second (Q2) transistors and a fifth transistor (Q10) having a base connected to collectors of said third (Q3) and fourth (Q4) transistors. A tuning detector according to claim 3, characterized in that said detuning mute band detecting section (MD) is constituted by a sixth transistor (Q7) having a base connected to an arbitrary dividing point (B) of said resistors (R1, R2) and a seventh transistor (Q6) having a base connected to said one terminal of said power source. A tuning detector according to one of claims 3 to 7, characterized in that said first (BF1) and second (BF2) buffer circuits are high-input-impedance buffer circuits.

Inventors: ITOH MASASHI KOUYADAI HEIGHTS, WATANABE SHUUJI, ITOH, MASASHI, WATANABE, SHUUJI, ITOH, MASASHI KOUYADAI HEIGHTS B-102, WATANABE, SHUUJI 1-1, OGURA, SAIWAI-KU
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H03J3
IPC codes: H03G3, H03J7, H03D3, H03J5, H03J3