JPH0946153A - Gain control circuit for fm receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a gain control circuit for an FM receiver in which a sufficiently and stable attenuation is obtained to suppress occurrence of distortion even when a large level input signal is received. SOLUTION: Attenuation circuits 121a and 122a are connected in series with a matching section 12 and the two-stages of the attenuation circuits 121a, 122a are operated based on an output of a high frequency amplifier section 14. That is, the attenuation circuits 121a, 122a are connected in series to secure a sufficient attenuation and operated based on an output of the high frequency amplifier section 14 to suppress occurrence of distortion in the high frequency amplifier section 14. Furthermore, the increase in a current is suppressed by giving an AGC signal to the two stages of the attenuation circuits 121a, 122a connected in series.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain control circuit for an FM receiver, and more particularly to a gain control circuit for an FM receiver capable of suppressing the occurrence of distortion even when a strong signal is input.

[0002]

2. Description of the Related Art In an FM receiver, when a strong signal is input, the high frequency amplifier circuit and the frequency conversion circuit are in a state of being used up to the non-linear portion, and the distortion increases. Therefore, it is common to apply AGC (Automatic Gain Control) to the high frequency amplifier circuit to suppress distortion.

FIG. 1 is a functional diagram of a conventionally used FM receiver, in which an FM broadcast wave is desired to be received by a tuning circuit 13 via an antenna 11 and a matching circuit 12.
M broadcast waves are selected. After that, it is amplified by the high frequency circuit 14 and goes to the frequency conversion circuit. The AGC signal is amplified by the high frequency circuit 14, and the signal level detected by the level detection circuit 15 is amplified by the drive circuit 16 and then fed back to the tuning circuit 13.

However, when the AGC signal is fed back to the tuning circuit 13, it is inevitable that the tuning characteristic of the tuning circuit 13 is lost and the selectivity is lowered. In order to solve this problem, an FM receiver that feeds back the AGC signal to the matching circuit 12 has been proposed.

[0005]

SUMMARY OF THE INVENTION AGC by this method
The circuit attenuates a signal by activating an attenuator composed of a capacitor and a PIN diode by an AGC signal. The required attenuation is A
The signal level at which the GC starts operation and the maximum value of the input signal can be determined, but the signal level at which the AGC starts operation is 90 dB, and the maximum value of the input signal is 130 d.
If it is B, the required attenuation amount is -40 dB.

However, a practical capacitor and P
The AGC circuit configured using the IN diode has an attenuation amount of about -30 dB at most, and it is difficult to secure a sufficient attenuation amount. Therefore, when a large input signal is received, distortion is avoided. The problem arises that you cannot do it. Conversely, if the capacitance of the capacitor is reduced to increase the amount of attenuation, the effect of stray capacitance generated between wiring or elements cannot be avoided, and stable AGC is achieved.
There is a problem that can not be realized.

The present invention has been made in view of the above problems, and a gain of an FM receiver capable of obtaining a sufficient and stable attenuation amount to suppress the occurrence of distortion even when a large input signal is received. An object is to provide a control circuit.

[0008]

According to a first aspect of the present invention, there is provided a gain control circuit for an FM receiver, wherein an impedance of an antenna for receiving an FM broadcast wave in front of a high frequency amplifying means and an impedance of an antenna and a feeder connected to the antenna. The matching means for matching and the F taken through the antenna and the matching circuit
A gain control circuit for an FM receiver used in an FM receiver, comprising: a tuning means for selecting an FM broadcast wave of a predetermined frequency from M broadcast waves, wherein the matching circuit has two or more stages connected in series. It includes an attenuator for attenuating the FM broadcast wave received by the antenna by a predetermined amount when the output signal of the amplifying device exceeds a predetermined threshold value.

In the gain control circuit of the FM receiver according to claim 2, each of the attenuating means receives the F signal received by the antenna.
The attenuator circuit is configured to attenuate the M broadcast wave by a predetermined amount, and the attenuator circuit operating unit that activates the attenuator circuit when the output of the high frequency amplifier unit is equal to or higher than a predetermined threshold value. It is connected in series to the output of the high frequency amplifying means.

In the gain control circuit of the FM receiver according to claim 3, the high frequency amplifying means comprises a high frequency amplifying circuit and a high frequency tuning circuit connected in series with the high frequency amplifying circuit, and is supplied to the attenuating circuit operating means. The output signal of the high frequency amplification stage is the output signal of the high frequency amplification circuit. The gain control circuit of the FM receiver according to claim 4 includes a tuning circuit attenuating circuit for attenuating the output of the matching circuit by a predetermined amount when the output signal of the high frequency amplifying means exceeds a predetermined threshold value. Including further.

According to a fifth aspect of the present invention, there is provided a gain control circuit for an FM receiver, wherein the matching circuit operating circuit activates the attenuating circuit operating means when the output signal of the high frequency amplifying means exceeds a first threshold value. And a tuning circuit operating circuit that activates the tuning circuit attenuating circuit when the output signal of the high frequency amplifying means is greater than or equal to a second threshold value that is greater than the first threshold value. .

In the gain control circuit of the FM receiver according to claim 6, the matching circuit operating circuit and the tuning circuit operating circuit are connected in series. In the gain control circuit of the FM receiver according to claim 7, the operation signal supplied to the matching circuit operation circuit is an output signal of the high frequency amplifier circuit, and the operation signal supplied to the tuning circuit drive circuit is the high frequency tuning circuit. This is the output signal of the circuit.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of an FM receiver equipped with a gain control circuit according to the present invention.
1, a matching unit 12, a tuning unit 13, a high frequency amplification unit 14, a frequency conversion unit 15, an intermediate frequency amplification unit 16, a detection unit 17, a power amplification unit 18, and a speaker 19.

The matching section 12 in this embodiment is composed of a first attenuating section 121 and a second attenuating section 122 connected in series to the first attenuating section 121. The attenuator may be connected in two or more stages. Furthermore, the first damping unit 1
21 is composed of a first attenuating circuit 121a and a first attenuating circuit operating circuit 121b, and a second attenuating section 122 is a second attenuating unit.
Attenuator circuit 122a and second attenuator circuit operation circuit 122b
It is composed of

FIG. 2 is a detailed circuit diagram of the front stage of the FM receiver according to the first embodiment, in which the first attenuation circuit 121a and the second attenuation circuit 122a are connected in series with the antenna 11. Further, a tuning unit 13 and a high frequency amplification unit 14 are arranged in the subsequent stage, and the output of the high frequency amplification unit 14 is supplied to the frequency conversion unit 15.

The high frequency amplifier 14 is a high frequency amplifier 14
1 and a high frequency tuning circuit 142. First
The attenuator circuit 121a includes a first capacitor 1a1 connected in series with the antenna 11 and a first capacitor 1a1.
Of the first diode 1a2 whose anode is connected to the output terminal and whose cathode is grounded.

The first attenuation circuit operating circuit 121b is the first
The first attenuation circuit operating diode 1b1 has a cathode connected to a connection point between the capacitor 1a1 and the first diode 1a2, and the AGC signal is supplied to the anode of the first attenuation circuit operating diode 1b1. Second
The attenuator circuit 122a includes a second capacitor 2a1 connected in series to the first attenuator circuit 121a, and a second diode 2a2 whose anode is connected to the output terminal of the second capacitor 2a1 and whose cathode is grounded. Composed.

The second damping circuit operating circuit 122b has a second
The second attenuator circuit operating diode 2b1 has a cathode connected to a connection point between the capacitor 2a1 and the second diode 2a2, and the AGC signal is supplied to the anode of the second attenuator circuit operating diode 2b1. In the first embodiment, the AGC circuit 20 is the level detection circuit 2 that detects the level of the output of the high frequency amplification unit 14.
01 and the AGC drive circuit 202 which outputs an AGC signal when the output of the level detection circuit 201 exceeds a threshold value.
It is composed of

The AGC signal output from the AGC circuit 20 is supplied in parallel to the anode of the first attenuator circuit operating diode 1b1 and the anode of the second attenuator circuit operating diode 2b1. That is, according to the first implementation response, AGC
When the AGC signal is output from the circuit 20, the first attenuating circuit operating diode 1b1 and the second attenuating circuit operating diode 2b1 are turned on, and the attenuating circuit composed of the capacitor and the diode is turned on and received by the antenna 11. The FM broadcast wave thus generated is output after being attenuated at a predetermined attenuation rate.

The capacitance of the first capacitor 1a1 and the second capacitor 2a1 is 30 pF, and the first diode 1a is
2 and the impedance of the second diode 2a2 to 4
If it is Ω, the attenuation rate for the FM broadcast wave of 83 MHz is 4 / (4 + 64) + 4 / (4 + 64) = − 49 dB.

In a practical attenuator circuit having one stage of attenuation circuit, it is difficult to set the capacitance of the capacitor to 15 pF or less, so the attenuation factor is at most -30 dB, whereas the attenuation factor should be increased. Not only that, when an attenuating circuit is connected in series, the capacitance of the capacitor can be increased to stabilize the circuit. However, since the AGC signal is supplied in parallel to the first attenuating circuit operating diode 1b1 and the second attenuating circuit operating diode 2b1,
The C drive circuit 144 is required to output twice the current.

However, in recent years, FM receivers have tended to be integrated circuits, and it is not always easy to increase the output current. The second embodiment is for solving the above-mentioned problems, and FIG. 3 is a detailed circuit diagram of a front stage of the FM receiver according to the second embodiment. That is, in the second embodiment, the first diode 1a2 and the second diode 2b1
By connecting and in series, the AGC drive circuit 144
It is possible to suppress an increase in the output current of the.

In the first and second embodiments, the output of the high frequency amplifier 141 which constitutes the high frequency amplifier 14 is supplied to the level detection circuit 201. This is because when the output of the high frequency tuning circuit 142 that constitutes the high frequency amplifying section 14 is supplied to the level detection circuit 143, the signal flattened by the tuning action of the high frequency tuning circuit 142 causes the AG
This is because it is difficult to maintain a sharp AGC operation because the C signal is generated.

The gain control circuit according to the present invention and the conventional AGC applied to the tuning unit 13 are used together to further increase the attenuation rate, and the AGC of the high frequency amplification unit 14 is omitted to generate the high frequency amplification unit 14. It is also possible to suppress distortion. FIG. 4 is a functional diagram of the third embodiment, in which the output of the AGC drive circuit 202 is also fed back to the tuning unit 13 and the tuning unit 13 also executes automatic gain control.

That is, the level of the output of the high frequency amplifier 141 is detected by the level detection circuit 201, and when it is higher than the level set by the setter 2021, the AGC drive amplifier 202 is detected.
2 outputs the AGC signal. However, applying AGC in the tuning unit 13 reduces the selectivity.

FIG. 5 is a functional diagram of a fourth embodiment for solving the above-mentioned problems, in which the matching section AGC drive circuit 2 is used.
03 and the tuning unit AGC drive circuit 204 are installed.
The level detection circuit 201 is commonly used by the two drive circuits. Then, the set value of the setter 2031 of the matching section AGC drive circuit 203 is set to a value smaller than the set value of the setter 2041 of the tuning section AGC drive circuit 204.

Therefore, while maintaining the tuning characteristics of the tuning unit 13, the matching unit 12 is subjected to AGC to suppress the occurrence of distortion, and when the FM broadcast wave becomes stronger, the tuning unit 1
AGC is applied to 3 to suppress the occurrence of distortion. In the third and fourth embodiments, since the AGC signal is supplied in parallel to the matching section 12 and the tuning section 13, the current increases.

FIG. 6 is a detailed circuit diagram of the fifth embodiment for solving the above-mentioned problems. The matching part attenuation circuit 12a includes a matching part capacitor 12a1 and a matching part diode 12a2.
And a matching portion resistor 12a3. Matching part AG
The C drive circuit 12b is composed of a matching section AGC diode 12b1.

The tuning section attenuating section 13a is a tuning section capacitor 1
3a1 and 13a2 and a tuning section diode 13a3. Further, the tuning unit drive circuit 13b is a tuning unit AG.
The tuning section AGC diode 13b1 is connected to the matching section diode 12a2 in series.

That is, the output of the high frequency amplifier 141 is the matching section A.
When the radio wave is supplied to the GC diode 12b1 and reaches a predetermined level or higher, the broadcast wave received by the antenna 11 is attenuated by the attenuator circuit including the matching section capacitor 12a1, the matching section diode 12a2, and the matching section resistor 12a3. Further, the output of the high frequency amplifier 141 becomes large and the tuning unit AGC
When the diode 13b1 becomes conductive, the output of the tuning unit 13 is attenuated by the tuning unit attenuating unit including the tuning unit capacitors 13a1 and 13a2 and the tuning unit diode 13a3.

That is, in the present embodiment, since the matching section AGC drive circuit 12b and the tuning section drive circuit 13b are connected in series, an increase in the AGC drive current is suppressed. In the fourth embodiment, more appropriate AGC can be applied by taking the AGC signal to the tuning unit 13 from the subsequent stage of the high frequency tuning circuit 142. FIG. 7 is a functional diagram of the sixth embodiment, in which the output of the high frequency tuning circuit 142 is passed through the second level detection circuit 205 and the AGC for tuning unit is used.
It is supplied to the drive circuit 204.

[0032]

According to the gain control circuit of the FM receiver of the first aspect, it is possible to apply a large amount of AGC while ensuring the tuning characteristic, and suppress the occurrence of distortion. According to the gain control circuit of the FM receiver according to the second aspect, it is possible to suppress an increase in AGC current even when a large amount of AGC is applied, and it is easy to realize an integrated circuit.

According to the gain control circuit of the FM receiver according to the third aspect, the high frequency amplifier having no tuning characteristic is used for the AG control.
By multiplying C, it becomes possible to secure an appropriate AGC. According to the gain control circuit of the FM receiver of the fourth aspect, it is possible to secure a sufficient amount of attenuation by applying AGC not only to the matching section but also to the tuning section.

According to the gain control circuit of the FM receiver of the fifth aspect, when the attenuation by the AGC of the matching section does not provide sufficient attenuation, the AGC is applied to the tuning section to suppress the generation of distortion. According to the gain control circuit of the FM receiver of claim 6, it is possible to suppress the increase of the AGC current by connecting the AGC of the matching section and the AGC of the tuning section in series.

According to the gain control circuit of the FM receiver according to claim 7, it is possible to realize an appropriate AGC by applying the AGC of the tuning section from the latter stage of the high frequency tuning section.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an FM receiver.

FIG. 2 is a detailed circuit diagram of the first embodiment.

FIG. 3 is a detailed circuit diagram of the second embodiment.

FIG. 4 is a functional diagram of a third embodiment.

FIG. 5 is a functional diagram of a fourth embodiment.

FIG. 6 is a detailed circuit diagram of the fifth embodiment.

FIG. 7 is a functional diagram of a sixth embodiment.

[Explanation of symbols]

 11 ... Antenna 12 ... Matching part 121a ... 1st attenuation circuit 121b ... 1st attenuation circuit operation circuit 122a ... 2nd attenuation circuit 122b ... 2nd attenuation circuit operation circuit 13 ... Tuning part 14 ... High frequency amplification part 15 ... Frequency conversion unit 16 ... Intermediate frequency amplification unit 17 ... Detection unit 18 ... Power amplification unit 19 ... Speaker

Claims (7)

[Claims] 1. An antenna for receiving an FM broadcast wave, a matching unit for matching impedances of the antenna and a feeder connected to the antenna, before the high-frequency amplification unit, and via the antenna and the matching circuit. A gain control circuit for an FM receiver used in an FM receiver, comprising: tuning means for selecting an FM broadcast wave having a predetermined frequency from the taken-in FM broadcast wave, wherein the matching circuit has two or more stages. A gain control circuit for an FM receiver including an attenuating means for attenuating a predetermined amount of an FM broadcast wave received by the antenna when an output signal of the high frequency amplifying means connected in series exceeds a predetermined threshold value. 2. An attenuator circuit for attenuating an FM broadcast wave received by the antenna by a predetermined amount, and the attenuating circuit when the output of the high frequency amplifying device exceeds a predetermined threshold value. 2. The gain control circuit for an FM receiver according to claim 1, further comprising: an attenuation circuit operating means for operating a circuit, wherein the attenuation circuit operating means is connected in series to an output of the high frequency amplification means. 3. The high-frequency amplification means comprises a high-frequency amplification circuit and a high-frequency tuning circuit connected in series to the high-frequency amplification circuit, and an output signal of the high-frequency amplification stage supplied to the attenuation circuit operation means is The gain control circuit of the FM receiver according to claim 1, wherein the gain control circuit is an output signal of the high frequency amplifier circuit. 4. The tuning circuit attenuating circuit for attenuating the output of the matching circuit by a predetermined amount when the output signal of the high frequency amplifying means exceeds a predetermined threshold value. Gain control circuit of the described FM receiver. 5. A matching circuit operating circuit for activating the attenuating circuit operating means when the output signal of the high frequency amplifying means exceeds a first threshold value, and the output signal of the high frequency amplifying means is the first 5. The gain of the FM receiver according to claim 4, further comprising: a tuning circuit operating circuit that activates the tuning circuit attenuating circuit when a second threshold value which is larger than the threshold value is exceeded. Control circuit. 6. The FM according to claim 5, wherein the matching circuit operating circuit and the tuning circuit operating circuit are connected in series.
Gain control circuit of receiver. 7. The operation signal supplied to the matching circuit operation circuit is an output signal of the high frequency amplifier circuit, and the operation signal supplied to the tuning circuit drive circuit is an output signal of the high frequency tuning circuit. The gain control circuit of the FM receiver according to claim 5.