JPH025061B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention provides an AM/FM system equipped with a stereo demodulation circuit.
Regarding FM radio receivers.

FIG. 1 shows the detection circuit section of a conventional AM/FM radio receiver. In an AM/FM radio receiver in which the AM detection output is extracted as an audio signal through a stereo demodulation circuit in the same way as the FM detection output, there is a capacitance between the FM detector 2 or AM detector 4 and the stereo demodulation circuit 6. Coupling capacitors 8 and 10 with different values are individually inserted and coupled, and an FM
The detection output or the AM detection output is selectively extracted. In this way, a coupling capacitor 8,
10 is inserted separately for FM detector 2 and AM
This is because the demodulation frequency characteristics of the detector 4 are different, and the capacitance of each coupling capacitor 8, 10 is C ₁ ,
Assuming C ₂ , the capacitance C ₁ on the FM detector 2 side is a large capacitance that is not affected by the input impedance of the stereo demodulation circuit 6, and the capacitance C ₂ on the AM detector 4 side constitutes a high-pass filter. It is necessary to set the capacitance to be smaller than the capacitance C ₁ (C ₁ >C ₂ ).

By the way, separately installing the coupling capacitors 8 and 10 on the output side of the FM detector 2 and AM detector 4 means that the FM detector 2, AM detector 4, and stereo demodulation circuit 6 are configured with a single integrated circuit. In this case, terminals (pins) for externally connecting the coupling capacitors 8 and 10 are required, and the device itself becomes expensive.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an AMFM radio receiver that reduces the required number of capacitors and has a simplified configuration.

That is, the AM/FM radio receiver of the present invention has the following features:
An FM detection circuit extracts an audio signal from an input signal by FM detection, and an FM detection circuit extracts an audio signal from an input signal by FM detection.
A differential pair consisting of an AM detection circuit that extracts an audio signal by detection, a first transistor corresponding to the FM detection circuit, and a second transistor corresponding to the AM detection circuit, and a mode switching input. a mode switching circuit that selectively operates a first or second transistor in response to flow an operating current to switch either the FM detection circuit or the AM detection circuit to an operating state; and an output of the FM detection circuit. A voltage source that sets a constant bias voltage between the circuit and the output circuit of the AM detection circuit via a resistor, and a capacitor inserted between the output point of the output circuit of the FM detection circuit and the stereo demodulation circuit. At the same time, by inserting the series circuit consisting of the capacitor and the resistor between the output point of the output circuit of the AM detection circuit and the stereo demodulation circuit, the FM detection circuit or the AM detection circuit and the stereo It is equipped with a high-pass filter coupled to the demodulation circuit.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the AM/FM radio receiver of the present invention, and the same parts as the circuit in FIG. 1 are given the same reference numerals.

Along with an FM detection circuit 14 that extracts an audio signal from an input signal by FM detection, an AM detection circuit 16 that extracts an audio signal from the input signal by AM detection is installed. An output circuit is added to the output side of the conventional FM detector 2 and AM detector 4 to make the output DC potentials the same. A capacitor 1 is connected between the output side of the FM detection circuit 14 and the input side of the stereo demodulation circuit 6.
8 is inserted and the two are connected,
Output side of AM detection circuit 16 and stereo demodulation circuit 6
A capacitor 18 and a resistor 20 are connected between the input side of
A high-pass filter 22 consisting of the following is inserted and the two are coupled.

With this configuration, the number of coupling capacitors is reduced to one, and the configuration can be simplified compared to a conventional coupling circuit. Furthermore, the output side of the FM detection circuit 14 and the output side of the AM detection circuit 16 are
Since the circuit is connected via the resistor 20, when constructed using an integrated circuit, the number of terminals (pins) can be reduced compared to conventional circuits. In particular, resistance 20
As a result, the coupling between the FM detection circuit 14 and the AM detection circuit 16 becomes weak, so that they are not affected by the FM signal during AM detection, and the same is true for the reverse case.

Next, FIG. 3 shows a specific example of the circuit configuration of the FM detection circuit 14 and the AM detection circuit 16 in the AM/FM radio receiver of the present invention.

An FM signal and an AM signal, which have been converted to an intermediate frequency from the intermediate frequency amplification section, are individually input to an input terminal 30 of the FM detection circuit 14 and an input terminal 32 of the AM detection circuit 16.

Then, the FM detection circuit 14 or the AM detection circuit 16 depending on each detection mode of FM detection or AM detection.
A mode switching circuit 24 is installed to selectively switch the mode into an operating state.
Constant current source 34 that regulates the operating current of the FM detector 2
A differential pair is constituted by a first transistor 36 connected to a constant current source 38 that regulates the operating current of the AM detector 4, and a second transistor 40 connected to a constant current source 38 that regulates the operating current of the AM detector 4. The respective emitters are connected in common, and a constant current source 42 for regulating the sum of each operating current is connected between the emitters and a reference potential point (ground). A reference bias voltage V _B is applied to the base of the transistor 36.
A voltage source 44 is connected to the base of the transistor 40, and a control input terminal 46 is connected to the base of the transistor 40 to apply a control voltage V _C for switching between FM and AM.

In addition, the output circuit 26 of the FM detector 2 includes an FM
Detector 2 and power supply terminal 4 to which driving voltage V _CC is applied
Diodes 50 and 52 are inserted between the diodes 8 and 8. The bases of transistors 54 and 56 are individually connected to the cathodes of each diode 50 and 52.
constitutes a voltage inverting circuit, and similarly, the diode 5
2 and transistor 54 also constitute a current inversion circuit. The emitter of each transistor 54, 56 is connected to a power supply line connected to a power supply terminal 48, and a capacitor 58,
60 is inserted. The base and collector of transistor 58 and the base of transistor 60 are shared, and transistors 58 and 60 constitute a current inversion circuit.

On the other hand, the output circuit 28 of the AM detector 4 includes diodes 62, 64 and transistors 66, 68, 7.
0,72, and its configuration is similar to that of the output circuit 26. A drive voltage V _CC similar to that applied to the power supply terminal 48 is applied to the power supply terminal 73 .

The output terminal 74 of the FM detection circuit 14 is connected to the collectors of the transistors 56 and 60.
An output terminal 76 of the AM detection circuit 16 is connected to the collectors of transistors 68 and 72, and a voltage source 82 for setting a constant bias voltage is connected between each output point via resistors 78 and 80. .

In the above configuration, when the value of the control voltage V _C applied to the control input terminal 46 is made higher than the reference bias voltage V _B of the voltage source 44, the transistor 40
Since the transistor 40 becomes conductive and an operating current flows through the transistor 40, the AM detector 4 enters the operating state. In this case, the output of the AM detector 4 flows as a signal current into the transistors 70 and 72 via the diodes 62 and 64 and the transistors 66 and 68, and the AM detection output is taken out from the output terminal 76.

Further, when the control voltage V _C is lowered from the reference bias voltage V _B , the transistor 36 becomes active, and an operating current flows through the transistor 36, so that the FM detector 2 becomes active. Then, the output of the FM detector 2 is connected to the diode 50,5.
2 and transistors 54 and 56, and as a result of current synthesis, an FM detection output can be taken out from the output terminal 74.

The AM detection output and the FM detection output obtained in this way are controlled to have an opposite phase relationship when a signal current is generated and when a signal current is not generated, and the output thereof is determined.

In this operation switching, each output circuit 26, 2
Since a constant bias voltage is applied to the collectors of the transistors 60 and 72 of No. 8 from a common voltage source 82 via resistors 78 and 80, each output point is equalized in DC potential. As a result, as mentioned above, when switching from FM mode to AM mode or from AM mode to FM mode, there is no change in the DC potential, so it is possible to reliably prevent the generation of abnormal sounds caused by switching, such as pump noise. be able to.

Since the output points of each output circuit 26 and 28 are connected to the common voltage source 82 in this way, the second
As shown in the figure, the output part on the FM detection circuit 14 side is connected to the stereo demodulation circuit 6 via the capacitor 18 in the high-pass filter 22, and the output part on the AM detection circuit 16 side is connected to the high-pass filter 22. It is connected to the stereo demodulation circuit 6 through a series circuit of a capacitor 18 and a resistor 20 forming the same.

Further, as shown in FIG. 4, the AM detection circuit 16
A low pass filter 88 consisting of a resistor 84 and a capacitor 86 is connected between the high pass filter 22 and the high pass filter 22.
A similar effect can be expected by inserting .

As explained above, according to this invention, FM
Since the detection circuit, AM detection circuit, and stereo demodulation circuit are coupled by a high-pass filter, the coupling capacitor installed in each conventional FM detector and AM detector can be omitted, and AM/FM radio reception is possible. In addition to simplifying the configuration of the device, mutual interference between the detection circuits can be prevented, and the output points of the output circuits of the FM detection circuit and AM detection circuit can be connected to a common voltage source. By converting it into a potential, it is possible to prevent potential fluctuations between the output points of the FM detection circuit and the AM detection circuit due to switching of the detection mode, and it is possible to suppress the generation of abnormal sounds.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional AM/FM radio receiver, Fig. 2 is a block diagram showing an embodiment of the AM/FM radio receiver of the present invention, and Fig. 3 is the same as shown in Fig. 2. A circuit diagram showing an FM detection circuit and an AM detection circuit in an AM/FM radio receiver,
FIG. 4 is a block diagram showing another embodiment of the AM/FM radio receiver of the present invention. 2...FM detector, 4...AM detector, 6...stereo demodulation circuit, 14...FM detection circuit, 16...AM detection circuit, 18...capacitor, 20...resistor, 22...
High-pass filter, 24...mode switching circuit, 2
6, 28... Output circuit, 36... First transistor, 40... Second transistor, 78, 80... Resistor, 82... Voltage source.

Claims (1)

[Claims] 1. An FM detection circuit that extracts an audio signal from an input signal by FM detection, an AM detection circuit that extracts an audio signal from the input signal by AM detection, and an FM detection circuit corresponding to the FM detection circuit. a differential pair consisting of a first transistor and a second transistor corresponding to the AM detection circuit, selectively operating the first or second transistor in response to a mode switching input to flow an operating current; a mode switching circuit that switches either the FM detection circuit or the AM detection circuit to an operating state; and a constant bias between the output circuit of the FM detection circuit and the output circuit of the AM detection circuit via a resistor. A capacitor is inserted between a voltage source that sets the voltage, an output point of the output circuit of the FM detection circuit, and the stereo demodulation circuit, and a capacitor is inserted between the output point of the output circuit of the AM detection circuit and the stereo demodulation circuit. By inserting a series circuit consisting of the capacitor and resistor in between,
An AM/FM radio receiver comprising: a high-pass filter that couples the FM detection circuit or the AM detection circuit with the stereo demodulation circuit.