JP3012741B2 - FM / AM receiving circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM / AM receiving circuit, and more particularly to a tuner IC constituted by one chip.

[0002]

2. Description of the Related Art In FM / AM receivers, it is required to reduce the number of parts in order to realize cost reduction and size reduction. For this reason, the FM / A is a single chip from the FM / AM front end to the audio signal output circuit.
M tuner ICs have been put to practical use. An example is shown in FIG. The FM / AM tuner IC includes a receiving unit and an audio circuit unit, and the receiving unit includes an FM receiving unit and an AM receiving unit. The FM receiver is an FM front end circuit 1,
It comprises an intermediate frequency amplification circuit 2 and a detection circuit 3. F
The M front end circuit 1 includes an RF amplifier, a mixing circuit, a local oscillation circuit, and the like. The intermediate frequency amplifying circuit 2
The FM signal converted to 0.7 MHz is amplified. The mixing circuit and the intermediate frequency amplifier are connected by an intermediate frequency transformer and a ceramic filter. Detection circuit 3 is 10.7 MH
The sum signal (main channel), difference signal (sub-channel), and 19
This is a circuit for extracting a composite signal composed of a kHz pilot signal. The AM receiver includes an AM front-end circuit 4, an intermediate frequency amplifier circuit 5, and a detection circuit 6. The AM front end circuit 4 includes an RF amplifier circuit, a mixing circuit, a local oscillation circuit, and the like. The intermediate frequency amplifier circuit 5 is an amplifier circuit having an intermediate frequency of 455 kHz. The detection circuit 6 is a general diode detection circuit, and is 455 kHz.
The detection signal is extracted from the AM wave of z. The FM detection signal and the AM detection signal are input to the switch circuit 7. The switch circuit 7 outputs one of the FM detection signal and the AM detection signal to the amplifier 8. The selected signal is amplified by the amplifier 8 and then output from the detection output terminal 24. The selection of the switch circuit 7 is controlled by a selection signal input from a selection signal terminal (not shown).

Here, the AM detection signal (sound signal) has a high-frequency portion suppressed by its band characteristic, and if reproduced as it is, a so-called "muffled" sound in which only the low-frequency portion is emphasized. Furthermore, in the case of AM broadcasting, 9 kHz (90
00 Hz), the broadcast frequency is allocated at intervals, so that the carrier of the broadcast station whose frequency is adjacent is 9000 Hz.
May be detected as an audio signal and become beat noise. Therefore, terminals 22 and 23 are provided between the AM detection circuit 6 and the switch circuit 7, a capacitor 31 and a resistor 32 are connected in series between the terminals, and the terminal 23 is grounded via the capacitor 33. The capacitor 31 is several tens n
It has a capacity of about F and forms a low cut filter with the output impedance of the AM detection circuit 6 so as to suppress a low frequency part and balance with a high frequency part. The resistor 32 and the capacitor 33 constitute a high-pass filter determined by f ≒ 1 / 2πRC, and remove a beat noise and a component in a high frequency range.

On the other hand, the audio circuit section mainly comprises an FM stereo signal demodulation circuit. This circuit is a switching type stereo signal demodulation circuit using a PLL.
The detection signal (especially F
M detection signal) is supplied to the decoder 12 via the amplifiers 10 and 11.
And supplied to the phase detection circuit 13. Decoder 12
Indicates that a signal obtained by adding a main channel signal and a sub channel signal is converted into an L channel signal and an R signal signal every 38 kHz half-wave.
This is a circuit for switching signals to channel signals. 3
The signal of 8 kHz is provided from the flip-flop 15. The phase detection circuit 13 has a frequency of 19k included in the FM detection signal.
Hz pilot signal and the 38 kHz signal are halved.
Is a circuit for comparing with a 19 kHz signal obtained by counting down. The DC component signal resulting from the comparison is provided to a voltage controlled oscillator (VCO) 14. The VCO 14 oscillates and outputs 76 kHz, which is four times the frequency of the pilot signal, under the control of the phase detection circuit 13. This 76 kHz oscillation wave is provided to flip-flop 15. The flip-flop 15 counts down the rectangular wave of 76 kHz to １ ／ and outputs it to the decoder 12 and the flip-flop 16. Since the 38 kHz rectangular wave given to the decoder 12 needs to have a duty ratio of exactly 50%,
The signal of kHz is counted down by a flip-flop.

[0005] Here, a coupling capacitor 35 is connected between the detection signal output terminal 24 and the detection signal input terminal 25. The capacitor 35 is for cutting the DC component, but forms a high-pass filter with the input impedance of the input stage (amplifiers 10 and 11). Use a large one. Further, the detection signal output terminal 24 is grounded via a capacitor 34. This capacitor 34 is a bypass capacitor for removing the FM intermediate frequency (10.7 MHz) and its harmonic components.
This capacitor forms a low-pass filter with the output impedance of the amplifier 8, but does not affect the audio frequency (including the FM sub-channel) by using a capacitor of sufficiently small capacity.

A capacitor 36 is connected between the amplifier 11 and the phase detection circuit 13 by terminals 26 and 27. This capacitor forms a high-pass filter with the input impedance of the phase detection circuit 13 and suppresses the FM audio signal (main channel). This is because the excessive audio signal is input to the phase detection circuit 13 so that the
This is to prevent the operation of L from becoming unstable.

The audio signals of the left and right two channels separated by the decoder 12 are output from audio signal terminals 28 and 29.
Also, if the AM detection signal is inputted from the detection signal input terminals decoder 1 2 acts as a simple amplifier, and outputs the same signal to the audio signal output terminal 28 and 29.

[0008]

SUMMARY OF THE INVENTION The above-mentioned FM / A
In the M tuner IC, only ten terminals are required in the illustrated one. In addition, if a power supply terminal, a ground terminal, an adjustment terminal, a switching signal input terminal, and the like are included, an extremely large number of terminals are required. Further, the number of bulk components to be externally attached is only six even in the illustrated case.

By the way, in the case of an IC-type receiving circuit,
Since the size of the circuit can be sufficiently reduced, the size of the IC is determined by the number of terminals, and there is a problem that if the number of terminals is large, miniaturization is hindered. Further, such a receiving circuit is extremely severe in terms of cost, and there is a demand for reducing the price of only one external component and the labor of the mounting process alone.

An object of the present invention is to provide an FM / AM receiving circuit in which the number of terminals is reduced and the number of components is reduced.

[0011]

According to the present invention, there is provided a detection signal output terminal for outputting an FM detection signal or an AM detection signal,
Connected to said detection signal output terminals, audio signal is input
A first detection signal input terminal for FM stereo separation
Connecting a second detection signal input terminal to which a pilot signal is input and the first detection signal input terminal to an audio circuit;
A variable resistor circuit for setting the input impedance to be high when outputting an FM detection signal and setting the input impedance to be low when outputting an AM detection signal; and a second detection signal input terminal, the phase detection circuit being provided in the audio circuit when the FM detection signal is input. and contact <br/> connection to the circuit, at the time of the AM detection signal input there by FM / AM reception circuit having a first switch circuit, and a sound signal output terminal of the two left and right channels connected to the audio circuit to ground The detection signal output terminal is grounded via an FM intermediate frequency removing capacitor, the detection signal output terminal is connected to a first detection signal input terminal by a coupling capacitor, and the detection signal output terminal is The second detection signal input terminal is connected to a low-cut capacitor.

[0012]

According to the FM / AM receiving circuit of the present invention, the above configuration allows the first detection signal input terminal to be connected to the first detection signal input terminal during FM reception.
With respect to the input FM audio signal, a low-pass filter is formed by the output impedance of the detection circuit and the capacitor for removing the FM intermediate frequency, and the FM intermediate frequency and harmonics are removed. Further, a high-pass filter is formed by the coupling capacitor and the high-resistance variable resistor circuit, and the extremely low-frequency portion and the DC component are cut. Further, a signal (for phase detection) input to the second detection signal input terminal is filtered by a high-pass filter including a low-cut capacitor and an input impedance of an audio circuit.
The signal of the main channel of M is removed and set in the audio circuit.
The phase is input to the phase detector circuit.

When receiving AM, an AM audio signal is input to the first detection signal input terminal. However, when the low-cut capacitor is grounded, a high frequency range such as beat noise is cut, and the coupling capacitor and the low resistance of the low resistance are connected. A high-pass filter is constituted by the variable resistor circuit, and the sound band can be balanced.

[0014]

FIG. 1 is a partial circuit diagram of an FM / AM tuner IC according to an embodiment of the present invention.

The same parts as those of the conventional FM / AM tuner shown in FIG. This I
C differs from the IC of FIG. 5 in the following points.

A check wave signal input terminal for directly inputting a signal (detection signal) of the AM detection circuit to the switch circuit 7 has 25 terminals.
a, a switch circuit 18 is provided between the two inspection wave signal input terminals 25b of 25a and 25b and the phase detection circuit 13;
During M reception, the detection signal input terminal 25b is connected to the phase detection circuit 1
3 and a variable resistor circuit 17 for changing the input impedance between the decoder 12 and the check signal input terminal 25a in which the detection signal input terminal 25b is grounded at the time of AM reception. This FM / AM tuner IC is set high and the input impedance is set low during AM reception.
The detection signal input terminal 25a is a terminal to which an audio signal (main channel, sub-channel, and AM detection signal) of the detection signal is input.
b is a terminal to which a 19 kHz pilot signal for FM stereo separation is input. The detection signal output terminal 24 and the detection signal input terminal 25a are connected to a coupling capacitor 37.
Connected by On the other hand, the detection signal output terminal 24 and the detection signal input terminal 25b are connected by a low cut capacitor 37 of about 0.01 μF. Further, a detection signal output terminal 2
4 is grounded via a capacitor 34 of about 100 pF.

During FM reception, the capacitor 34 forms a low-pass filter having a cutoff frequency of about 300 kHz with the output impedance of the preceding stage (detection circuit).

The low-pass filter cuts the intermediate frequency (10.7 MHz) component of the FM detection signal and its higher harmonic components. This FM detection signal is input to the detection output input terminal 25a via the coupling capacitor 36.
Here, since the input impedance of the terminal 25a (decoder 12) is set to be high (several tens of kΩ) by the variable resistance circuit 17, the transmission frequency of the coupling capacitor 37 is extremely low, as shown in (25a) of FIG. It becomes a frequency characteristic. The FM detection signal from which the signal of the main channel has been cut by the low cut capacitor 38 is input to the detection signal input terminal 25b. This frequency characteristic is shown in FIG. The switch circuit 18 is a phase detector
It is connected to the circuit 13 side. Thereby, 19 kHz
Is less affected by the audio signal and is input to the phase detection circuit 13.

On the other hand, at the time of AM reception, since the switch circuit 18 is switched to the ground side, a low-pass filter is formed by the output impedance of the capacitor 38 and the terminal 24. This low-pass filter cuts a high sound range such as beat noise.

Further, at this time, since the variable resistance circuit 17 switches to the low resistance side (several hundred Ω to several kΩ), the cutoff of the high-pass filter composed of the variable resistance circuit 17 and the coupling capacitor 37 increases. , The bass range of the audio signal is cut. These low-pass filters,
AM input to the decoder 12 by the high-pass filter
The detection signal (audio signal) has frequency characteristics as shown in FIG. At the time of AM reception, the decoder 12 operates simply as an amplifier.

FIG. 4 shows a specific circuit example of the variable resistor circuit 17 and the switch circuit 18 of the FM / AM tuner IC. The variable resistance circuit 17 has a high resistance 17b as shown in the figure.
And the low resistance 17d are connected in parallel, the low resistance 17d can be opened and closed by an analog switch 17a, and further connected to the decoder 12 via a buffer transistor 17c. The analog switch 17a has a structure in which collectors and emitters of two bipolar transistors are connected to each other. The analog switch 17a is turned on when an "H" signal is input to the base, and the low resistance 17d is connected in the circuit to lower the input impedance.

The switch circuit 18 connects an analog switch 18a and a resistor 18b in parallel between the detection signal input terminal 24b and the ground.
4b and a PLL system (phase detection circuit 13) are connected by a buffer transistor 18c. When the analog switch 18a is off, the input impedance is substantially the value of the resistor 18b, and this impedance and the low cut capacitor 38 constitute a high-pass filter. On the other hand, at the time of AM reception, the analog switch 18a
Is turned on, the detection signal input terminal 24b is grounded.
The capacitor 38 forms a low-pass filter with the output impedance of the previous stage. Detection signal input terminal 2
4b, the input of the signal to the PLL system is cut off.

The analog switches 17a and 18
a, and the other switch circuits operate in synchronization with a single AM / FM switching signal.

[0024]

As described above, according to the present invention, it is possible to reduce the number of pins of an IC in an FM or AM receiving circuit formed into an IC and to reduce the number of external components. Become.

[Brief description of the drawings]

FIG. 1 is an FM / AM tuner I according to an embodiment of the present invention.
Diagram showing C

FIG. 2 is a diagram showing frequency characteristics of a detection signal input terminal of the FM / AM tuner IC;

FIG. 3 is a diagram showing frequency characteristics of a detection signal input terminal of the FM / AM tuner IC;

FIG. 4 is a diagram showing a specific circuit of the FM / AM tuner IC;

FIG. 5 is a diagram showing a conventional FM / AM tuner IC.

[Explanation of symbols]

 17-Variable resistance circuit 18-Switch circuit 24-Detection signal output terminal 25a, 25b-Detection signal input terminal 34-High frequency component cut capacitor 36-Coupling capacitor 37-Low cut capacitor

Claims (1)

(57) [Claims] 1. A detection signal output terminal that outputs an FM detection signal or an AM detection signal, and is connected to the detection signal output terminal, and receives an audio signal.
A first detection signal input terminal for FM stereo separation
A second detection signal input terminal to which a pilot signal is input , and the first detection signal input terminal are connected to an audio circuit, and the input impedance is set high when outputting an FM detection signal and low when outputting an AM detection signal. A variable resistance circuit, and a second detection signal input terminal connected to a phase detection circuit provided in the audio circuit when an FM detection signal is input;
An FM / AM receiving circuit comprising: a first switch circuit that is grounded when an M detection signal is input; and two-channel left and right audio signal output terminals connected to the audio circuit, wherein the detection signal output terminal is an FM intermediate Grounded via a frequency removing capacitor, the detection signal output terminal and the first detection signal input terminal are connected by a coupling capacitor, and the detection signal output terminal and the second detection signal input terminal are low cut. F connected by a capacitor
M / AM receiving circuit.