JPH0241958Y2 - - Google Patents

Description

[Detailed description of the invention] A. Field of the invention The present invention relates to an FM/AM receiver that uses a phase-locked loop (PLL) circuit as an FM stereo demodulation circuit.

B. Prior art In this type of FM/AM receiver, when receiving AM
When the voltage controlled oscillator circuit that makes up the PLL circuit is operating, a bead is generated between the AM local oscillation signal and reception interference. , when receiving AM
The operation of the voltage controlled oscillator of the PLL circuit is stopped.

That is, in Fig. 1, 1 is composed of an integrated circuit, and includes an FM intermediate frequency amplification circuit and an FM detection circuit.
A circuit section including an intermediate frequency amplification circuit for AM and an AM detection circuit; 2 is an FM stereo demodulation circuit composed of an integrated circuit and includes a PLL circuit (not shown); 3 is a PLL in the FM stereo demodulation circuit 2; This is a control voltage input terminal for stopping the operation of the voltage controlled oscillation circuit that constitutes the circuit.
The voltage controlled oscillator circuit is configured to stop oscillating when a voltage of 1.5V or higher is applied.

S ₁ , S ₂ , and S ₃ are the first, second, and third
This is a band changeover switch. The first band changeover switch _S1 has a common terminal COM connected to the power supply line 4, and a fixed terminal FMST and FMST on the FM stereo side.
The FM monaural side fixed terminal (FMMO) is connected to the power line 5 of the FM front end section (not shown), and the AM side fixed terminal AM is connected to the power line 6 to the AM front end section (not shown). There is.
The second band switching switch _S2 has a common terminal COM connected to the power supply line 4, an FM monaural side fixed terminal FMMO connected to the control voltage input terminal 3 for stopping the operation of the voltage controlled oscillator circuit via a resistor R,
The fixed terminal FMST on the FM stereo side and the fixed terminal AM on the AM side are idle terminals. A backflow prevention diode is connected between the AM side fixed terminal AM of the first band selection switch _S1 and the FM monaural side fixed terminal FMMO of the second band selection switch _S2 .
D ₁ is connected. The third band selection switch _S3 has a common terminal COM connected to the FM stereo demodulation circuit 2.
Connected to the input terminal of the FM stereo side fixed terminal
FMST and FM monaural side fixed terminal FMMO are connected to the output terminal of the FM detection circuit in circuit section 1,
The AM side fixed terminal AM is connected to the output terminal of the AM detection circuit in the circuit section 1.

In the circuit of FIG. 1 configured in this way,
When receiving FM stereo [when the movable terminals MT of switches S ₁ , S ₂ , and S ₃ are set between the common terminal COM and the fixed terminal FMST on the FM stereo side], voltage is applied to the control voltage input terminal 3. Not done. Therefore, at this time, the voltage controlled oscillation circuit operates, and the FM stereo demodulation circuit 2 performs stereo demodulation operation.

During FM monaural reception [when each movable terminal MT of switches S ₁ , S ₂ , S ₃ is set between the common terminal COM and the FM monaural side fixed terminal FMMO], the power supply line 4 is connected to switch S ₂ A voltage is applied to the control voltage input terminal 3 through the resistor R, and the oscillation operation of the voltage controlled oscillation circuit is stopped, resulting in FM monaural operation.

When receiving AM [Movable terminals of switches S ₁ , S ₂ , and S ₃ ]
When MT is set between the common terminal COM and the AM side fixed terminal AM, voltage is applied from the power supply line 4 to the control voltage input terminal 3 via the switch S ₁ , the diode D ₁ and the resistor R. As a result, the oscillation operation of the voltage controlled oscillation circuit is stopped, and beats are prevented from occurring between the AM local oscillation signal and the AM local oscillation signal.

In the circuit shown in Figure 2, the second band selection switch
The FM monaural side fixed terminal FMMO of _S2 is connected to the AM side fixed terminal AM, and the connection point 7 is connected to one end of the resistor R. The diode used in the circuit of FIG. 1 is not used. The other circuit configurations are the same as the circuit shown in FIG.

In the circuit shown in FIG. 2, the operation during FM stereo radio reception and FM monaural reception is exactly the same as the operation of the circuit shown in FIG. 1. When receiving AM, switch S ₂ is connected from power supply line 4 in the same way as when receiving FM monaural.
Since a voltage is applied to the control voltage input terminal 3 via the resistor R and the resistor R, the operation of the voltage controlled oscillation circuit is stopped.

C. Problems with the Prior Art However, in the circuit shown in FIG. 1, the voltage applied to the control terminal ₃ of the voltage controlled oscillation circuit during AM reception drops by about 0.7V due to the backflow prevention diode D1. Therefore, receivers with low voltage sources, e.g. rated 3V
When the power supply voltage drops to 2V at the receiver of the voltage source, only a voltage of 1.3V is applied to the control voltage input terminal 3, so the operation of the voltage controlled oscillator circuit does not stop and the AM local oscillator A beat occurs between the signals.

For this reason, low voltage source receivers use a circuit as shown in FIG. 2 that does not use a reverse current prevention diode. However, when a small band changeover switch is used, it is difficult to regulate the changeover timing inside the switch, so there is a risk that a beat or oscillation noise may occur during the changeover of the switch.

In other words, while switching the band switching switch from a mode other than the AM band to the AM band side, for example, the common fixed terminal of the first band switching switch _S1
If the movable terminal MT short-circuits between COM and the fixed terminal AM on the AM side, and the common fixed terminal COM of the second band selection switch S ₂ and the fixed terminal AM on the AM side become open, the local oscillation for AM occurs. circuit and
Both voltage controlled oscillation circuits of the FM stereo demodulation circuit 2 become operational, and a beat is generated. Furthermore, if the band changeover switch is quickly switched to avoid this, there is a risk that abnormal noise such as oscillation noise will be generated.

D. Purpose of the invention In view of the above, the purpose of the present invention is to obtain an FM/AM receiver that eliminates the above-mentioned drawbacks with a simple circuit configuration.

E. Example of the invention An example of the invention is shown in FIG. 3. In FIG. 3, the same figure numbers as in FIGS. 1 and 2 will be used for the same figures as in FIGS. 1 and 2.

In the circuit shown in Figure 3, the first band selection switch
A backflow prevention diode _D1 is connected between the AM side fixed terminal AM of _S1 and the AM side fixed terminal AM of the second band switching switch _S2 . The rest of the configuration is exactly the same as the circuit shown in FIG.

In the circuit shown in Fig. 3, when receiving FM stereo, no voltage is applied to the control voltage input terminal 3 for stopping the operation of the voltage controlled oscillator circuit, as in the case of the circuits shown in Figs. The oscillation circuit operates and performs FM stereo demodulation. Also, during FM monaural reception, voltage is applied from the power supply line 4 to the control voltage input terminal 3 via the switch S ₂ and the resistor R, and the voltage controlled oscillation circuit is activated. The oscillation operation stops and the FM monaural state becomes available.

When receiving AM, the switch is switched from power supply line 4.
Since a voltage is applied to the control voltage input terminal 3 via S ₂ and the resistor R, the operation of the voltage controlled oscillation circuit is stopped. At this time, since no voltage drop occurs between the power supply line 4 and the control voltage input terminal 3, even in a receiver using a low voltage source (for example, rated 3V),
A voltage sufficient to stop the voltage controlled oscillation circuit can be supplied to the control voltage input terminal 3 until the voltage is reduced by about 60% (about 1.8 V).

Also, while changing the band changeover switch from a mode other than the AM band to the AM band side, the common fixed terminals COM and AM of the first band changeover switch _S1
The movable terminal MT first shorts between the side fixed terminals AM and the common fixed terminal of the second band changeover switch _S2 .
Even if the connection between COM and AM side fixed terminal AM becomes open, the switch can be disconnected from the power supply line 4.
Since a voltage is applied to the control voltage input terminal 3 via S ₁ , diode D ₁ and resistor R, the oscillation operation of the voltage controlled oscillation circuit stops and a beat occurs between the AM local oscillation signal and the AM local oscillation signal. There isn't. By the way, when the voltage is reduced in a receiver with a low voltage source, while switching the band switch from a mode other than the AM band to the AM band side, the AM local oscillation circuit and voltage control are momentarily disconnected due to the voltage drop caused by diode _D1 . The oscillator circuit may operate at the same time, causing abnormal noise, but at this time the voltage of the voltage source is decreasing, and the level of the AM local oscillation signal and the gain of the low frequency amplifier are decreasing. Since the level of the abnormal noise is low and the abnormal noise only occurs momentarily, it does not pose a problem to the auditory sense.

In the embodiment shown in Fig. 3, each band switching switch S ₁ , S ₂ , S ₃ is a three-point switch for FM stereo side, FM monaural side, and AM side, but this is not necessarily the case. There is no need to use a separate band changeover switch. For example, as shown in Figure 4, FM
2-point band changeover switch for side and AM side
S ₁ ′, S ₂ ′, S ₃ ′ and a stereo-monaural changeover switch S ₄ may also be used.

Furthermore, in the embodiments shown in FIGS. 3 and 4, the operating voltage is constantly supplied from the power supply line 4 to the circuit section 1 constituted by an integrated circuit, but the present invention is not limited to such embodiments. An intermediate frequency amplification circuit and detection circuit for FM and an intermediate frequency amplification circuit and detection circuit for AM are provided separately, and operating voltage is selectively supplied to these circuits via a band selection switch. Good too.

Furthermore, the output terminals of the FM and AM detection circuits and the FM
The present invention can also be applied to FM and AM receivers that do not have a band switching switch between the input terminal of the stereo demodulation circuit and instead supply the FM and AM detection outputs to the input terminal of the FM stereo demodulation circuit. It can be used.

F. Effects of the invention As described above, according to the invention, even when the voltage is reduced in a receiver using a low voltage source, the voltage controlled oscillation that constitutes the PLL circuit of the FM stereo demodulation circuit can be reliably performed during AM reception. Since the oscillation operation of the circuit stops,
There is no risk of beats occurring between the AM local oscillation signal and the AM local oscillation signal. Further, according to the present invention, abnormal noises caused when switching bands can also be eliminated.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the main part showing an example of a conventional FM/AM receiver, Fig. 2 is a circuit diagram of the main part showing another example of a conventional FM/AM receiver, and Fig. 3 is a circuit diagram of the main part showing an example of a conventional FM/AM receiver.
Main part circuit diagram showing an example of an FM, AM receiver,
FIG. 4 is a circuit diagram showing another embodiment of the FM/AM receiver of the present invention. 3... Control voltage input terminal, 4... Power supply line, 5... FM front end power line, 6
... AM front end power line, S ₁ , S ₂ ,
S ₁ ′, S ₂ ′...First and second band changeover switch, D ₁
……diode.

Claims (1)

[Scope of utility model registration request] A first band switching device in which the common fixed terminal is connected to the power supply line, the AM side fixed terminal is connected to the power line of the AM front end section, and the FM side fixed terminal is connected to the power line of the FM front end section. The switch has a common fixed terminal connected to a power supply line, and at least an AM side fixed terminal connected to a control voltage input terminal for stopping operation of a voltage controlled oscillation circuit constituting an FM stereo demodulation PLL circuit. Equipped with a band changeover switch,
At least when receiving an AM band, the operation of the voltage controlled oscillation circuit is stopped by supplying power to the control voltage input terminal for stopping operation.
An FM/AM receiver comprising a diode connected between an AM side fixed terminal of the first band changeover switch and an AM side fixed terminal of the second band changeover switch.