JPH0321086Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a signal meter drive circuit for an AM/FM receiver that can selectively receive AM broadcasts and FM broadcasts, and is particularly applicable to AM broadcast receiving stations. This invention relates to a signal meter drive circuit that can output a signal corresponding to the electric field strength of an FM broadcast receiving station and a signal corresponding to the electric field strength of an FM broadcast receiving station to a single output terminal connected to the signal meter.

(b) Conventional technology On page 227 of the ``'85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuits'' published on March 20, 1985, there is a description of the technology for FM/AM tuners.
ICLA1265 is listed. The ICLA1265 is
As shown in Figure 2, there is an AM signal generation circuit 1 inside which generates a signal according to the electric field strength when receiving AM broadcasting, and an FM signal generating circuit 1 which generates a signal according to the electric field strength when receiving FM broadcasting. Signal generation circuit 2
The output signals of the AM and FM signal generation circuits 1 and 2 are commonly connected to the first external pin 3. The first external pin 3 is connected to an output terminal 4 to which a signal meter (not shown) is connected, and is also connected to a first resistor 5 and a first capacitor 6, one end of which is grounded.
Further, the first external pin 3 is connected to a second resistor 8 and a second capacitor 9, one end of which is connected to a second external pin 7, and the second external pin 7 is connected to a switch 10 in the IC. Connected. In Figure 2, FM
When receiving a broadcast, the switch 10 is
The FM signal generating circuit 2 generates a DC current I ₁ corresponding to the electric field strength of the receiving station, and the output terminal 4 receives R ₁ I ₁ (where R ₁ is connected to the first resistor 5).
An output voltage of (resistance value) is generated. Further, when receiving AM broadcasting, the switch 10 is switched to the AM position shown in the figure, and the AM signal generation circuit 1 generates a DC current I ₂ corresponding to the electric field strength of the receiving station, and the output terminal 4 receives R ₁ R _{2 .} An output voltage of /R ₁ +R ₂ I ₂ (where R ₂ is the resistance value of the second resistor 8) is generated. Therefore, when the circuit shown in FIG. 2 is actually used, the values of the first and second resistors 5 and 8 must be adjusted to compensate for variations. Adjustment for reception of FM broadcasting is performed by varying the value of the first resistor 5. To adjust the reception of AM broadcasts,
This is done by varying the values of the first and second resistors 5 and 8. By performing the above adjustment, the output signal obtained at the output terminal 4 becomes appropriate for the signal meter, and when the output signal is used as a stop signal for auto search, its stop sensitivity is Become appropriate.

(c) Problems to be solved by the invention However, in the signal meter drive circuit shown in Fig. 2, the first resistor 5 is used for adjustment during FM broadcast reception and
Since it is also used for adjusting when receiving broadcasts, it is necessary to first adjust when receiving FM broadcasts, and then adjust when receiving AM broadcasts, which poses a problem in that the adjustment order is limited. Also, since the variations in the AM signal generation circuit 1 are not so large, there are cases where no adjustment is made, but if the configuration is as shown in Figure 2, the first resistor 5 is varied to make adjustments when receiving FM broadcasts. This inevitably necessitated adjustment when receiving AM broadcasts, and there was a problem in that it was not possible to reduce the number of adjustment steps. Furthermore, the first resistor 5 is variable to mainly adjust the FM broadcast reception, and the second resistor 8 is variable to mainly adjust the AM broadcast reception. However, the output current of the AM signal generation circuit 1 must be made sufficiently large, but such conditions have the problem of severely restricting circuit design.

(d) Means for solving the problem The present invention has been made in view of the above-mentioned points. The present invention is characterized in that it includes an impedance conversion circuit that has an output terminal connected to the impedance conversion circuit and becomes inactive when the output signal of the first signal generation circuit is not present.

(e) Effect According to the present invention, for example, when receiving AM broadcasting, the first
The output signal generated from the signal generation circuit can be led to the output terminal via the impedance conversion circuit, and the output signal generated from the second signal generation circuit when receiving FM broadcasting can be directly led to the output terminal. Therefore, the signal meter connected to the output terminal can display the electric field strength of AM broadcasting and FM broadcasting. Furthermore, the output signals of the first and second signal generation circuits can be adjusted independently.

(F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
11 is a second signal generation circuit that generates an output signal according to the electric field strength when receiving AM broadcasting; 12 is FM
a second signal generating circuit that generates an output signal according to the electric field strength when receiving a broadcast; 13 is a first resistor connected to the output end of the first signal generating circuit 11; 14;
is a first capacitor connected in parallel to the first resistor 13; 15 is a second resistor connected to the output terminal of the second signal generating circuit 12; and 16 is a second capacitor connected in parallel to the second resistor 15. , 17 is an input terminal connected to the output terminal of the first signal generating circuit 11;
a voltage follower circuit having an output end connected to the output end of the second signal generation circuit 12 and operating as an impedance conversion circuit; and 18 an output connected to the output end of the second signal generation circuit 12. It is a terminal.

Now, when AM broadcasting is received, the first signal generation circuit 1
If an output current I ₁ is generated at the output terminal of the voltage follower circuit 17, a voltage of F ₁ I ₁ (however, R ₁ is the resistance value of the first resistor 13) is generated at the input terminal of the voltage follower circuit 17. The voltage at the output end of the voltage follower circuit 17 also becomes R ₁ I ₁ . Then, since the voltage R ₁ I ₁ is applied to the signal meter from the output terminal 18,
The signal meter can display the electric field strength. At this time, since the output impedance of the voltage follower circuit 17 is sufficiently small, the output voltage obtained at the output terminal 18 is not affected by the second resistor 15. Further, by adjusting the value of the first resistor 13, the value of the output voltage obtained at the output terminal 18 can be adjusted. Incidentally, when receiving AM broadcasting, since no output signal is generated from the second signal generating circuit 12 for FM broadcasting, the second signal generating circuit 12 does not adversely affect the output voltage.

Next, the FM broadcast is received, and the second signal generation circuit 12
If _an output _{current I 2} is generated at the output terminal _of An output voltage of (resistance value) is generated. At that time, no output signal is generated from the first signal generation circuit 11 for AM, and the voltage follower circuit 17 stops its operation when no input signal is present, and the output impedance is set to be high impedance. Therefore, the voltage follower circuit 1
7 has no adverse effect on the output signal obtained at the output terminal 15. By adjusting the value of the second resistor 15, the value of the output voltage obtained at the output terminal 18 can be adjusted.

In the embodiment shown in FIG. 1, the impedance seen from the output terminal 18 becomes the output impedance (several tens of Ω) of the voltage follower circuit 17 when the output signal of the first signal generation circuit 11 is generated.
When the output signal of the second signal generating circuit 12 is generated, the resistance value of the second resistor 15 (several kilohms) is reached, so the time constant of the filter consisting of the first resistor 13 and the first capacitor 14 is C ₁ R ₁ (However, C ₁ is the capacitance of the first capacitor 14) and the time constant of the filter consisting of the second resistor 15 and the second capacitor 16
C ₂ R ₂ (C ₂ is the capacity of the second capacitor 16)
can be set independently. Therefore, the alternating current component caused by the modulation current contained in the output currents of the first and second signal generating circuits 11 and 12 can be removed by filters having time constants that are independently set. Therefore, the output current during AM reception, which has a large AC component, can be removed using a filter with a large time constant.

FIG. 3 is a circuit diagram showing a specific circuit example of the present invention. In FIG. 3, the voltage follower circuit 17 includes first and second transistors 19 and 20 that are differentially connected, and third and fourth transistors that are inserted between the collectors of the first and second transistors 19 and 20. The current mirror circuit 23 includes transistors 21 and 22. If this voltage follower circuit 17 is used, the voltage R ₁ I ₁ generated by the output current generated from the first signal generation circuit 11 is applied to the base of the first transistor 19, and is transferred to the second transistor 20 with the same magnitude.
is generated at the base of the output terminal 18. Further, if the output current of the first signal generation circuit 11 does not exist, the second transistor 20 is turned off and the output impedance of the voltage follower circuit 17 becomes high impedance, so the output current of the second signal generation circuit 12 I The voltage R ₂ I ₂ generated by ₂ is generated at the output terminal 15 as it is.

In the embodiment, the first signal generating circuit 11 generates an output signal when receiving an AM broadcast, and the second signal generating circuit 12 generates an output signal when receiving an FM broadcast. The second signal generating circuit 12 may generate output signals when the signal generating circuit 11 receives AM broadcasting. In addition, as an impedance conversion circuit, instead of the voltage follower circuit 17 shown in FIG. An emitter follower transistor connected to the emitter follower transistor may also be used. In that case, the circuit configuration can be greatly simplified. Furthermore, in FIG. 1, the voltage follower circuit 17 is connected to the first and second
Since it is integrated into the IC together with the signal generation circuits 11 and 12, the number of external pins and external circuits does not change at all from the conventional one.

(g) Effect of the invention As stated above, according to the invention, AM and
A signal corresponding to the electric field strength when receiving FM broadcasting can be derived to a single output terminal to drive a single signal meter. At this time, the output signals when receiving the AM and FM broadcasts can be adjusted independently of each other. Therefore, AM with less variation
The resistance value is fixed for the output signal during broadcast reception, and the adjustment process can be omitted. Furthermore, since the time constant of the filter for removing the alternating current component can be set independently, it is also possible to increase only the time constant of the filter for the output signal when receiving an AM broadcast that includes a large amount of the alternating current component.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional signal meter drive circuit, and FIG. 3 is a circuit diagram showing a specific example of the circuit shown in FIG. Explanation of main figure numbers, 11...first signal generation circuit,
12... Second signal generation circuit, 13... First resistor,
15...Second resistor, 17...Voltage follower circuit, 18...Output terminal.

Claims (1)

[Scope of utility model registration request] A first signal generation circuit that generates a signal according to the electric field strength when receiving one of AM or FM broadcasts, and a second signal generation circuit that generates a signal according to the electric field strength when receiving the other AM or FM broadcast. and an input end connected to the output end of the first signal generation circuit and an output end connected to the output end of the second signal generation circuit, when there is no output signal of the first signal generation circuit. It consists of an impedance conversion circuit that becomes inactive, and an output terminal connected to the output terminal of the second signal generation circuit, and the output terminal uses the output signal of the first or second signal generation circuit as a signal meter drive signal. Signal meter drive circuit generated in