JPS6326569B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a band switching device that can prevent transient noise that occurs when switching bands in a multiband radio receiver.

Conventional configurations and their problems In general, transient noise that occurs when switching bands not only gives an unpleasant feeling to the listener, but also causes damage to the speaker in extreme cases. Therefore, it is necessary to prevent the transient noise by cutting off the signal supply to the acoustic output section while the transient noise is occurring. Conventionally, a method of preventing this by using a mechanical switching means using a band changeover switch with a complicated contact structure has been proposed.
It is difficult to use this mechanical switching means to perform an operation to prevent transient noise only when switching bands, due to mechanical accuracy and lifespan.

OBJECTS OF THE INVENTION An object of the present invention is to provide a band switching device that prevents transient noise that occurs during band switching and has good characteristics that do not have any influence on the signal system in a steady state.

Composition of the Invention In order to achieve the above object, the present invention includes a non-shooting type band switching switch that responds to band switching, and a first time constant circuit that starts operating simultaneously with the switching operation of the band switching switch. , a second time constant circuit that continues to operate for a certain period of time even after the first time constant circuit is inactive, and a signal cutoff circuit for muting control by the outputs of the first and second time constant circuits; By controlling to the cut-off state, transient noise at the time of band switching is prevented.

DESCRIPTION OF EMBODIMENTS FIG. 1 shows an embodiment of a band switching device according to the present invention, and FIG. 2 shows its operating waveforms. In FIG. 1, 1 is a non-shooting type band switching switch that switches in response to band switching, 2 is a first time constant circuit that starts operating simultaneously with the switching operation of band switching switch 1, and 3 is a first time constant circuit. A second time constant circuit that operates at the same time as the time constant circuit 2 starts operating, and continues to operate for a certain period of time even after the first time constant circuit 2 stops operating; 4 is a signal input terminal; 6 is a low voltage terminal; The frequency amplifier 5 temporarily supplies a signal to the low frequency amplifier 6 by using the switching operation of a transistor or the like during a period when the muting control outputs of the first time constant circuit 2 and the second time constant circuit 3 are equal to or higher than V _T . This is a signal passing cutoff circuit that cuts off the signal automatically.

Here, the first time constant circuit 2 includes differentiating circuits 8 and 19, diodes 12 and 13, and a resistor 1.
4 and 15, and a control transistor 16, and the second time constant circuit 3 is composed of a capacitor 17 and a resistor 18. The input sides of the differentiating circuits 8 and 17 are connected to the FM and AM side terminals of the band switching switch 1, respectively, and detect voltage fluctuations at each terminal during band switching, and generate detection outputs A and B. and
It is composed of resistors 9 and 11 and a capacitor 10. The operation of the band switching device of this embodiment configured as described above will be described below. First, when the contact 1a of the band changeover switch 1 is connected to the FM side terminal and is in the FM band receiving state, the differentiating circuits 8 and 19 connected to the FM and AM side terminals do not operate, and the points A and B The point is held at the potential of the power supply voltage V _CC . As a result, diodes 12 and 13 become non-conducting, and control transistor 16 also becomes non-conducting.
The first time constant circuit 2 and the second time constant circuit 3 are in a non-operating state, that is, the potential at point c is equal to the ground potential. Therefore, the FM signal supplied to signal input terminal 4
The demodulated signal of the band receiver is transmitted to the signal passing cutoff circuit 5.
(in this case signal passing state) and low frequency amplifier 6
The signal is then output from the speaker 7.

Next, when the AM band switching operation is started at time t ₀ , since the band switching switch 1 is a non-shooting type, the contact 1a is not connected to either the FM side terminal or the AM side terminal for a minute time Δt. After Δt, the contact 1a is not connected to the AM side terminal. Therefore, the potential at point A at time t ₀
VA is ≒ from the power supply voltage V _CC as shown in Figure 2 A.
It instantly falls to the potential given by the relationship of V _CC · R ₁₁ / (R ₁₁ + R _T ), and rises to the power supply voltage V _CC with the time constant given by the relationship of ≈C ₁₀ (R ₁₁ + R _T ). (However, under the condition of R ₁₄ ≫ R ₁₂ , R _T = R ₉
(Given by the relationship R ₁₄ + R _15. At this time, C ₁₀
are the capacitance values of capacitor 10, R ₉ , R ₁₁ , R ₁₄ , R ₁₅
is the resistance value of resistors 9, 11, 14, and 15, and _r12 is the forward resistance of diode 12). That is,
The control transistor 16 is conductive at time _t0 ,
The charge accumulated in the capacitor 17 during normal operation is instantly discharged. Therefore, the potential V _C at point c rises to V _CC instantaneously, and the control transistor 16 becomes (V _CC −
V _D ) R ₁₅ / (R ₁₄ + R ₁₅ )≧0.7 (where V _D is the forward voltage of the diode 12, and 0.7 is the base-emitter voltage that makes the control transistor 16 conductive) _. The first time constant circuit 2 is electrically conductive for a period of time, and the first time constant circuit 2 continues to operate.

Here, even if the control transistor 16 becomes non-conductive after Δt ₁ and the first time constant circuit 2 becomes inactive, the second time constant circuit 3 still maintains Δt ₂ due to the time constants C ₁₇ and R ₁₈ .
continues to operate for a certain period of time. (However, C ₁₇ is the capacitance value of the capacitor 17, and R ₁₈ is the resistance value of the resistor 18.) This situation is shown in Figure 2C. Here, since the contact 1a of the band changeover switch 1 is connected to the AM side contact after Δt, the output of the differentiating circuit 19, that is, the potential VB at point _B , is 2V at the same time as shown in FIG. It rises to _CC , and the time constant C ₁₀ (R ₉ +
R ₁₁ ), it gradually decreases to V _CC . However, since the V _B is a potential that reverse biases the diode 13, the diode 13 is in a non-conductive state, that is, it has no effect on the operation of the transistor 16 described above. Therefore, the periods of Δt ₁ and _{Δt 2} are set to R ₉ , R ₁₁ , R ₁₄ , R ₁₅ , _{R 18} ,
By setting C ₁₀ and C ₁₇ to predetermined values, Δt ₁
By keeping the signal passing cutoff circuit 5 in the cutoff state for a period of +Δt ₂ , it is possible to prevent transient noise that occurs when switching bands. Here, the relationship between Δt ₁ and Δt ₂ is such that Δt ₁ <Δt in order to prevent malfunction of the first time constant circuit 2 when switching from the FM band to the AM band or from the AM band to the FM band very quickly. It is desirable to set the relationship such that ≪Δt ₂ .

In addition, in this example, there are two types of FM and AM.
As explained in terms of bands, switching of multiple bands,
It goes without saying that the present invention can also be used in audio equipment that combines the signal reproducing circuit section of a cassette tape.

Effects of the Invention As is clear from the above explanation, the present invention can provide a practically excellent band switching device that can completely prevent transient noise that occurs during band switching and can be expected to have a reliable effect with a simple configuration. It is something.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a band switching device according to the present invention, and FIG. 2 is an operational waveform diagram of each main part thereof. DESCRIPTION OF SYMBOLS 1...Band changeover switch, 2...First time constant circuit, 3...Second time constant circuit, 5...Signal passage cutoff circuit.

Claims (1)

[Claims] 1. A non-shooting type band changeover switch that responds to a band changeover operation, a first time constant circuit that starts operating at the same time as the changeover operation of the band changeover switch, and a first time constant circuit that starts operating at the same time as the first time constant circuit operates. and a second time constant circuit that continues to operate for a certain period of time even after the first time constant circuit is inactive, and outputs a muting control signal from the outputs of the first and second time constant circuits. A band switching device characterized in that it is configured to control a passing cutoff circuit to a cutoff state.