JPS6136416B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a muting circuit that performs muting using two control voltage systems with different bandwidths, and is particularly suitable for use in an FM receiver without a tuning indicator. .

Conventional FM receivers often use a tuning meter or light-emitting diode to indicate the tuning point at the tuning point, but these receivers without a tuning indicator listen to the actual sound to determine the approximate tuning point. was. If the muting circuit is operating at this time, the audio may be cut off near both ends of the tuning bandwidth due to frequency drift. For this reason, it is difficult to obtain an optimal tuning point, and stable reception cannot be achieved.

The purpose of the present invention is to eliminate such problems,
To provide a muting circuit that facilitates tuning operation even in the case of a simple FM receiver without a tuning indicator or the like when the muting circuit is operated.

The muting circuit of the present invention uses two voltage systems with different bandwidths as control voltages for muting, and has a two-stage tuning bandwidth, which facilitates the tuning operation and provides stable reception against frequency drift. It is something that makes it possible. In order to configure the tuning bandwidth in two stages by using two control voltage systems with different bandwidths, one of the control voltages converts the center voltage of the frequency discriminator into a voltage with narrow band characteristics, and converts this voltage into a voltage with narrow band characteristics. is applied to the muting circuit via a capacitor, and the other side applies a voltage with broadband characteristics extracted from the front stage of the detector to the muting circuit. Therefore, the tuning bandwidth near the optimal tuning point is determined by the narrowband control voltage, and when performing a tuning operation, at both ends of the tuning bandwidth, the narrowband control voltage is temporarily supplied to the muting circuit through the capacitor. This activates the muting circuit to prevent low frequency output signals, and when the capacitor charging current stops flowing, the muting operation is canceled by the wideband control voltage, so that the frequency drift is eliminated when tuned to both ends of the tuning bandwidth. This is to prevent muting due to the narrow band control voltage from occurring even if such a phenomenon occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A mutating circuit according to the present invention will be explained below with reference to embodiments shown in the drawings. In FIG. 1, reference numeral 1 denotes an intermediate frequency amplifier, and an FM intermediate frequency signal is supplied to its input terminal 9. A frequency discriminator 2 is supplied with the output signal of the intermediate frequency amplifier 1 and detects it, and a detected output signal is supplied to its output terminal 3. Reference numeral 4 denotes a voltage converter to which the detected DC voltage of the frequency discriminator 2 is supplied and converts it into a narrow band voltage, and the output voltage is a predetermined value above and below the center frequency ₁ of the intermediate frequency, as shown in Fig. 2B. A positive voltage is generated at frequencies ₂ and ₃ apart, and an output signal is generated in which the output voltage becomes zero at Δ between frequencies ₂ and ₃ . This is done by double-wave rectifying the detected DC voltage of the frequency discriminator 2 shown in Figure 2A, supplying that voltage to an amplifier having a predetermined threshold voltage (for example, the voltage V ₁ in Figure 2A), and This can be obtained by making the output voltage of the amplifier zero when the output voltage of the discriminator 2 is below the threshold voltage. 5
A rectifier is supplied with an intermediate frequency signal from the intermediate frequency amplifier 1 and rectifies it to generate a first muting control signal, and its output signal is supplied to the muting circuit 6. The output signal of the voltage converter 4 is supplied through a capacitor 7 as a second muting control signal to a muting circuit 6, and the muting circuit 6 is also connected to the output terminal 3.

In such a circuit, the muting circuit 6
operates to attenuate the signal supplied to the output terminal 3 when the output voltage of the rectifier 5 is below a predetermined level, and does not perform the attenuation effect when the output voltage of the rectifier 5 exceeds a predetermined level. When the output voltage of device 4 is supplied, output terminal 3
operates to attenuate the signal supplied to the
When the output voltage of the voltage converter 4 changes from positive to zero or from zero to positive as shown in FIG. Attenuate the signal supplied to 3. capacitor 7
When the transient current that flows through the terminal 3 stops flowing, the muting circuit stops operating and an output is output to the terminal 3. Further, since the output voltage of the voltage converter 4 is zero in the frequency range Δ, the muting circuit 6 does not operate in this range and the output is output to the terminal 3. That is, when the output voltage of the voltage converter 4 is applied to the muting circuit 6, no output is produced. Next, in the band characteristics of the intermediate frequency amplifier 1 as shown in Fig. 3, the center frequency of the intermediate frequency signal is the frequency
When the frequency is below ₄ and above ₅ , the signal level of the intermediate frequency signal supplied to the rectifier 5 becomes small, and the output voltage of the rectifier 5 also becomes small, and the muting circuit 6 operates to reduce the signal supplied to the output terminal 3. Attenuate.

FIG. 4 shows a specific circuit example of the muting circuit according to the present invention, in which a part of the intermediate frequency signal amplified by the intermediate frequency amplifier 1 is rectified by the rectifier 5,
The signal is smoothed by a resistor 15 and a capacitor 16 and is supplied to the base of the first switching transistor 10. The collector of the transistor 10 is a resistor 17
It is connected to the power supply through the transistor 11, and further connected to the base of the second switching transistor 11. The base of the transistor 11 is connected to the voltage converter 4 through a capacitor 7, and is further grounded through a discharge diode 8. The collector of the transistor 11 is connected to a power supply through a resistor 18 and further connected to the base of a third switching transistor 12. Further, the emitter of the second switching transistor 11 and the emitter of the third switching transistor 12 are grounded through a common resistor 21. The collector of the transistor 12 is connected to a power supply through a resistor 19, and further connected to the base of a muting transistor 14 through a Zener diode 13. Further, the emitter of the transistor 14 is grounded, the collector is connected to the power supply through the resistor 20, and further connected to the output terminal 3.

In such a circuit, the output voltage of the rectifier 5 increases or decreases depending on the band characteristics of the intermediate frequency amplifier shown in FIG. 3, and when the center frequency of the intermediate frequency signal is outside the band of the intermediate frequency amplifier 1, the output voltage becomes small. Become. There is a predetermined threshold voltage between the base and emitter of the transistor 10 for making the transistor 10 conductive, and the transistor 10 is operated so that this threshold voltage becomes the output voltage of the rectifier 5 at frequencies ₄ and ₅ in FIG. By setting or by supplying a reverse bias voltage between the base and emitter of the transistor 10, the transistor 10 is cut off when the center frequency of the intermediate frequency signal becomes a frequency of frequency ₄ or lower and frequency ₅ or higher. Further, in the frequency band Δ' between the two frequencies ₄ and ₅ , the transistor 10 is conductive. When the transistor 10 is conductive, the transistor 11 is turned off, the transistor 12 is turned on, and the transistor 14 is turned off, stopping the muting operation. Furthermore, when transistor 10 is in a cut-off state, transistor 14 is conductive and attenuates the signal supplied to output terminal 3.

When the center frequency of the intermediate frequency signal is outside the frequency range Δ' in FIG. attenuates the signal supplied to the

Next, when the center frequency is near the tuning point of the frequency range △, the output voltage of the rectifier 5 is large, the transistor 10 becomes conductive, and the transistor 14 is therefore cut off, and the output terminal 3 receives a normal signal. is supplied.

When the center frequency moves from this state to between frequencies ₂ and ₄ and between frequencies ₃ and ₅ (the shaded band in Figure 3), the output terminal of the voltage converter 4 will appear as shown in Figure 2B. A high voltage appears at , and a charging current flows through the capacitor 7 due to this voltage. The current flowing through the capacitor 7 becomes a base bias current of the transistor 11, which inverts the transistor 11 from a cut-off state to a conduction state. Therefore, the transistor 14 becomes conductive and attenuates the signal supplied to the output terminal 3. The current flowing through the capacitor 7 is transient, and when the charging of the capacitor 7 is finished, the current stops flowing, and the transistor 11 becomes cut off again, causing the transistor 14 to be cut off. Frequency range△
In the range of , output is immediately output to output terminal 3, but
In the shaded area, the output is cut off and then output again. That is, if the tuning operation is made very slow, the output will be output continuously within the frequency range Δ', but if the tuning operation is performed at a normal speed, the output will be output only within the frequency range Δ. Therefore, it is easy to find an accurate tuning point, and due to frequency drift, even if it moves to the shaded area, there will be no output at the output terminal 3, but after a few seconds (this can be changed by selecting the charging time constant of the capacitor 7). (possible) output will appear again.

As described above, according to the present invention, the difficulty in finding the tuning point and the tuning deviation at both ends of the tuning point, which occur in a simple FM receiver without a tuning indicator, etc., can be solved by using two systems of control voltages. It is possible to easily select the matching point.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the muting circuit according to the present invention, FIGS. 2 and 3 are waveform diagrams of various parts, and FIG. 4 is a circuit diagram showing an embodiment of the muting circuit according to the present invention. . 1: Intermediate frequency amplifier, 2: Frequency discriminator, 4:
Voltage converter, 5: rectifier, 6: mutating circuit.

Claims (1)

[Claims] 1: an intermediate frequency amplifier that amplifies the FM signal; a frequency discriminator that detects the output signal of the intermediate frequency amplifier; and a rectifier that extracts the intermediate frequency signal from the intermediate frequency amplifier and rectifies it to obtain a DC voltage;
The output signal of the frequency discriminator is supplied, and the output signal is turned to approximately zero voltage within a predetermined band narrower than the bandwidth of the intermediate frequency amplifier around the center frequency, and to a DC voltage of a predetermined amplitude outside the predetermined band. a voltage converter to be converted; and a first voltage converter that is supplied with the DC output voltage of the rectifier and becomes conductive when the level of the DC output voltage exceeds a predetermined value, and is cut off when the level of the DC output voltage exceeds the predetermined level. a switching circuit, and a first switching circuit controlled by the output of the first switching circuit.
a second switching circuit that is in a cutoff state when the switching circuit is conductive and is in a conduction state when it is cut off; and a second switching circuit that supplies the output voltage of the voltage converter to the second switching circuit as a control voltage, a capacitor that makes the second switching circuit conductive for a certain period of time when the frequency is outside the predetermined band and within the band of the intermediate frequency amplifier; and the second switching circuit is controlled by the output signal of the second switching circuit. A muting circuit using two control voltage systems having different bandwidths, characterized in that the switching circuit comprises a muting circuit that cuts off a low frequency output signal of the frequency discriminator when conductive.