JPS6024706A - Muting circuit - Google Patents

Abstract

PURPOSE:To eliminate generation of distortion and ensure excellent muting attenuation by applying a muting controlling signal to the automatic gain controlling circuit of an AM receiving circuit through a reverse current prevention diode. CONSTITUTION:When the muting controlling signal is low level, the reverse current prevention diode 16 is reversely biased. Accordingly, even if input level to an integrated circuit 11 for AM receiving changes, detection output is maintained nearly constant. An audio signal is transmitted from detection output of the circuit 11 to next circuit through a coupling condenser 14 and a resistance 15. When the muting controlling signal becomes high level, the reverse current prevention diode 16 conducts and an AGC signal is applied forcibly to the circuit 11, and accordingly, detection output of the circuit 11 is cut off. Therefore, the audio signal is not transmitted to next circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improvement in a muting circuit for an AM radio receiver.

Conventional configurations and their problems Radio receivers employ muting circuits to reduce shock noise when switching reception bands, or for other purposes.

A conventional muting circuit will be explained below.

FIG. 1 is a block diagram of a conventional muting circuit. 1 is an integrated circuit for AM reception, 2 is a resistor for automatic gain control (hereinafter abbreviated as AGC), 3 is a capacitor that determines the response speed of AGg, 4 and 6 are audio signal coupling capacitors, and 5 is an audio signal coupling capacitor. A resistor 7 determines the output level, a muting transistor 7, and a resistor 8 that maintains the collector of the muting transistor 7 at ground potential.

Regarding the muting circuit configured as above,
The operation will be explained below.

First, an AM modulated signal input to the AM integrated circuit 1 is detected and output as a detected output. Part of the detection output is A
Via the GC resistor 2, it is added to the capacitor 3 for determining the response speed of the AGC 7, and is also connected to the AM integrated circuit 1.
is applied to the AGC input terminal of. Due to the action of this AGC, even if the input level to the AM integrated circuit 1 changes, the detected output is kept almost constant.

Here, when the muting control signal is not applied to the base of the muting transistor 7, the audio signal is transmitted from the detection output of the AM integrated circuit 1 via the coupling capacitor 4, the resistor 6, and the coupling capacitor 6.
It is transmitted to the next circuit. At this time, a high resistance is used as the resistor 8 so as not to affect the transmitted audio signal. Next, when a voltage of 10 is applied to the base of the muting transistor 7 as a muting control signal, the muting transistor 7 is turned on, and the audio signal is transmitted between the resistor 5 and the collector-emitter of the muting transistor 7. divided by saturation resistance and attenuated,
It is not transmitted to the next circuit.

However, in the above configuration, when the amplitude of the audio signal to be transmitted becomes large, the base of the muting transistor 7 changes even if no muting control signal is applied to the base of the muting transistor 7.

Due to the characteristics of the diode between the collectors, the audio signal is attenuated and distortion occurs.

Even if the muting transistor 7 is turned on, the audio signal cannot be completely blocked unless the saturation resistance is finite. Furthermore, the circuit becomes complicated and the material and assembly costs increase.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned conventional problems, and an object thereof is to provide a muting circuit that can secure excellent muting attenuation with a simple configuration. This is a muting circuit configured to connect the signal to the AGC circuit of the AM receiving circuit via a backflow prevention diode. By controlling the AGC with the muting control signal, excellent muting is achieved without distortion. The amount of attenuation can be secured.

DESCRIPTION OF THE EMBODIMENTS FIG. 2 is a block diagram of a muting circuit in one embodiment of the present invention. In Fig. 2, 11 is an integrated circuit for AM reception, 12 is a resistor for AGC, 13 is a capacitor for determining the response speed of AGC, 14 is a coupling capacitor, 15 is
is a resistor that determines the output level of the audio signal, and these are the same configuration as the conventional example. 16 is a backflow prevention diode that prevents the AGC circuit from being affected when the muting control signal is at a low level.

The operation of the muting circuit of this embodiment configured as described above will be described below.

First, when the muting control signal is at a low level, the reverse current prevention diode 16 is reverse biased, so the A
The GC circuit operates in the same manner as the conventional example shown in FIG. 1, and even if the input level to the AM receiving integrated circuit 11 changes, the detected output is kept almost constant. The audio signal is transmitted from the detection output of the AM reception integrated circuit 11 to the next circuit via a coupling capacitor 14 and a resistor 15. Next, when the muting control signal becomes high level, the reverse current prevention diode 16 conducts and forces the AM reception integrated circuit 11 to
Since the GC signal is applied, the detection output of the AM receiving integrated circuit 11 is cut off. Therefore, the audio signal is not transmitted to the next circuit.

With a configuration that exceeds the effects of the invention, AGC can be controlled by muting control signals.
By controlling this, it is possible to realize a muting circuit that does not generate distortion, maintains an excellent muting attenuation amount, and has a small number of parts and a low assembly cost.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional muting circuit,
FIG. 2 is a circuit diagram of a muting circuit in one embodiment of the present invention. 11...AM reception integrated circuit, 12...
・AGC resistor, 13... Capacitor for determining AGC response speed, 14... Coupling capacitor, 16... Resistor for determining output level, 16...
...Reverse current prevention diode.

Claims (1)

[Claims] A muting circuit configured to apply a muting control signal to an automatic gain control circuit of an AM receiving circuit via a backflow prevention diode.