JPS6322738B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal circuit for in-vehicle radios, wireless devices, etc., and particularly to a circuit for removing impulsive noise such as ignition noise of automobiles.

Conventionally, this is the first impulsive noise removal circuit of this type.
I got what is shown in the figure. In the figure, 1 is a preamplifier circuit, 2 is a delay amplifier circuit, 3 is a gate circuit that is inserted into the signal path and is cut off based on the detection output of a noise detection circuit, which will be described later, 4 is an output circuit, and 5 is a noise detection circuit. , 6 is a trigger circuit for controlling the gate circuit 3, and 7 is a capacitor 8 attached to the noise detection circuit 5.
This is a noise sensitivity adjustment circuit composed of a resistor 9 and a resistor 9. These days, most of the units 1 to 6 are integrated circuits, and their operations are well known, so a detailed explanation of their operations will be omitted.

The noise sensitivity adjustment circuit 7 is connected to a noise detection circuit 5, which is usually constituted by a differential amplifier circuit or the like, and is connected to one base input of a transistor of the differential amplifier circuit. That is, by delaying the signal input to one base with a time constant determined by a capacitor and a resistor, and adjusting this time constant, the amplification degree of the differential amplifier circuit can be changed. It is well known that the degree of amplification increases if the time constant is decreased.

However, when a fixed time constant is set using the capacitor 8 and resistor 9 in this way, the following problems occur. In other words, the noise level of the receiver increases when the electric field is weak, so if the sensitivity is set to be high by the noise sensitivity adjustment circuit 7, the noise will cause malfunction, and even though there is no impulsive noise, the receiver's noise level will increase. The signal is intermittent, giving an undesirable result to the auditory sense. Alternatively, continuous noise may reduce the sensitivity of the noise detection circuit 5.
If the AGC function is added, even if ignition noise etc. enters the receiver, it will not be possible to remove it.

On the other hand, if the time constant of the noise sensitivity adjustment circuit 7 is set so as not to malfunction due to noise even in a weak electric field, impulsive noise such as ignition noise will occur in a medium electric field or higher. The removal effect is small, and the original function is degraded.

As described above, in the conventional circuit, it has not been possible to set the constants of the noise sensitivity adjustment circuit so that the noise removal effect is optimal for both a weak electric field and an electric field of medium electric field or higher.

Since the present invention provides an impulsive noise removal circuit that eliminates the above-mentioned drawbacks, an embodiment of the present invention will be described below with reference to FIG. 2.

In FIG. 2, numerals 1 to 9 are the same as those shown in FIG. Reference numeral 10 denotes an integrated intermediate frequency amplifier circuit, which includes an intermediate frequency detection circuit, that is, a so-called signal meter drive circuit that outputs a DC voltage according to the electric field level. Resistor 11 is an emitter resistance element in the output stage of the signal meter drive circuit, and its output terminal is led to the connection point of capacitor 8 and resistor 9 via diode 13 and resistor 12, which are variable resistance elements.

In this embodiment, the resistor 12 and diode 13 constitute a control circuit 16.

Now, if the electric field level of the station that the receiver is receiving is low, a corresponding DC voltage is output from the signal meter drive circuit of the intermediate frequency detection circuit 10, but that voltage is small enough to make the diode 13 conductive. Therefore, the time constant of the noise sensitivity adjustment circuit 7 is determined by the capacitor 8 and the resistor 9, and the sensitivity is set in advance using this time constant so that the impulsive noise removal circuit does not malfunction due to receiver white noise, etc. Therefore, the quality of the desired signal does not deteriorate in the case of a weak electric field, and moreover, it works effectively against impulsive noise such as ignition noise.

Next, when the electric field level of the receiving station increases, the output voltage of the signal meter drive circuit increases, making the diode 13 conductive, so that the resistor 12 is connected via the output impedance (usually low impedance) of the output stage of the signal meter drive circuit. and grounded. As a result, the time constant of the noise sensitivity adjustment circuit 7 is
It is determined by the capacitor 8 and the parallel resistance value of the resistors 9 and 12 plus the output impedance, and the time constant is small, so the sensitivity is higher than the noise detection sensitivity in a weak electric field. By setting this sensitivity so that it does not malfunction due to receiver white noise, etc., it is possible to obtain the usual noise removal effect. That is, as the electric field becomes stronger, the white noise level of the receiver decreases, making it possible to increase the noise detection sensitivity.Moreover, impulsive noise is suppressed by the limiter effect of the intermediate frequency amplification circuit 10, and the noise is reduced. Since the amount of noise detected by the detection circuit 5 becomes smaller, it becomes necessary to increase the sensitivity. In other words, a more effective noise removal effect can be obtained.

FIG. 3 shows an embodiment in which a time constant circuit composed of a resistor 14 and a capacitor 15 is connected in parallel with a resistor 9. In FIG. That is, in this embodiment, the control circuit 16 includes a time constant circuit 17 made up of a resistor 14 and a capacitor 15. As a result, a time constant is provided for the sensitivity switching of the noise sensitivity adjustment circuit 7,
This prevents the sensitivity from frequently switching due to changes in the electric field level, and also reduces so-called pop tubes that appear in the output circuit due to switching.

As described above, according to the present invention, a control circuit is provided which controls the time constant of the noise sensitivity adjustment circuit to be smaller as the input electric field level increases when a signal corresponding to the input electric field level is input. This has the effect of providing an impulsive noise removal circuit that can always effectively remove noise regardless of its strength.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional impulsive noise removal circuit, FIG. 2 is a block diagram showing an embodiment of the impulsive noise removal circuit of the present invention, and FIG. 3 is a schematic diagram showing another embodiment of the invention. FIG. 3...gate circuit, 5...noise detection circuit, 7...
... Noise sensitivity adjustment circuit, 10 ... Intermediate frequency detection circuit, 13 ... Diode, 16 ... Control circuit, 1
7...Time constant circuit. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A noise detection circuit, a gate circuit inserted into a signal line and cut off based on the detection output of the noise detection circuit, a noise sensitivity adjustment circuit attached to the noise detection circuit, and an input circuit. An impulsive noise removal circuit comprising: a control circuit which receives a signal corresponding to an electric field level and controls the time constant of the noise sensitivity adjustment circuit to be smaller as the input electric field level becomes larger. 2. The control circuit includes a variable resistance element that becomes conductive or non-conductive depending on the output voltage depending on the input electric field level, and controls the time constant of the noise sensitivity adjustment circuit by making the variable resistance element conductive or non-conductive. An impulsive noise removal circuit according to claim 1. 3. The impulsive noise removal circuit according to claim 1 or 2, wherein the noise sensitivity adjustment circuit is a time constant circuit consisting of a resistor and a capacitor. 4 The variable resistance element is composed of a diode, and this diode is connected between the intermediate frequency detection circuit that outputs a DC voltage according to the input electric field level and the connection point between the resistor and the capacitor of the time constant circuit. An impulsive noise removal circuit according to claim 2 or 3. 5. The impulsive noise removal circuit according to claim 4, wherein the control circuit includes a time constant circuit comprising a resistor and a capacitor connected in parallel to the resistor constituting the time constant circuit.