JPS5844672Y2 - Noise removal circuit - Google Patents

Description

[Detailed description of the invention] The present invention relates to a noise removal circuit in a radio receiver.
In particular, the present invention relates to a noise removal circuit that removes impulsive noise generated from an automobile's ignition system and mixed into a radio receiver.

The prior art will be explained with reference to FIGS. 1 to 3.

As impulsive noise removal circuits mixed into radio receivers, circuits that limit the frequency and voltage to below a certain value have conventionally been employed.

Figure 1 shows its block configuration diagram and waveform diagrams of various signals.The input signal 1 is differentiated by a differentiator 2, the signal resulting from this differentiation is passed through an amplitude limiter 3, and then integrated by an integrator 4. Then, the output signal 5 is obtained.

N(1) to N(4) are impulsive noises, 5(1) to 5(4)
) shows the waveform diagram of each part of the signal, among which N(1
) and 5(1) are N(2) and 5(2
) is after leaving the differentiator 2, N(3) and 5(3) are after leaving the amplitude limiter 3, N(4) and 5(4) are after leaving the integrator 4, They are respective waveform diagrams.

The threshold value (clipping level) of the amplitude limiter 3 is set to be larger than the differential output of the signal component, so that the amplitude limiting effect only works on components with extremely large differential outputs, such as impulsive noise. Become.

In contrast, signal components simply pass through the processes of differentiation and integration and are transmitted without distortion.

Fig. 2 shows a block diagram of a conventional device that controls the threshold voltage of an amplitude limiter according to the received electric field so that the signal component is not distorted due to changes in the electric field strength. The receiver's intermediate frequency signal or automatic gain control signal is used to detect the electric field.

FIG. 2 shows a case where the output signal obtained by detecting the intermediate frequency signal 6 of the receiver by the detector 7 is taken into the threshold control circuit 8 to control the threshold voltage of the amplitude limiter 3.

Figure 3 shows the threshold voltage characteristics in this case.
The horizontal axis shows the electric field strength, and the vertical axis shows the level of each part signal in level.

Curves 11 and 12 are the SN characteristics of the receiver, with curve 11 showing the average level of signal S and curve 12 showing the level of background noise N.

Curve 13 shows the output characteristics of the DC voltage obtained by detecting the receiver's intermediate frequency signal 6 with detector 7, and curve 14 is the result of amplifying curve 13 by threshold control circuit 8. The voltage becomes the threshold voltage.

Further, curve 15 is the output characteristic of impulsive noise such as automobile ignition noise, and as the electric field strength increases, the output is suppressed by the effect of automatic gain control.

Note that the curve 16 is the peak level of the signal S shown in the curve 11.

In the above-mentioned noise removal circuit, the threshold voltage must be 14
must be higher than the peak level 16 of the signal S, as shown in Figure 3, but there is a contradictory relationship that it is better to make the threshold voltage as small as possible in order to improve the noise removal effect. .

However, for the reasons described below, the magnitude relationship of the threshold voltage 14 with respect to the peak level 16 of the signal S can be determined depending on the relationship with the electric field strength, depending on whether to emphasize the removal effect or the distortion characteristics.

That is, in the weak/medium electric field region from no signal, (a) the SN is poor and the clarity of the signal S is low, so some distortion caused by the amplitude limiter cannot be detected, and (b) the impulsive noise level is low. Since the threshold voltage is high, the threshold voltage 1 is
4 can be set lower than the peak level 16 of the signal S, and in a region where the electric field strength is large beyond the medium electric field region, (a) the S/N ratio is improved, the clarity of the signal S is high, and there is no distortion. (b) Impulsive noise is suppressed by automatic gain control of the receiver. Therefore, emphasis is placed on the distortion characteristics, and the threshold voltage 14 is set to the peak level 16 of the signal S.
It must be set larger to avoid distortion.

The present invention was developed in view of the above points, and aims to improve the noise removal effect and reduce signal distortion more effectively. It is characterized by a configuration in which the threshold control circuit is provided with a switching circuit that automatically switches in response to the magnitude of the electric field, and when the received electric field is weak, the threshold voltage is such that distortion cannot be detected audibly. The aim is to increase the noise removal effect in a small way, and to increase the threshold voltage when the received electric field is strong to prevent distortion from occurring.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 4 is a block configuration diagram of one embodiment of the present invention, and 8(1)
is a threshold control circuit that sets a negative amplitude threshold, 8 (
2) is a threshold control circuit that sets a positive amplitude threshold;
9(1) and 9(2) are switching circuits for the negative side and positive side threshold control circuits 8(1) and 8(2), respectively, and 10(1) and 10(2) are diodes, respectively. Other symbols are the same as in FIG. 2.

The switching circuits 9(1) and 9(2) adjust the received electric field according to the magnitude of the DC voltage obtained by detecting the intermediate frequency signal 6 of the receiver with the detector 7, that is, the magnitude of the received electric field. When the electric field is weak or medium, the amplification degree of the threshold control circuits 8(1) and 8(2) is lowered to reduce the threshold voltage, and when the received electric field enters the strong electric field region, the threshold voltage is lowered. Control circuit 8 (1)
, 8(2) and increases the threshold voltage.

FIG. 5 is a circuit configuration diagram of a specific embodiment of the present invention corresponding to FIG. 4, and the same reference numerals are used for parts having the same function.

C1 and C2 are capacitors and DI. D2 is a diode, R
3 and RI R9, RIO are resistors.

21 and 22 are the threshold control circuit 8(1) in FIG.
An amplifier with a function corresponding to 8(2), and an IC in the embodiment.
(integrated circuit).

10B, -B are DC drive power supplies for amplifiers 21 and 22, EB
is a bias DC power supply.

The amplifier 21 is designed to output a voltage having a polarity opposite to that of the output voltage of the amplifier 22.

The diode D2 in the switching circuit 9 is a switching diode, and the resistors R, , R2, and R3 together with the resistor R8 determine the amplification degree of the amplifier 22.

Assuming that the resistance values of the resistors R, , R2, R3, and R3 also use the same signs as the resistors, the threshold voltage, that is, the output voltage of the amplifier 22, Eo is now E.

-EB・(R1+R2)/R2, and in the range where Eo is smaller than that, the switching diode D
2 is in the off state, the amplification degree A1 of the amplifier 22 is, and the output signal of the detector 7 is outputted as a threshold value via the amplifier 21.

In a range larger than this, the switching diode D
2 is in the on state, and the amplification degree A2 of the amplifier 22 is A2=-R3/R3, so that 1A21>A,I.

Since the reference voltage of the amplifier 21 increases, the output of the amplifier 21 increases.

In other words, it outputs a high threshold value.

Figure 6 shows the SN characteristics and threshold voltage characteristics when the present invention is implemented, and curves 12, 14 and 16 are the level of background noise N and the threshold voltage, respectively, as in the case of Figure 3. Although the voltage and the peak level of the signal S are shown, the shape of the threshold voltage curve 14 in FIG. 6 is different from that in FIG. 3.

That is, in FIG. 6, the amplification degree A1 of the threshold control circuit is small in the weak/medium electric field region, and the threshold voltage curve 14
is below the peak level curve 16 of the signal S, but in the strong electric field region, the amplifier A2 is large and the threshold voltage curve 14 exceeds the peak level curve 16 of the signal S.

In addition, in the above-mentioned embodiment, the case where the amplification degree of the threshold control circuit is switched between two stages A1 and A2 according to the magnitude of the received electric field has been described, but the switching circuit provided in the threshold control circuit By further increasing the number, it is also possible to adopt a system in which the degree of amplification is switched in multiple stages depending on the magnitude of the received electric field.

FIG. 7 shows the amplification degrees of A, A2. It shows an example of the relationship between the threshold voltage curve 14 and the peak level curve 16 of the signal S when switching to three stages A3.

As explained above, according to the present invention, the amplification degree of the threshold control circuit is automatically switched according to the magnitude of the received electric field, so that noise can be reduced in the weak and medium received electric field regions. It is possible to improve the rejection effect, and at the same time eliminate the audibly perceptible signal distortion in the strong electric field region.

[Brief explanation of the drawing]

Figure 1 is a conventional noise removal circuit and a signal waveform diagram for explaining its operation. Figure 2 is a block diagram of a conventional device that controls the threshold voltage according to the received electric field. Figure 4 is a block diagram of an embodiment of the present invention, Figure 5 is a circuit diagram of an embodiment corresponding to Figure 4, and Figures 6 and 7 are diagrams of the present invention. It is a threshold voltage characteristic diagram when implemented. Explanation of symbols 1... Input signal, 2...
Differentiator, 3... Amplitude limiter, 4... Integrator, 5... Output signal, 6... Intermediate frequency signal, 7...・Detector, 8(1), 8(2)・
...Threshold control circuit, 9(1), 9(2)...
... Switching circuit, 11 ... Signal average level curve, 12 ... Background noise level curve,
14...Threshold voltage curve, 16...
Signal peak level curve 21.22...Amplifier.

Claims (1)

[Scope of utility model registration request] A differentiator is connected between the receiver's detection circuit and the low-frequency amplification stage to differentiate the detection circuit output signal, and a threshold control circuit automatically adjusts the amplitude of this differentiated output signal in response to the magnitude of the received electric field. In a noise removal circuit that limits the frequency/amplitude product to below a certain value, we avoid the cascade connection of an amplitude limiter that suppresses the amplitude below a threshold value set by the amplitude limiter and an integrator that integrates the output signal of this amplitude limiter. . A noise removal circuit, characterized in that the threshold control circuit is provided with a switching circuit that automatically switches the amplification degree of the threshold control circuit in response to the magnitude of the received electric field.