JPH01162019A - Noise suppressing circuit - Google Patents

Abstract

PURPOSE:To widely decrease externally fitting parts in case an integrated circuit is obtained by utilizing the delay time of a celamic filter CF, which is originally used in an FM radio receiver, without using a delay circuit by an LPF. CONSTITUTION:For the IF signal of an intermediate frequency amplifying step to receive a pulsative noise, distortion is received in the amplitude of the signal. The IF signal before passing through the celamic filter CF is inputted to a first detecting circuit ND1. The first detecting circuit shapes the IF signal to receive the amplitude distortion and the envelop of the signal is detected. The amplitude distortion of the IF signal due to the pulsative noise can be generated in both positive and negative direction. Full-wave shaping is executed by a second detecting circuit for the distortion in these both directions. Since an FM detecting output to receive the pulse noise can be obtained by detecting the IF signal to pass through the filter CF, the output is delayed only by a delay time by this filter CF. When this pulse noise appears in a gate circuit G, since the gate circuit G is closed by a single stable multi-MM, the noise does not appears in an output edge.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a noise control circuit, and particularly to a circuit for preventing pulsed noise such as ignition noise from being emitted from a speaker in a vehicle-mounted FM receiver.

[Conventional technology]

In in-vehicle FM radios, pulse noise generated from the vehicle, particularly the engine ignition system, is a factor that impedes reception quality, and therefore there are conventional circuits for removing such noise.

FIG. 2(a) is a block diagram of the pull pull, and the dashed line frame No.
is a noise suppression circuit. The configuration of a normal FM receiver is antenna ANT, high frequency amplification stage 2 RF, intermediate frequency amplification stage: IF, FM detection stage: DET, low frequency amplification stage: AF,
Speaker: inserted between the F'M detection stage DET and the low frequency amplification stage AF of a system consisting of SP.

The noise suppression circuit NG consists of two systems. One of them is buffer amplifier A1. low pass filter LPF
, a signal system consisting of a gate circuit G, a capacitor C1, a buffer amplifier A2, and a bypass filter HPF,
The control system consists of a noise detection circuit ND and a monostable multivibrator MM.

In this circuit, when an FM detection output signal containing pulse noise is applied to the input terminal of the noise suppression circuit NO, on the one hand A 1
．． A through the path of L P F , G , A 2
It is input to F. On the other hand, pulsed noise is detected in the path of HPF and ND, and this detection signal is a monostable multi-MM
is triggered to create a pulse with a constant width, and this pulse creates a gate circuit G. Pulse noise superimposed on the FM detection output is delayed for a certain period of time by a low-pass filter LPF, and when the pulse noise appears in the gate circuit G, the gate circuit is closed by the end-stable multi-MM, so that it does not appear at the output terminal. The noise-removed portion is held by a capacitor C connected to the output terminal of the gate circuit G, and is returned to the original waveform before noise is mixed.

The low-pass filter LPF is for delaying pulse noise superimposed on the FM detection output for a certain period of time. Without this delay, the timing of the gate control pulse generated by the control system would be delayed at the same time as the pulse noise superimposed on the FM detection output appears or due to delays in the control system circuit, and the noise suppression effect would not be sufficient. I can't get it. The bypass filter HPF is for extracting only heavy pulse noise from the FM detection output.

Generally, the frequency band of the FM detection output signal is 76K, considering up to 53KHz%SCA in the case of stereo broadcasting.
It is Hz. On the other hand, the pulse noise handled by the noise suppression circuit is a component of approximately 100 KHz or higher. Therefore, the low pass filter LPF.

The cutoff frequency of the bypass filter HPF is 76 to 1
It is set between 00KHz. If these filters are first-order (6dBloc) filters or low-pass filters, it will not be possible to obtain sufficient delay time for pulsed noise, and the cutoff frequency must be lowered to obtain the delay time. But then FM
The phase of the sub-signals in the composite signal also lags, resulting in poor stereo vision. In the case of a bypass filter, the SCA signal and sub-signals cannot be sufficiently attenuated, and these signals are detected as pulse noise, resulting in malfunction.

For this reason, fourth-order active filters are used as the low-pass filter LPF and bypass filter HPF of the conventional noise suppression circuit, as shown in FIGS. 2(b) and 2(c), respectively.

[Problem that the invention seeks to solve]

The above-mentioned conventional noise suppression circuit is integrated into many integrated circuits, and the low-pass filter LPF, bypass filter HPF, and top IC of these ICs are as shown in FIG. 2(b), CC), respectively. It has a fourth-order active filter configuration, and all of the above-mentioned I
In C, Fig. 2(b).

The resistor and capacitor shown in (c) have high resistance and I
Since the capacity is too large to be incorporated into the C, it is necessary to add it as an external component. In other words, there is a drawback that a total of 16 external parts for C and 'R are required for the LPF and HPF.

The present invention aims to provide a noise suppression circuit that can significantly reduce the number of external components when integrated into a circuit.

[Means for solving problems]

The present invention includes a gate circuit inserted in the signal path between the FM detection stage and the low frequency stage, and when pulse noise mixed in the signal arrives, the gate circuit is closed to prevent the noise from passing through. In the noise suppression circuit of an FM radio receiver, F
At least one or more band 3 wave filters (ceramic filters) are inserted in the F signal path of the M radio receiver, and the I signal before passing through the at least one band band 3 wave filter is inserted.
A high-frequency triple wave generator is connected to the output of the first detection circuit and the first detection circuit that rectify the F-fold signal, and the output signal of the high-frequency triple wave generator is connected to the first detection circuit and the second circuit. It consists of a monostable multi whose output is the trigger.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1(a) is a block diagram of an embodiment of the present invention. In this figure, the same symbols as those shown in FIG. 2 are the same or equivalent parts. These explanations are vague.

The broken line NO is a noise suppression circuit. The noise suppression circuit NC consists of two systems. One of them is a signal system in which buffer amplifier A 1g gate circuit G, capacitor C, and buffer amplifier A2 are connected in sequence, and the other is a first detection circuit ND 1 , high-frequency triple wave filter HPF, and second detection circuit. ND2. This is a noise detection system in which monostable multi-MMs are connected in sequence.

Next, the operation will be explained.

The signal appears at the output of the high frequency amplification stage RF and is input to the IP amplification stage IF through the IF multiplied signal band triple waver CF. A filter called a ceramic filter is often used in the band triwave filter CF, and two to four filters are usually inserted in the IF signal path to obtain the required band.

Now, with the IF multiplied signal IF amplification stage IF, the FM detection DE
The signal is input to T and subjected to FM detection. The FM detection output is input to the low frequency stage AF via the buffer amplifier AH, the gate circuit G, and the buffer amplifier A2.

The amplitude of the IP multiplied signal subjected to pulse noise is distorted. The first IF multiplied signal before passing through the ceramic filter CF
is input to the detection circuit MDI.

The first detection circuit rectifies the IP times signal subjected to the heart width distortion and detects its envelope. When an FM signal is rectified, its modulation signal (audio signal) is demodulated, albeit slightly. It is removed by a bypass filter HPF connected next.

Pulse noise superimposed on the input F signal appears in the output of the bypass filter HPF. This is input to the second detection circuit ND2. IF multiplied signal amplitude distortion due to pulse noise can occur in both positive and negative directions. Both of these signals are full-wave rectified by the second detection circuit so that they can serve as triggers for the next-stage monostable Malta MM. The output signal of the second detection circuit is used as a trigger.

The FM detection output that received the pulsed noise is obtained by detecting the IF multiplied signal that has passed through the ceramic filter CF.
When the pulsed noise appears in the gate circuit G after being delayed by the delay time by the ceramic filter CF, it does not appear at the output terminal because the gate circuit G is closed by the monostable multi-MM.

〔Effect of the invention〕

As explained above, the present invention has a configuration that utilizes the delay time of the ceramic filter CF originally used in FM radio receivers, without using the delay circuit using the low-pass filter LPF used in the conventional example. Therefore, when the conventional example is implemented as an IC, there is an effect that the LPF, which requires the resistor and capacitor shown in FIG. 2 as external components, can be made unnecessary.

Furthermore, when converting the conventional example into an IC, it is necessary to take out the output terminal of the buffer amplifier A1 and the input/output terminals of the low-pass filter LPF as external terminals of the IC, but in the present invention, these are no longer necessary, and when integrated into an IC. This has the effect of reducing the number of terminals.

Furthermore, the bypass filter HPF in the present invention has a first
This is to remove the audio signal component demodulated at the same time when the detection circuit MDI rectifies the F signal, and the level of the audio signal at this time is much lower than the original FM detection output. Therefore, the bypass filter in the present invention does not require a fourth-order active filter as shown in FIG. 2, and can be a single-stage passive filter as shown in FIG. 1(b), and when integrated into an IC. This has the effect of further reducing the number of external parts.

[Brief explanation of the drawing]

FIG. 1(a) is a block diagram of an embodiment of the present invention, FIG. 1(b) is a circuit diagram of a bypass filter used in the embodiment, FIG. 2(a) is a block diagram of a conventional example, and FIG. Figure 2 (b)
, (c) is a circuit diagram of a conventional LPF and HPF'. ANT: antenna, RF: high frequency amplification stage, IF: IF wave amplification stage, DET: group/FM detection circuit, NO: noise suppression circuit, A1
・・Group・Noise control circuit L LPF・・・Low pass filter, G・・Gate circuit, A2・・・Buffer amplifier, HPF・・・・Bypass filter, ND・・・Group・Noise detection circuit L MM... Monostable multi, AP... Low frequency amplification stage, SP...
...Speaker, NDl...First noise detection circuit, ND2...Second noise detection circuit. Agent Patent Attorney Oto Hara ((1) (b) M1 times

Claims (1)

[Claims] A radio that includes a gate circuit in a signal path between an FM detection stage and a low frequency stage, and closes the gate circuit when pulse noise mixed in the signal arrives to prevent the noise from passing through. In the noise suppression circuit of the receiver, at least one or more bandpass filters are inserted in the IF signal amplification path of the FM radio receiver, and the IF signal before passing through the at least one or more bandpass filters. a first detection circuit that rectifies the signal; a high-pass filter connected to the output of the first detection circuit; a second detection circuit that rectifies the output signal of the high-pass filter; A noise suppression circuit having a monostable multivibrator triggered by the output signal of the detection circuit No. 2.