JP2000091932A - Pulse noise canceller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely eliminate a pulse noise regardless of a simple configuration. SOLUTION: An output of a mixer 25 of an AM receiver 22 in the noise canceller 21 gives a noise detection input to a noise amplifier 31. The noise amplifier 31 has a low pass filter characteristic that reduces a local oscillation frequency and its harmonies generated from a voltage controlled oscillator 26. A pulse noise processing circuit 32 detects a pulse noise from an output of the noise amplifier 31 and a sample-and-hold circuit 33 controls a switch circuit 30. An intermediate frequency component included in an output of a mixer 25 receives a delay in the case of amplifying the intermediate frequency component selectively by an intermediate frequency amplifier 27. A switch circuit 30 surely eliminates the pulse noise included in an audio signal subject to the delay and detected by a detector 28.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a pulse noise canceller for reducing pulse noise particularly when an AM signal is received.

[0002]

2. Description of the Related Art Single sideband modulation (S) is used for broadcasting and communication in a relatively low frequency band such as a medium wave (MF) and a short wave (HF).
Amplitude modulation (AM) including SB) is used. AM
In this case, the frequency band required for transmission corresponds to the frequency band of a low-frequency signal indicating the content of broadcasting or communication, and is generally narrower than the frequency band required for transmitting an FM signal. However, when pulse noise is superimposed on the received AM signal, it is demodulated and output in the same manner as the AM signal. In a tuner mounted on a car, noise generated from the engine of the car, noise generated from a transmission line on the side of a running road, or noise generated from an engine of another vehicle running on a road. And the like, and the need for a pulse noise canceller that reduces the effect increases.

FIG. 7 shows a schematic electrical configuration of an AM receiver 2 including a noise canceller 1 on which the present invention is based. The AM receiver 2 is of a superheterodyne type, and after an AM signal received by an antenna 3 is amplified by a high-frequency amplifier (hereinafter sometimes abbreviated as “RF AMP”) 4, a mixer (hereinafter “MIX”) is used. Is sometimes abbreviated as "). The mixer 5 is also provided with a local oscillation signal from a voltage-controlled oscillator (hereinafter sometimes abbreviated as “VCO”) 6 as a local oscillation signal, and outputs an AM high-frequency signal and a local oscillation signal to the output from the mixer 5. And the components of the sum and the difference. Of the output from the mixer 5,
The signal of the difference frequency is supplied to an intermediate frequency amplifier (hereinafter, "IF
AMP ”). The AM intermediate frequency signal amplified by the intermediate frequency amplifier 7 is detected by a detector 8 to obtain a low frequency signal. The output of the detector 8 is amplified by a low frequency amplifier 9.

The noise canceller 1 includes a switch circuit 10 provided between a detector 8 and a low frequency amplifier 9.
In the switch circuit 10, the output of the mixer 5 is amplified by the noise amplifier 11, pulse noise is detected by the pulse noise processing circuit 12, and the switching of the switch circuit 10 is controlled by the sample and hold circuit 13. In the switch circuit 10, switching control is performed to transmit or cut off the audio signal output from the detector 8 to the low-frequency amplifier 9 as it is in accordance with the detection state of the pulse noise.

In the noise canceller 1, the point at which the noise amplifier 11 inputs a signal for detecting pulse noise is between the output of the mixer shown as A in FIG. 7 and the input of the intermediate frequency amplifier 7. , B, between the output of the intermediate frequency amplifier 7 and the input of the detector 8 or between the output of the detector 8, indicated by C, and the input of the switch circuit 10.

In the superheterodyne AM receiver 2, band characteristics such as selectivity are obtained by a band filter such as a ceramic filter provided in the intermediate frequency amplifier 7. The use of a bandpass filter with good selectivity in an intermediate frequency amplifier introduces a delay in the signal. When a signal for noise detection is input to the noise canceller 1 from points B and C, pulse noise generated in an audio output and
Even if the timing of the pulse noise at the time of noise detection is the same and the noise canceller 1 operates, there is a response delay time, so that the pulse noise cannot be sufficiently reduced. Generally, in the intermediate frequency amplifier 7, a ceramic filter or the like is inserted on the input side to which the output from the mixer 5 is given.
If the noise is taken out before such a filter and applied to the noise amplifier 11, the sample-and-hold circuit is not affected by the time delay of the ceramic filter or the like, and when the pulse noise appears in the output of the detector 8. The circuit 13 can perform time-preceding control such as switching off the switch circuit 10.

For the reasons described above, the noise canceller 1 as shown in FIG. 7 is used, and FIG. 8 shows a more detailed configuration of the pulse noise processing circuit 12. The pulse noise processing circuit 12 includes a voltage control amplifier (VCA) 14, an automatic gain (AGC) circuit 15, and a processing circuit 16. When an AM broadcast is being received, a carrier having a maximum level at an intermediate frequency of, for example, 450 kHz is also input.
AGC control is performed by the voltage control amplifier 14 and the automatic gain adjustment circuit 15. The processing circuit 16 detects pulse noise.

FIG. 9 shows a signal spectrum at a point A in FIG. FIG. 9A shows an ideal spectrum. In the ideal spectrum, the carrier has a maximum level around 450 kHz, which is an intermediate frequency, and pulse noise is superimposed on the carrier. However, the actual spectrum is shown in FIG.
As shown in (b), not only the carrier component of the intermediate frequency but also the local oscillation signal from the voltage controlled oscillator (VCO) 6 and its harmonic component are included. If a filter similar to the intermediate frequency amplifier 7 is inserted in the noise canceller 1, the influence of the local oscillation signal and its harmonic components as shown in FIG. 9B can be reduced. However, since a filter having a steep characteristic causes a delay in a signal, the operation of the noise canceller 1 causes a delay in response to pulse noise.

Japanese Patent Application Laid-Open No. 7-231268 discloses a high-pass filter (HPF) used as a pulse noise canceller in an FM tuner. In this prior art, in order to perform FM signal reception with higher sound quality in an FM tuner, a signal for a noise canceller is extracted from the output side of the FM detector, and an audio signal output from the FM detector is further temporally delayed by a delay circuit. , Causing a noise cancellation operation in the switching gate circuit. It is considered that the high-pass filter is provided for reliably detecting the pulse noise component.

[0010]

The noise canceller 1 as shown in FIGS. 7 and 8 is preferable in terms of timing for detecting noise, but has a problem in that it is affected by a local oscillation signal and its harmonic components. . In the pulse noise processing circuit 12 shown in FIG. 8, the sensitivity as the noise canceller 1 is adjusted by feedback control by the automatic gain control circuit 15 according to the level of the intermediate frequency carrier obtained by converting the AM high frequency signal. The local oscillation signal from the voltage controlled oscillator 6 and its harmonic components are always generated at the time of receiving the AM signal regardless of the presence or absence of the carrier. Therefore, in the local oscillation signal and its harmonic components,
The gain control of the voltage control amplifier 14 is always performed by the automatic gain control circuit 15, and as a result, the sensitivity of the noise canceller 1 is reduced.

Even if a high-pass filter is provided as in JP-A-7-231268, the local oscillation signal and its harmonic cannot be removed. Also, as in this prior art,
When extracting a signal for noise detection after the detector, it is necessary to provide a delay circuit or the like in the audio signal from which noise is to be removed.

An object of the present invention is to provide a pulse noise canceller which can remove the influence of a local oscillation signal and its harmonics with a simple configuration and effectively remove pulse noise.

[0013]

SUMMARY OF THE INVENTION The present invention relates to a pulse noise canceller for removing a pulse noise mixed in a high frequency signal when a high frequency signal is received by a superheterodyne method. Amplifying an output from a mixer that mixes and converts the mixed signal into an intermediate frequency signal, detects pulse noise, and reduces a local oscillation signal and its harmonic components. And a switch circuit that is provided after selectively amplifying and detecting the intermediate frequency component of the output from the amplifier, and that shuts off the detection output during the period when the noise amplifier is detecting the pulse noise. This is a pulse noise canceller.

According to the present invention, the noise amplifier detects pulse noise by amplifying an output from a mixer that mixes a high-frequency signal and a local oscillation signal and converts the signal into an intermediate frequency signal. Since the noise amplifier has low-pass filtering characteristics for reducing the local oscillation signal and its harmonic components, it is possible to detect pulse noise without being affected by the local oscillation signal and its harmonic components. . During the time when the noise amplifier is detecting pulse noise, the switch circuit selectively amplifies the intermediate frequency component of the output from the mixer and cuts off the detected output. It can be suppressed reliably. The noise amplifier only needs to have low-pass filtering characteristics that are not affected by the local oscillation signal and its harmonic components.Therefore, the low-pass filtering characteristics do not need to be steep and can be easily configured. Signal delay can be avoided.

Further, in the present invention, the noise amplifier has a low-pass filter including a resistor and a capacitor on an input side.

According to the present invention, since a low-pass filter constituted by a resistor and a capacitor is provided on the input side of the noise amplifier, a local oscillation signal and a signal having its harmonic components reduced are input to the noise amplifier. Automatic gain control and the like can be performed without being affected by the local oscillation signal and its harmonic components.

In the present invention, the noise amplifier has a low-pass filter constituted by a resistor and a capacitor on an output side.

According to the present invention, since the low-pass filter constituted by the resistor and the capacitor is provided on the output side of the noise amplifier, the low-pass filter is not directly connected to the output side of the mixer, but is connected directly to the mixer. The load can be reduced.

The present invention further comprises a voltage-controlled oscillation circuit for generating the local oscillation signal, wherein a cutoff frequency of the low-pass filter is changed in conjunction with a tune bias voltage applied to the voltage-controlled oscillation circuit. It is characterized by the following.

According to the present invention, the local oscillation signal is generated by the voltage-controlled oscillation circuit, and the cutoff frequency of the low-pass filter is changed in conjunction with the tune bias voltage for changing the frequency. , Suitable low-pass characteristics can be obtained.

[0021]

FIG. 1 shows a schematic configuration of a noise canceller 21 as one embodiment of the present invention. The noise canceller 21 is used by the AM receiver 22 to reduce the frequency of pulse noise. The AM receiver 22 includes:
Antenna 23 for receiving AM signal radio waves, high frequency amplifier 2 for amplifying at the frequency of the AM signal received by antenna 23
4. A mixer 25 to which the high-frequency amplifier 24 is input, a voltage-controlled oscillator 26 which supplies a local oscillation signal to the mixer 25, and an intermediate frequency signal which is a difference between an AM signal output from the mixer 25 and the local oscillation signal is selectively amplified. Intermediate frequency amplifier 27,
A detector 28 for detecting the output of the intermediate frequency amplifier 27 and a low frequency amplifier 29 for amplifying the audio signal detected by the detector 28 are included.

The noise canceller 21 includes a switch circuit 30 provided between the detector 28 and the low-frequency amplifier 29,
A noise amplifier 31 that amplifies the output of the mixer 25, a pulse noise processing circuit 32 that detects pulse noise from the output of the noise amplifier 31, and a sample hold that controls the switch circuit 30 in accordance with the detection output of the pulse noise processing circuit 32 The circuit 33 is included.

As shown in FIG. 2, the noise amplifier 31 of the present embodiment has a low-pass filter (hereinafter referred to as “LP”) on the input side.
F) (which may be abbreviated as F). The low-pass filter 40 forms an integration circuit including the resistor 41 and the capacitor 42, and has an attenuation characteristic of 6 dB per octave. The frequency at which the resistance value of the resistor 41 becomes equal to the impedance of the capacitor 42 is the cutoff frequency, and the 3 dB signal is attenuated as compared with the low frequency component. The AM receiver 22 of FIG.
When a frequency AM audio broadcast is received in the frequency range of 1620 kHz and the intermediate frequency is 450 kHz, the local oscillation frequency is a frequency of 1 MHz or more. If the cut-off frequency of the low-pass filter 40 is set to about 1 MHz, the local oscillation signal and its harmonic components can be sufficiently suppressed when receiving the frequency band AM broadcast of the medium wave AM audio broadcast. The noise amplifier 31 includes a capacitor 4
The signal from which the DC component has been cut is input via the low-pass filter 40. A bias voltage for operating the noise amplifier 31 is provided by a bias power supply 44.

FIG. 3 shows another embodiment of the present invention.
The configuration in which the low-pass filter 40 is provided on the output side of the noise amplifier 31 is shown. In the present embodiment, portions corresponding to the embodiment shown in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant description will be omitted. In the present embodiment, the noise amplifier 31
Is connected to a low-pass filter 40 via a DC cut capacitor 45. Low pass filter 40
Is applied to a voltage control amplifier 46 in a pulse noise processing circuit 32 similar to the voltage control amplifier 14 in FIG. In the present embodiment, the low-pass filter 40 is provided in the noise amplifier 31 on the output side instead of the input side. When the low-pass filter 40 is provided on the input side of the noise amplifier 31 as in the embodiment of FIG. 2, the low-pass filter 40 is directly connected to the output side of the mixer 25 of FIG.
When the input frequency becomes higher than the cut-off frequency, the input impedance of the low-pass filter 40 composed of the resistor 41 and the capacitor 42 decreases to the same level as the resistance value of the resistor 41. For this reason, the impedance connected to the output of the mixer 25 of FIG. 1 becomes low, and the load of the mixer 25 becomes heavy. In the embodiment shown in FIG. 3, it is possible to avoid a situation where the load on the mixer 25 becomes heavy due to the low-pass filter 40.

FIG. 4 shows a low-pass filter 5 having a variable cutoff frequency according to still another embodiment of the present invention.
0 is provided on the input side of the noise amplifier 31. The AM receiver 22 shown in FIG.
4 to receive medium-wave AM broadcast in the range of z to 1620 kHz
To convert to an intermediate frequency of 50 kHz, the voltage controlled oscillator 26 changes the local oscillation frequency in the range of 972 kHz to 2079 kHz. Therefore, it is ideal if the cutoff frequency of the low-pass filter 50 is changed according to the received frequency. In the voltage controlled oscillator 26, since the tune bias voltage and the reception frequency have a correspondence relationship as shown in FIG. 5A, the tune bias voltage is used to change the reception frequency as shown in FIG. The cutoff frequency can be changed accordingly. In FIG. 4, a voltage variable capacitance element 51 such as a variable capacitance diode is used for the low-pass filter 50. To apply the tune bias voltage to the voltage variable capacitance element 51, a processing circuit 52 is provided. The processing circuit 52 applies the tune bias voltage as it is or a voltage corresponding to the tune bias voltage to the voltage variable capacitance element 51,
The cutoff frequency of 0 is changed according to the reception frequency.

FIG. 6 shows a schematic configuration of still another embodiment of the present invention. In the present embodiment, the low-pass filter 50 is provided on the output side of the noise amplifier 31. That is, while the embodiment of FIG. 4 is obtained by improving the low-pass filter 40 of the embodiment of FIG. 2 so that the cutoff frequency is variable according to the tune bias voltage, the present embodiment differs from the embodiment of FIG. The low-pass filter 40 of the embodiment shown in FIG. 3 is replaced with a low-pass filter 50 whose cutoff frequency is variable according to a tune bias voltage.

In each of the embodiments described above, the low-pass filtering characteristics of the noise amplifier 31 are obtained by the passive low-pass filters 40 and 50 that do not include any active elements. An active low-pass filter including itself may be configured to reduce the local oscillation signal and its harmonic components. Although the local oscillation signal is varied by applying a tune bias voltage to the voltage controlled oscillator 26, the local oscillation signal is generated by configuring a phase locked loop (PLL) circuit or the like including the voltage controlled oscillator, or generated by a frequency synthesizer. It can also occur in a system.

[0028]

As described above, according to the present invention, the noise amplifier for amplifying the output from the mixer and detecting the pulse noise has low-pass filtering characteristics. An operation of reducing the influence of harmonics and reliably removing pulse noise can be performed. The low-pass filtering characteristic can be obtained only by adding a relatively simple component to the noise amplifier, and the signal can be received in a favorable state by reducing the influence of pulse noise.

According to the present invention, a low-pass filter including a resistor and a capacitor is provided on the input side of the noise amplifier, so that the operation of the noise amplifier is not affected by the local oscillation signal and its harmonics. can do.

According to the present invention, the low-pass filter constituted by the resistor and the capacitor is provided on the output side of the noise amplifier, so that the load on the mixer can be reduced.

According to the present invention, the cutoff frequency of the low-pass filter provided in the noise amplifier can be changed in accordance with the reception frequency, so that an appropriate cutoff frequency is automatically set in accordance with the reception frequency. To improve the detection characteristics of pulse noise.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic electrical configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a partial electrical configuration of a noise canceller 21 in the embodiment of FIG.

FIG. 3 is a block diagram showing a partial electrical configuration of a noise canceller according to another embodiment of the present invention.

FIG. 4 is a block diagram showing a partial electrical configuration of a noise canceller according to still another embodiment of the present invention.

FIG. 5 is a graph showing a relationship among a reception frequency, a tune bias voltage, and a cutoff frequency in the embodiment of FIG.

FIG. 6 is a block diagram showing a partial electrical configuration of a noise canceller according to still another embodiment of the present invention.

FIG. 7 is a block diagram showing a schematic electrical configuration of an AM receiver 2 including a noise canceller 1 on which the present invention is based.

FIG. 8 is a block diagram partially showing a detailed configuration of the noise canceller 1 of FIG. 7;

FIG. 9 is a graph showing a spectrum of a signal input to the noise amplifier 11 of FIG.

[Explanation of symbols]

 Reference Signs List 21 noise canceller 22 AM receiver 25 mixer 26 voltage controlled oscillator 27 intermediate frequency amplifier 28 detector 29 low frequency amplifier 30 switch circuit 31 noise amplifier 32 pulse noise processing circuit 33 sample hold circuit 40, 50 low pass filter 41 resistor 42, 43 , 45 capacitor 46 voltage control amplifier 51 voltage variable capacitance element 52 processing circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5K020 CC01 DD21 EE04 EE06 GG04 HH11 KK08 5K052 AA01 BB02 CC01 DD21 EE02 EE12 EE22 FF13 GG03 GG33 GG47

Claims (4)

[Claims] 1. A pulse noise canceller for removing a pulse noise mixed in a high-frequency signal when a high-frequency signal is received by a superheterodyne method, wherein the high-frequency signal and a local oscillation signal are mixed into an intermediate-frequency signal. A noise amplifier having low-pass filtering characteristics for amplifying an output from the mixer to be converted to detect pulse noise and reducing a local oscillation signal and its harmonic components; and an intermediate frequency component among the outputs from the mixer. And a switch circuit that is provided after selectively amplifying and detecting a signal, and that switches off a detection output during a period when the noise amplifier is detecting the pulse noise. 2. The pulse noise canceller according to claim 1, wherein the noise amplifier has a low-pass filter including a resistor and a capacitor on an input side. 3. The pulse noise canceller according to claim 1, wherein the noise amplifier has a low-pass filter including a resistor and a capacitor on an output side. 4. A voltage-controlled oscillation circuit for generating the local oscillation signal, wherein a cutoff frequency of the low-pass filter is changed in conjunction with a tune bias voltage applied to the voltage-controlled oscillation circuit. The pulse noise canceller according to claim 2 or 3, wherein