JP3335414B2 - Amplitude modulated adjacent interference canceler by frequency conversion. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulation circuit for orthogonally transforming an amplitude-modulated signal, and more particularly to an amplitude-modulated adjacent interference removing apparatus for removing interference of an adjacent channel.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field are disclosed in JP-A-61-273005 and JP-A-4-150307.
In Japanese Patent Application Laid-Open Publication No. H10-264, there is a device that describes a demodulation circuit that performs quadrature modulation of amplitude modulation. By the way, when an amplitude-modulated signal is received and demodulated, radio waves from a distant broadcasting station are mixed as interference of an adjacent channel due to the influence of the ionosphere or the like, thereby deteriorating sound quality. For this reason, when the occurrence of adjacent channel interference is detected when an amplitude-modulated signal is received, there is a case where the high band is limited by a filter in the case of envelope detection.
As another example, in the case of an ISB (Independent Side Band), a process is performed in which a side wave that has been disturbed is manually switched to a channel that does not disturb and removed.

[0003]

However, in the conventional adjacent interference removal device, the sound quality is deteriorated because the interference wave cannot be sufficiently cut by the filter, or the circuit configuration of the filter becomes complicated when it is attempted to completely cut it. There was a problem that many adjustments were required. The manual switching by the ISB has a problem that when the demodulation device is mounted on a vehicle, an operation other than driving is forced, and furthermore, there is a problem that it cannot be directly used in a demodulation circuit for orthogonally transforming a signal under amplitude.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an amplitude-converted adjacent interference elimination apparatus that can easily improve sound quality and that does not require manual operation in view of the above problems.

[0005]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides an amplitude modulation adjacent interference canceling device by frequency conversion of a demodulation circuit for orthogonally transforming an amplitude-modulated signal by an orthogonal transform oscillator and demodulating the signal. An adjacent interference removal filter and an amplitude conversion adjacent interference removal device are provided. The adjacent interference removal filter is provided after the orthogonal transform of the signal to be amplitude, and includes a low-pass filter that varies a cutoff frequency to remove an adjacent channel.

The amplitude-converted adjacent interference canceller converts the frequency of the amplitude-converted signal to detect an adjacent channel, and at the time of detection, determines whether or not the adjacent channel is in the lower side wave or the upper side wave. The oscillation frequency is controlled to increase or decrease by a fixed amount, and at the same time, the cutoff frequency of the adjacent interference removal filter is reduced by a fixed amount.

Further, the amplitude-converted adjacent interference canceling device converts the amplitude-converted signal at a frequency shifted by a predetermined amount from the frequency of the quadrature transform oscillator, and extracts a carrier wave frequency of an adjacent channel of an upper wave or a lower wave. An adjacent channel extracting unit, an adjacent channel detecting unit that detects a carrier of an adjacent channel of the extracted upper wave or lower side wave, and an oscillation frequency of the orthogonal transform oscillator when an adjacent channel is detected in the lower side wave. Is increased by a fixed amount and the adjacent channel is detected in the upper wave, the oscillation frequency of the quadrature transform oscillator is reduced by a fixed amount, and when the adjacent channel is detected in any of the side waves, the frequency of the adjacent interference removal filter is reduced. And a control unit for lowering the cutoff frequency by a certain amount. Further, the adjacent interference removal filter may be formed by an adaptive filter. After the orthogonal transformation of the modulated signal, a capacitor is provided to cut a DC component and simultaneously detect a change in frequency. If the frequency change is equal to or less than a certain value, a certain amount is used as a control signal for the amplitude conversion adjacent interference removal device. To control the oscillation frequency of the orthogonal transform oscillator.

[0008]

According to the amplitude modulation adjacent interference canceling device by frequency conversion of the present invention, the adjacent interference canceling filter composed of the low-pass filter is provided after the orthogonal modulation of the amplitude-modulated signal, so that the cutoff frequency is reduced. It can be made variable to prevent adjacent channels from being demodulated and demodulated. The frequency of the amplitude-modulated signal is converted to detect an adjacent channel, and at the time of detection, the oscillation frequency of the quadrature conversion oscillator is increased or decreased by a certain amount depending on whether the adjacent channel is on the lower side wave or the upper side wave. , The adjacent channel is outside the pass band of the adjacent interference filter, so that side waves of the adjacent channel can be reliably removed. At the same time, by reducing the cutoff frequency of the adjacent interference rejection filter by a certain amount, the passband of the adjacent interference rejection filter is widely used when there is no adjacent channel, but only when the adjacent channel exists, Control is performed so that the band becomes narrower and adjacent channels are removed.

Further, since the adjacent interference rejection filter is formed by an adaptive filter, when the level of the carrier wave of the adjacent channel is large, the cut frequency of the adjacent interference rejection filter changes quickly, and when the level is small, The change of the cut frequency becomes slower. As a result, it is possible to alleviate the adverse effects caused by uniformly cutting the high frequency components of the broadcast wave. After the orthogonal transformation of the modulated signal, a capacitor is provided to cut a DC component and simultaneously detect a change in frequency. If the frequency change is equal to or less than a certain value, a certain amount is used as a control signal for the amplitude conversion adjacent interference removal device. Can also control the oscillation frequency of the quadrature transform oscillator by superimposing it on the frequency conversion, thereby expanding the application range of the amplitude modulation adjacent interference removal device by the present frequency conversion.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a first example of an apparatus for removing amplitude-modulated adjacent interference by frequency conversion according to an embodiment of the present invention, which is used for an amplitude-modulated demodulation circuit. First, an outline of the demodulation circuit shown in FIG. The demodulation circuit is based on a superheterodyne system, and includes an antenna 1 for receiving a broadcast wave and converting it into a received signal.
A high-frequency amplifier 2 connected to the high-frequency amplifier 2 for amplifying a received signal; an intermediate-frequency mixer 3 connected to the high-frequency amplifier 2 and mixing with a local oscillation signal to form an intermediate-frequency signal (for example, fo = 450 KHz); An intermediate wave local oscillator 4 for supplying a local signal to the intermediate frequency mixer 3, a band filter 5 connected to the intermediate frequency mixer 3 for passing only the intermediate frequency signal and removing unnecessary frequencies, 5, a quadrature transform mixer 6 and 7 for forming in-phase and quadrature baseband signals, a quadrature transform oscillator 8 for forming a local signal and outputting to the quadrature transform mixer 6, and a connection to the quadrature transform oscillator 8. A 90-degree phase shifter 9 that shifts the phase of the local signal by 90 degrees and outputs the resulting signal to the quadrature mixer 7 and low-pass filters connected to the quadrature transform mixers 6 and 7, respectively, to remove signals of adjacent channels. You An adjacent interference removing filter 10 and 11
And amplifiers 12 and 13 connected to the adjacent interference rejection filters 10 and 11, respectively, and the amplifiers 12 and 13
A / D converter (Analog to Digital Converter) 1 that is connected to each and converts analog signals to digital signals
4 and 15; squaring operators 16 and 17 for squaring each digital signal of the A / D converters 14 and 15; and an adder 18 for adding the result of squaring by the squaring operators 16 and 17. A square root operator 19 for performing a square root operation on the result of the addition performed by the adder 18, and a square root operator 1
D / A converter (Digital to Analog Co.) which converts the digital signal, which is the result of the square root operation at 9, into an analog signal
a low-pass filter 21 connected to the D / A converter 20 for removing unnecessary frequencies; and a power amplifier 22 connected to the low-pass filter 21 for power amplification.
And a speaker 23 driven by the power amplifier 22 to reproduce sound. In Japan, AM broadcasting has a frequency between channels of 9 KHz.
The pass band of the intermediate frequency filter 5 of the receiver is usually 450 ±
It is set to 7 KHz. According to the demodulation circuit, the modulated signal can be demodulated without distortion even if there is a synchronization shift such as a frequency shift or a phase shift between the received signal and the orthogonal transform oscillator 8.

Further, if an adjacent channel is received during reception of a signal in a sideband with respect to a desired carrier, interference occurs due to side waves associated with the carrier.
The demodulation circuit is connected to the output of the bandpass filter 5, detects an adjacent channel, and controls the cut-off frequency of the adjacent interference canceling filters 10 and 11 to be narrow at the time of the detection. A voltage-controlled resonance circuit that changes a resonance frequency in response to a control signal of the amplitude conversion adjacent interference removal device and causes the orthogonal conversion oscillator to oscillate at the resonance frequency.

FIG. 2 is a diagram showing the amplitude-converted adjacent interference canceling device 24 of FIG. The amplitude conversion adjacent interference removing device 24 shown in this drawing, the intermediate frequency signal from the bandpass filter 5 a extractor of neighbor channels (fo = 450KH
z), a conversion frequency oscillator 31 for generating a signal of a conversion frequency (f1 = 451 Khz) shifted by Δf (for example, 1 KHz), and a frequency conversion mixer for mixing output signals of the bandpass filter 5 and the conversion frequency oscillator 31 32. Further, the amplitude-converted adjacent interference removal device 24 includes a low-pass filter 33 connected to the frequency conversion mixer 32 for removing unnecessary frequencies. The adjacent-channel detector is connected to the low-pass filter 33. And band-pass filters 34 and 35, and rectifiers 36 and 37 connected to the band-pass filters 34 and 35, respectively, for converting alternating current to direct current. Are higher than a predetermined value, the comparators 38 and 39 output a fixed level, the inverter 40 inverts the output, and the outputs of the inverter 40 and the comparator 39 are added, and the voltage is calculated by the addition signal. An adder 41 for controlling the control type resonance circuit 25;
And an adder 42 for controlling the adjacent interference removal filters 10 and 11 based on the added signal.

FIG. 3 is a diagram for explaining the characteristics of the amplitude-converted adjacent interference removal device 24. This figure (a) shows that when the desired reception frequency is, for example, 1008 KHz, the carrier frequency position of the adjacent adjacent channel is 1017 KHz.
An example of occurrence at 999 KHz is shown. As shown by the dotted line in FIG. 3A, adjacent channels cause interference. In order to extract the carrier signal of this adjacent channel, FIG.
As shown in (2), when this carrier frequency is subjected to frequency conversion such as direct conversion using the same frequency as that of the bandpass filter 5, the converted frequency of the upper and lower side signals coincides with 9 KHz. The wave and the lower side wave cannot be distinguished, and the interference channel cannot be detected. Therefore, as shown in FIG. 3C, the frequency of the converted frequency oscillator 31 is changed to the intermediate frequency f of the bandpass filter 5.
Direct conversion is performed at, for example, 1009 KHz by shifting from o by Δf (for example, 1 KHz).
Thus, in the above example, when the carrier frequency position of the interfering adjacent channel is 1017 KHz, the position is 8 KHz after direct conversion, so that the center frequency fc1 of the band-pass filter 34 is 8 KHz.
When the carrier frequency position of the disturbing adjacent channel is 999 KHz, the position is 10 KHz after the direct conversion. Therefore, the center frequency fc2 of the band-pass filter 35 is set to 10 Hz. There is a difference of 2 KHz between the two, which can be distinguished by the band-pass filters 34 and 35.

[0014] upper sideband only adjacent channel is present, the AC signal is present is No. exchange Nagareshin passing a band-pass filter 34 which passes through the band pass filter 35 is absent. Therefore, the rectifier 36 outputs a DC signal of a certain level, but the rectifier 37 does not output a DC signal.
When the level of the rectifying unit 36 becomes equal to or more than a certain value, the comparator 38 outputs a signal of a certain level and outputs an inverted signal by the inverter 38. Since the level of the rectifying unit 37 does not exceed the certain value, the comparator 39 The output level from 39 is zero, and is output to the voltage-controlled resonance circuit 25 via the adder 41. Thereby, the orthogonal transform oscillator 8 changes its output frequency to Δ
f, for example, by 1 KHz. That is, specifically, the output is controlled to a frequency of 450 KHz to 449 KHz. Thus, the frequency of the adjacent channel in the upper wave at the output of the orthogonal transform mixers 6 and 7 is 9K
Hz to 10 KHz.

The adder 4 is operated based on the output signal from the comparator 38.
2, the cutoff frequencies of the adjacent interference removal filters 10 and 11 are controlled. FIG. 4 is a diagram for explaining the characteristics of the adjacent interference removal filters 10 and 11 of FIG. As shown by the dotted and solid lines in FIG.
When the signal of the adder 42 is present, the cut frequency of 1 decreases from, for example, 7 KHz to 6 KHz, and at the same time, the frequency of the carrier of the interfering wave changes from 9 KHz to 10 KHz, whereby the side wave changes from a dotted line to a solid line. Therefore, the disturbed portion is not demodulated. The above is the case where there is an adjacent channel in the upper side wave. However, when there is an adjacent channel in the lower side wave, the characteristics are reversed, and the orthogonal transform oscillator 8 increases its output frequency by 1 KHz. Is controlled to output a frequency from 450 KHz to 451 KHz. Thereby, the frequency of the adjacent channel in the lower side wave at the output of the orthogonal transform mixers 6 and 7 is 9 kHz.
To 10 KHz. Thus, the side wave of the adjacent channel can be reliably removed.

The adder 4 is operated based on the output signal from the comparator 39.
2, the cutoff frequencies of the adjacent interference removal filters 10 and 11 are controlled. As shown by the dotted line and the solid line in FIG. 4, the cut frequency of the adjacent interference removal filters 10 and 11 is, for example, 7 KHz to 6
KHz and at the same time the frequency of the interfering carrier is 9
From 10 KHz to 10 KHz.

FIG. 5 is a diagram showing a configuration of the voltage control type resonance circuit 25 of FIG. As shown in the figure, the voltage control type resonance circuit 25 changes the resonance frequency in response to a signal from the amplitude conversion adjacent interference removal device 24. FIG. 6 is a diagram illustrating the use of an adaptive filter which is a modification of the adjacent interference removal filters 10 and 11 of FIG. The adjacent interference removal filters 10 and 11 shown in FIG. 1 are analog filters, which change the cutoff frequency by switching the circuit configuration in accordance with a signal from the amplitude-converted adjacent interference removal device 24, as shown in FIG. Such a digital adaptive filter 50 may be provided instead of the amplitude modulation adjacent interference canceling device by frequency conversion in FIG. The digital-to-analog converter is omitted. The filter variable control 51 changes the cut-off frequency of the adaptive filter. The filter variable control 51 adds the output of the adder 43 obtained by adding the outputs of the rectifiers 36 and 37 in the amplitude-converted adjacent interference remover 24 shown in FIG. When the error signal is large, the cutoff frequency can be changed quickly, and when the error signal is small, the cutoff signal can be changed slowly. Thus, the high-frequency component of the broadcast wave is cut in accordance with the carrier wave level of the adjacent channel, so that the adverse effects caused by uniformly cutting the high-frequency component of the broadcast wave can be reduced.

FIG. 7 is a diagram showing a second example of an apparatus for removing amplitude-adjacent-conversion adjacent interference by frequency conversion used in an amplitude-modulated direct conversion demodulation circuit according to an embodiment of the present invention. This embodiment is described in Japanese Patent Application Laid-Open No. 61-2730.
The demodulation circuit shown in this figure is based on the direct conversion system, and the quadrature conversion oscillator 8 has a function of selecting a broadcasting station as compared with FIG. And a phase locked loop (PLL) 26 for inputting the N value (division ratio) of the above, and an added signal obtained by adding the signals of the phase locked loop 26 and the amplitude-converted adjacent interference removal device 24 to control the orthogonal transform oscillator 8. And an adder 27. For this reason, it is changed to the amplitude conversion adjacent interference removal device 24-1 as follows. FIG. 8 is a diagram showing the configuration of the amplitude-converted adjacent interference removal device 24-1 in FIG.

2 differs from the configuration of FIG. 2 in that the output from the phase locked loop 26 is Δf (for example, 1K
Hz), and a voltage-controlled resonance circuit 45 that changes the resonance frequency of the oscillator 31 ′ based on the addition signal of the adder 44. Thus, it is possible to remove the detection interference of the adjacent channel even by direct conversion detection.

FIG. 9 is a diagram showing a third example of an amplitude conversion adjacent interference canceling device by frequency conversion according to an embodiment of the present invention, which is used for a direct conversion demodulation circuit of amplitude modulation. This embodiment is based on the one described in Japanese Patent Application Laid-Open No. 4-150307. In this drawing, those additionally different from the configuration of FIG. The band-pass filters 60 and 61 provided between them, and the amplifier 1
The capacitors 62 and 63 provided between the A / D converters 14 and 15 and the A / D converters 14 and 15 respectively, and the data of the A / D converters 14 and 15 are branched to discriminate the frequency and detect the frequency deviation. Frequency correction means 64 for correcting the oscillation of the orthogonal transform oscillator 8 so that the components necessary for reproduction are AC.
There is. In the embodiment shown in this figure, when the DC offset voltage is included in the output signals of the orthogonal transform mixers 6 and 7 and the amplifiers 12 and 13 due to the temperature change, the offset voltage is demodulated with the synchronization shift. Capacitors 62 and 63 are provided to prevent the output from being distorted, but the capacitors 62 and 63 also remove DC components necessary for demodulation when the phase difference between the received signal and the local oscillation signal is constant. It solves the problem of doing. Hereinafter, the frequency correction unit 64 will be described. FIG. 10 is a diagram showing a configuration of the frequency correction means 64 of FIG.

In this figure, a frequency correcting means 64 receives frequency data which is input in-phase and quadrature data and obtains a phase amount changed in a unit time, that is, a frequency deviation, and a low frequency band which averages the deviation by the frequency discrimination means 270. A pass filter 280, an adding means 290 for adding the averaged data and a fixed offset value, a coefficient means 300 for converting the level after the addition, and converting the level-converted digital data into an analog signal and converting the result into the analog signal. And a D / A converter 160 for newly inputting to the adder 27. According to the frequency correction means 64, the signal output from the frequency discriminator 270 is higher than the frequency at which the local oscillation frequency of the orthogonal transform oscillator 8 is subtracted from the carrier frequency by the minute frequency corresponding to the offset value added by the addition means 290. Then, the value becomes a negative value so as to lower the frequency of the orthogonal transform oscillator 8, and if the frequency becomes lower than the frequency of the orthogonal transform oscillator 8, the value becomes a positive value so that the frequency of the orthogonal transform oscillator 8 becomes higher.
The voltage control type resonance circuit 25 calculates the voltage value from the face lock loop 26, the voltage value from the amplitude-converted adjacent interference removal device 24-1, the frequency and the frequency of the quadrature conversion oscillator 8 detected by the frequency discrimination means 270. Control is performed so that resonance occurs at the frequency obtained by adding the deviation and the offset value.

[0022]

As described above, according to the present invention, an adjacent interference removing filter comprising a low-pass filter is provided after the orthogonal transform of an amplitude-modulated signal, and the cutoff frequency is reduced by detecting an adjacent channel. At the same time, the frequency of the adjacent channel moves outside the cut frequency, so that the adjacent channel can be reliably removed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a first example of an amplitude-modulated adjacent interfering wave removing apparatus using frequency conversion according to an embodiment of the present invention, which is used in an amplitude-modulated demodulation circuit.

FIG. 2 is a diagram showing amplitude conversion adjacent interference removal 24 by frequency conversion in FIG. 1;

FIG. 3 is a diagram illustrating characteristics of an amplitude-converted adjacent interference removal device 24 by frequency conversion.

FIG. 4 is a diagram illustrating characteristics of adjacent interference removal filters 10 and 11 of FIG. 1;

FIG. 5 is a diagram showing a configuration of a voltage control type resonance circuit 25 of FIG. 1;

FIG. 6 illustrates the use of an adaptive filter which is a modification of the adjacent interference removal filters 10 and 11 of FIG.

FIG. 7 is a diagram showing a second example used in the amplitude-modulated direct conversion demodulation circuit, which is an amplitude-modulated adjacent interference wave removing apparatus by frequency conversion according to the embodiment of the present invention.

8 is a diagram showing a configuration of an amplitude-modulated adjacent interference removal apparatus 24-1 by frequency conversion in FIG. 7;

FIG. 9 is a diagram illustrating a third example of the amplitude-modulated adjacent interfering wave removing apparatus by frequency conversion used in the amplitude-converted direct conversion demodulation circuit according to the embodiment of the present invention.

FIG. 10 is a diagram illustrating a configuration of a frequency correction unit 64 of FIG. 9;

[Explanation of symbols]

 5: Bandpass filter 6, 7: Quadrature transformation mixer 8: Quadrature transformation oscillator 10, 11: Adjacent interference removal filter 24, 24-1 ... Amplitude conversion adjacent interference removal device 25, 45 ... Voltage controlled resonance circuit 31, 31 '... Conversion frequency oscillator 32 ... Frequency conversion mixer 34,35 ... Band pass filter 36,37 ... Rectifier 38,39 ... Comparator 41,42,43,44 ... Adder 50 ... Adaptive filter 51 ... Filter variable control

Claims (4)

(57) [Claims] An amplitude-modulated signal is converted into a quadrature transform oscillator (8).
In the amplitude modulation adjacent interference removal apparatus by frequency conversion of a demodulation circuit that performs orthogonal transformation and demodulation, a low-pass filter that is provided after the orthogonal transformation of the amplitude-receiving signal and that changes a cutoff frequency to remove an adjacent channel Adjacent interference rejection filter (10, 1
1) and the amplitude-modulated signal is shifted from the frequency of the desired wave by a predetermined amount.
Direct conversion by frequency detects an adjacent channel and increases the oscillation frequency of the orthogonal transform oscillator (8) by a certain amount according to whether the adjacent channel is on the lower side wave or the upper side wave at the time of detection. Or at the same time, the adjacent interference rejection filter (10,
11) An amplitude-modulated adjacent interference removing device by frequency conversion, comprising: an amplitude-converted adjacent interference removing device (24, 24-1) for lowering a cutoff frequency by a fixed amount. 2. The apparatus according to claim 1, wherein
4, 24-1): adjacent channel extraction for converting the signal to be amplitude at a frequency shifted from the frequency of the quadrature transform oscillator (8) by a fixed amount and extracting the carrier frequency of the adjacent channel of the upper side wave or the lower side wave. And an adjacent channel detection unit (34, 35, 3) for detecting a carrier of an adjacent channel of the extracted upper wave or lower wave.
6, 37), when the adjacent channel is detected in the lower side wave, the oscillation frequency of the orthogonal transform oscillator (8) is increased by a fixed amount, and when the adjacent channel is detected in the upper wave, the orthogonal transform oscillator is used. A control unit configured to reduce the oscillation frequency of (8) by a fixed amount and to reduce the cutoff frequency of the adjacent interference removal filter (10, 11) by a certain amount when an adjacent channel is detected in any of the side waves; 39, 40, 41, 4
2. The apparatus according to claim 1, further comprising the following steps: 3. The adjacent interference rejection filter (10, 1).
2. An apparatus according to claim 1, wherein 1) is formed of an adaptive filter. Wherein detecting the change in phase after cutting a DC component by providing a capacitor after orthogonal transformation of the modulated signal, the target amplitude conversion adjacent interference removing device control amount corresponding to the phase change ( 2. The amplitude modulation adjacent interference removal apparatus by frequency conversion according to claim 1, wherein the oscillation frequency of the orthogonal transform oscillator (8) is controlled by being superimposed on the control signal of (24, 24-1).