JPH1117497A - Band pass filter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a filter circuit with an excellent standardizing frequency characteristic through the use of a low pass filter with few degrees by combinating an orthogonal wave detector with an orthogonal modulator. SOLUTION: The orthogonal wave detector 24 for shifting the frequency of an input signal Vin to the neighborhood of a DC frequency band, the low pass filter 26 and the orthogonal modulator 28 for modulating the signal output of the low pass filter 26 into a prescribed frequency band are provided so that the signal is outputted from the orthogonal modulator 28 to an output terminal 30 only in the specified frequency band. That is, the orthogonal wave detector 24 mixes the input signal Vin with the sine wave of a center frequency to execute band passing and shifts it to the DC frequency band. A high frequency signal is interrupted by the low pass filter 26 and only a low frequency signal is made to pass concerning the orthogonally wave-detected signal. Then, the signal which is made to execute low passing is modulated into the center frequency to execute band passing by mixing the output signal of the low pass filter 26 with the sine wave of the center frequency to execute band passing so as to orthogonally modulate it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit which allows only a specific frequency component to pass and does not attenuate or pass other frequency components. In particular, the present invention relates to a quadrature modulation band-pass filter circuit combining a low-pass device constituted by a digital or analog filter and an arithmetic unit.

[0002]

2. Description of the Related Art Conventionally, in a band-pass filter circuit, only a specific frequency component of an input signal is passed by connecting a low-pass device and a high-pass device in series or converting the low-pass device.

FIG. 8 shows a conventional bandpass filter circuit 1.
0 is a block diagram of FIG. In the figure, a band-pass filter circuit 10 includes an eighth-order low-pass filter 14 composed of standardized capacitors C1 to C8, a resistor R, and an amplifier 12 having a gain of 1.
And an eighth-order high-pass filter 18 composed of normalized resistors R1 to R8, a capacitor C, and an amplifier 16 with a gain of 1. Was passed as.

[0004]

However, in order to obtain the desired standardized frequency Ω, the bandpass filter circuit 10 as described above depends on the pass frequency band and the cutoff angular frequency, but the order of the filter is small. However, there is a disadvantage that the circuit size increases. Also, as the frequency band through which the input signal passes becomes narrower, the accuracy of the cutoff frequency of the low-pass device and the high-pass device is required, and there is a problem that the order of the filter increases accordingly.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to use a low-pass filter having a low order to provide a band having excellent characteristics of a standardized frequency. It is to provide a pass filter circuit.
Another object of the present invention is to provide a band-pass filter circuit having a higher normalized frequency characteristic as the pass frequency band is narrower.

[0006]

In order to solve the above-mentioned problems, a band-pass filter circuit according to the present invention comprises a quadrature detector for shifting the frequency of an input signal to a vicinity of a DC frequency band, and a signal of the quadrature detector. It comprises a low-pass device that passes the low-frequency component of the output, and a quadrature modulator that modulates the signal output of the low-pass device into a predetermined frequency band.

A first multiplier for multiplying the input signal by the first sine wave; and a second multiplier for multiplying the input signal by a second sine wave having a predetermined angular phase shift from the first sine wave. A multiplier, a first low-pass device that passes a low-frequency component of a signal output of the first multiplier, and a second low-pass device that passes a low-frequency component of a signal output of the second multiplier A third multiplier for multiplying the signal output of the first low-pass device with the first sine wave, and a third multiplier for multiplying the signal output of the second low-pass device with the second sine wave. 4 and an adder for adding the signal outputs of the third and fourth multipliers.

Further, the second sine wave is delayed by 90 degrees in phase from the first sine wave.

[0009]

In the present invention having the above configuration, by combining a quadrature detector and a quadrature modulator, an excellent bandpass filter can be obtained even with a low-pass filter having a small order.

[0010]

Preferred embodiments of the present invention will be described below with reference to the drawings. There is no particular limitation,
This device is configured for a band pass filter using quadrature modulation.

FIG. 1 is a block diagram of a band-pass filter circuit 20 according to an embodiment of the present invention. In the figure,
The band-pass filter circuit 20 includes a quadrature detector 2 that shifts the frequency of the input signal Vin to near the DC frequency band.
4, a low-pass filter 26 for passing a low-frequency component of the signal output of the quadrature detector 24, and a quadrature modulator 28 for modulating the signal output of the low-pass filter 26 into a predetermined frequency band. The signal output Vout can be output from the modulator 28 to the output terminal 30 only in a specific frequency band. The orthogonal detector 24 mixes the input signal Vin with a sine wave having a center frequency to be band-passed and shifts the mixed signal to a DC frequency band.
This quadrature-detected signal can be cut off the high-frequency signal by the low-pass filter 26 and only the low-frequency signal can be passed.
The output signal of the low-pass filter 26 and the sine wave of the center frequency to be band-passed are mixed and quadrature-modulated by the quadrature modulator 28, whereby the signal that has been low-passed to the center frequency to be band-passed can be modulated. .

FIG. 2 is a block diagram of a band-pass filter circuit 36 according to another embodiment of the present invention. In the figure, a band-pass filter circuit 36 receives an input signal COS2π (f ± f1) from an input terminal 38 and
A first multiplier 40 for multiplying OS2π (f ± f1) by the COS wave 42 and S having a 90 ° phase shift from the COS wave 42
A second multiplier 44 for multiplying the IN wave 46 by the input signal COS2π (f ± f1), and a first low-pass filter (LPF) 4 for passing a low-frequency component of the signal output of the first multiplier 40
6, a second low-pass filter (LPF) 48 that passes low-frequency components of the signal output of the second multiplier 44, and a signal output of the first low-pass filter 46 are multiplied by the COS wave 52. The signal output of the third multiplier 50 and the signal output of the second low-pass
A fourth multiplier 54 for multiplying the N wave 56 and an adder 58 for adding the signal outputs of the third and fourth multipliers 50 and 54
And an input signal COS having a center frequency f and a bandwidth f1.
2π (f ± f1) and distribute the divided signal to the frequency f
Is multiplied by the COS wave 42 and shifted to the DC frequency band. Similarly, the input signal COS2π (f ± f1) is
Can be shifted to the DC frequency band by multiplying by the SIN wave 46 of the LPFs 46, 48
The high frequency component is removed by the
And the fourth multiplier 50, 54 outputs a COS wave 5 having a frequency f.
It can be quadrature modulated by multiplying by 2 or SIN wave 56. The quadrature-modulated signal is added by the adder 58 to output only the f-band signal of the target frequency to the output terminal 6.
It can be output from 0.

FIG. 3 is a characteristic diagram of the spectrum of the input signal COS2π (f ± f1) received by the band-pass filter circuit 36. The input signal has an amplitude characteristic of the plus side frequency 62 on the right side with respect to the DC frequency of 0 Hz and an amplitude characteristic of the minus side frequency 64 on the left side, and can be expressed by the following equation (1).

[0014]

(Equation 1) FIG. 4 is a characteristic diagram of the output signal spectrum of the first multiplier 40 of the bandpass filter circuit 36. The output signal of the first multiplier 40 is ± f1 near a DC frequency of zero Hz.
Amplitudes 68 and 7 of the minus and plus frequencies of
0, which has an amplitude characteristic of a plus side frequency 66 and a minus side frequency 72 centered on a frequency twice as high as the center frequency f to be band-passed, and can be expressed by the following equation 2.

[0015]

(Equation 2) FIG. 5 is a characteristic diagram of the output signal spectrum of the second multiplier 44 of the bandpass filter circuit 36. The output signal of the second multiplier 44 is ± f1 near a DC frequency of zero Hz.
Of the positive side frequency 74 and the negative side frequency 76, and the positive side frequency 78 and the negative side frequency 80 centered on a frequency twice the center frequency f to be band-passed. be able to.

[0016]

(Equation 3) When the output signals of the first and second multipliers 40 and 44 are passed through first and second low-pass filters 46 and 48 of the seventh order, the first sine wave of the following equation 4 is obtained. Output 8 of COS wave 42 side
The output 84 of the second and second sine waves on the SIN wave 46 side can be obtained.

[0017]

(Equation 4) The signal passed through the low-pass filter 46 is converted to a third multiplier 50.
, The signal having passed through the low-pass filter 48 is multiplied by the SIN wave 56 in the fourth multiplier 54, and each signal can be quadrature-modulated. be able to.

[0018]

(Equation 5) The output 86 on the COS wave side is the output of the third multiplier 50, and the output 88 on the SIN wave side is the output of the fourth multiplier 54. The result of adding these outputs 86 and 88 by the adder 58 90 can be determined by the following number.

[0019]

(Equation 6) FIG. 6 is a characteristic diagram of a spectrum that models a frequency to be band-passed when designing the band-pass filter circuit 36. The center frequency f of the band-passed signal is 3
4KHz, high frequency cutoff frequency f1H is 35.8KHz,
The frequency f2H that attenuates the low-frequency cutoff frequency f1L by 32.2 KHz and the high-frequency and low-frequency cutoff frequencies by 60 dB.
And f2L are set to 36 KHz and 32 KHz, respectively, the band-pass filter circuit 36 satisfying this condition.
Is 36KHz / 25.8KHz,
1.0558865. This normalized frequency has characteristics higher than those of the 8th-order simultaneous Chebyshev filter. In the bandpass filter using the eighth-order filter,
Although a total of 16th-order filters are required, according to the present embodiment, the same or better characteristics are obtained with a total of 14th-order filters.

FIG. 7 is a frequency characteristic diagram of a seventh-order low-pass filter by quadrature modulation according to an embodiment of the present invention. The normalized frequency Ω of this low-pass device can be 1.1111. Also, when the high frequency cutoff frequency is 300 Hz centered on a DC frequency of 0 Hz, the width from the high frequency to the low frequency cutoff frequency is 600 Hz and the pass band is 300 Hz, so that 600/300 = 2.0 is standardized. A frequency Ω can be achieved, and a sufficient frequency characteristic of an eight-order simultaneous Chebyshev type filter can be obtained. Therefore, the smaller the bandwidth for passing the signal, the larger the normalized frequency Ω, and the order of the filter can be reduced.

In the low-pass device and the multiplier, the order of the filter can be reduced in an analog circuit or a digital circuit in the same manner as described above.

[0022]

Since the present invention is configured as described above, it has the following effects.

Since the input signal is modulated on the DC side and then filtered by the low-pass device, the order of the low-pass device can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a bandpass filter circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram of a bandpass filter circuit according to another embodiment of the present invention.

FIG. 3 is a spectrum diagram of an input signal of a bandpass filter circuit according to another embodiment of the present invention.

FIG. 4 is a spectrum diagram of a multiplied output signal of a bandpass filter circuit according to another embodiment of the present invention.

FIG. 5 is a spectrum diagram of a multiplied output signal of a bandpass filter circuit according to another embodiment of the present invention.

FIG. 6 is a model diagram of an output signal of a bandpass filter circuit according to another embodiment of the present invention.

FIG. 7 is a spectrum diagram of an output signal of a low-pass filter according to another embodiment of the present invention.

FIG. 8 is a block diagram of a conventional bandpass filter circuit.

[Explanation of symbols]

20, 36 band-pass filter circuits, 24 quadrature detectors, 26 low-pass filters, 28 quadrature modulators, 38 input terminals, 40, 44, 50, 54 multipliers, 46, 48
Low-pass filter, 58 adder, 60 output terminals.

Claims (3)

[Claims] 1. A band-pass filter circuit for passing a predetermined frequency component of an input signal, wherein the quadrature detector shifts the frequency of the input signal to a vicinity of a DC frequency band, and a low-frequency signal output from the quadrature detector. A band-pass filter circuit comprising: a low-pass device that passes a component; and a quadrature modulator that modulates a signal output of the low-pass device into a predetermined frequency band. 2. A band-pass filter circuit for passing a predetermined frequency component of an input signal, comprising: a first multiplier for multiplying the input signal by a first sine wave; A second multiplier that multiplies the input signal by a second sine wave deviated from the first signal, a first low-pass device that passes a low-frequency component of a signal output from the first multiplier, and the second A second low-pass device that passes a low-frequency component of the signal output of the multiplier, a third multiplier that multiplies the signal output of the first low-pass device with the first sine wave, A fourth multiplier that multiplies the signal output of the second low-pass filter with the second sine wave; and an adder that adds the signal outputs of the third and fourth multipliers. A band-pass filter circuit characterized by the above-mentioned. 3. The phase of the second sine wave is delayed by 90 degrees from the phase of the first sine wave.
3. The band-pass filter circuit according to 1.