WO2008038107A2

WO2008038107A2 - Band-pass filter and receiver including the same

Info

Publication number: WO2008038107A2
Application number: PCT/IB2007/002796
Authority: WO
Inventors: Hiroki Okada
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-09-27
Filing date: 2007-09-26
Publication date: 2008-04-03
Also published as: JP2008085653A; WO2008038107A3

Abstract

A band-pass filter includes: a distributor (24) for distributing an input signal into two signals; a first low-pass filter (28) that passes a component of one of the two signals into which the input signal is distributed by the distributor (24), the component having a frequency lower than a cutoff frequency fl; a phase shifter (30) that converts a signal output from the first low-pass filter (28) so that the phase thereof is shifted by 180°; a mixer (32) that combines the other of the two signals into which the input signal is distributed by the distributor (24) with a signal output from the phase shifter (30); and a second low-pass filter (34) that passes a component of a signal output from the mixer (32), the component having a frequency lower than a second cutoff frequency f2 that is higher than the cutoff frequency fl of the first low-pass filter (28).

Description

BAND-PASS FILTER AND RECEIVER INCLUDING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a band-pass filter and a receiver including the same, and more specifically to a band-pass filter suitable to make the cutoff frequencies and the passband width continuously variable.

2. Description of the Related Art

[0002] A band-pass filter of which the cutoff frequencies are variable is provided with a plurality of electric current sources (see Japanese Patent Application Publication No. 1-169704, for example). In this filter, the cutoff frequencies are adjusted by varying the amount of electric current flowing through the electric current sources. [0003] In the above band-pass filter, it is necessary to provide electric current sources, in each of which the amount of electric current is variable, to adjust the cutoff frequencies, which results in a disadvantage that the filter itself becomes more complicated in configuration and larger.

SUMMARY OF THE INVENTION

[0004] An object of the invention is to provide a simple band-pass filter of which the cutoff frequencies and the passband width are continuously variable.

[0005] A first aspect of the invention is embodied by a band-pass filter that includes: a distribution means for distributing an input signal into two signals; a first low-pass filter that passes a component of one of the two signals into which the input signal is distributed by the distribution means, the component having a frequency lower than a first cutoff frequency; a mixer that combines the other of the two signals with a signal obtained by reversing a phase of a signal output from the first low-pass filter; and a second low-pass filter that passes a component of a signal output from the mixer, the component having a frequency lower than a second cutoff frequency that is higher than the first cutoff frequency.

[0006] In this aspect, when the low-pass filtered signal output from the first low-pass filter, and the other of the two signals into which a signal is distributed by the distribution means are combined in opposite phase in the mixer, these signals cancel each other, so that the signal level after the combination is very low, in the passband of the first low-pass filter. On the other hand, the signal level is maintained in the other frequency band. The second low-pass filter has a cutoff frequency higher than the cutoff frequency of the first low-pass filter, and therefore, the signal that results from the combination in the mixer and has passed the second low-pass filter has only the signal components of which the frequencies are in the range from the cutoff frequency of the first low-pass filter to the cutoff frequency of the second low-pass filter. Thus, when the cutoff frequency of the first low-pass filter and the cutoff frequency of the second low-pass filter are independently variable, it is possible to realize continuous variation of the cutoff frequencies and the passband width of the band-pass filter, using a simple configuration.

[0007] A second aspect of the invention is embodied by a band-pass filter that includes: a first low-pass filter that passes a component of an input signal, the component having a frequency lower than a first cutoff frequency; a first A/D converter that quantizes a signal output from the first low-pass filter into a digital signal; a second low-pass filter that passes a component of an input signal, the component having a frequency lower than a second cutoff frequency higher than the first cutoff frequency; a second A/D converter that quantizes a signal output from the second low-pass filter into a digital signal; and a subtraction means for subtracting an output of the first A/D converter from an output from the second A/D converter.

[0008] In this aspect, when the digital signal resulting from quantization, performed by the first A/D converter, of the low-pass filtered signal output from the first low-pass filter is subtracted from the digital signal resulting from quantization, performed by the second A/D converter, of the low-pass filtered signal output from the second low-pass filter having a cutoff frequency higher than the cutoff frequency of the first low-pass filter, the signal components having frequencies out of the range from the cutoff frequency of the first low-pass filter to the cutoff frequency of the second low-pass filter are cancelled, and the signal after the subtraction by the subtraction means includes only the signal components having frequencies within the above frequency range. Thus, when the cutoff frequency of the first low-pass filter and the cutoff frequency of the second low-pass filter are independently variable, it is possible to realize continuous variation of the cutoff frequencies and the passband width of the band-pass filter, using a simple configuration.

[0009] In the above band-pass filters, the first cutoff frequency of the first low-pass filter and the second cutoff frequency of the second low-pass filter may be independently variable.

[0010] A third aspect of the invention is embodied by a band-pass filter including two low-pass filters, in which cutoff frequencies of the two low-pass filters are used as an upper limit and a lower limit of a passband, wherein the cutoff frequencies of the two low-pass filters are independently variable.

[0011] In this aspect, cutoff frequencies of the two low-pass filters that are used as the upper limit and the lower limit of the passband of the band-pass filter are independently variable. Thus, it is possible to realize continuous variation of the cutoff frequencies and the passband width of the band-pass filter, using a simple configuration.

[0012] A fourth aspect of the invention is a receiver that includes the band-pass filter according to any one of the first to third aspects and that is mounted in a vehicle.

[0013] According to the invention, a receiver mounted in a vehicle, including a simple band-pass filter of which the cutoff frequencies and the passband width are continuously variable, is provided.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG. 1 is a configuration diagram of a band-pass filter according to a first embodiment of the invention;

FIG. 2 is a configuration diagram of a receiver including the band-pass filter of the embodiment;

FIGS. 3A and 3B show a specific configuration and characteristics, respectively, of first and second low-pass filters that the band-pass filter of the embodiment includes; FIG. 4 shows frequency-gain characteristics at several points in the band-pass filter of the embodiment;

FIG. 5 is a configuration diagram of a band-pass filter of a second embodiment of the invention; and

FIGS. 6 A and 6B are diagrams for explaining operation of the band-pass filter of the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0015] Specific embodiments of the invention will be described below with reference to drawings. [0016] FIG. 1 shows a configuration diagram of a band-pass filter 10 that is a first embodiment of the invention. FIG. 2 is a configuration diagram of a receiver 12 including the band-pass filter 10 of this embodiment. The band-pass filter (hereinafter referred to as the BPF) 10 of this embodiment is incorporated into the receiver 12 that is mounted in a vehicle, for example. [0017] The receiver 12 includes: a receiving antenna 14; an amplifier 16 connected to the receiving antenna 14; a local oscillator 18; a mixer 20 connected to the amplifier 16 and the local oscillator 18; the BPF 10 connected to the mixer 20; and an A/D converter 22 connected to the BPF 10. The amplifier 16 amplifies the signal received by the receiving antenna 14 and supplies it to the mixer 20. The local oscillator 18 generates a local oscillation signal that has a frequency higher than a desired reception frequency by a predetermined frequency (10.7 MHz, for example), and supplies the local oscillation signal to the mixer 20. The mixer 20 performs frequency conversion of the signal amplified by the amplifier 16 to a signal of intermediate frequency with the use of the local oscillation signal generated by the local oscillator 18. The A/D converter 22 converts a certain frequency signal that has passed the BPF 10 into a digital signal in accordance with a predetermined sampling clock.

[0018] The BPF 10 includes a distributor 24 connected to the output of the mixer 20. The distributor 24 distributes the input signal, received from the mixer 20, to two lines 26a and 26b. A first low-pass filter (hereinafter referred to as the first LPF) 28 and a phase shifter 30 are provided in series on the first line 26a. The first LPF 28 has a predetermined cutoff frequency fl as described later and passes the components of the input signal caused to flow from the distributor 24 through the line 26a that have frequencies lower than the cutoff frequency fl . The phase shifter 30 converts the signal that has passed the first LPF 28 so that the phase thereof is shifted by 180°.

[0019] A mixer 32 is connected to the phase shifter 30 on the line 26a-side. The mixer 32 is also directly connected to the distributor 24 on the line 26b-side. The mixer 32 receives the signal from the phase shifter 30 and the signal that the distributor 24 receives from the mixer 20. The mixer 32 combines the received signals. A second low-pass filter (hereinafter referred to as the second LPF) 34 is connected to the mixer 32. The second LPF 34 has a predetermined cutoff frequency f2 as described later and passes the components of the input signal received from the mixer 32 that have frequencies lower than the cutoff frequency f2. It should be noted that the cutoff frequency f2 is always set so as to be higher than the cutoff frequency fl of the first LPF 28. [0020] The A/D converter 22 is connected to the second LPF 34. The A/D converter 22 performs an analogue-to-digital conversion of the signal that has passed the second LPF 34, that is, the signal that has passed the BPF 10, in accordance with the predetermined sampling clock, for quantization. The signal obtained through the quantization by the A/D converter 22 is used in the following reception process. [0021] FIG. 3 A shows a specific configuration of the first and second LPFs 28 and

34 that the BPF 10 of this embodiment includes, and FIG. 3B shows the characteristics thereof. In this embodiment, each of the first and second LPFs 28 and 34 includes a resistor 36 connected between the input and the output in series, and a capacitor 38 connected between the output and ground.

[0022] The resistor 36 is a variable resistance resistor, such as a trimmer potentiometer and a pin diode. The resistance of the resistor 36 of the first LPF 28 and that of the resistor 36 of the second LPF 34 are variable independently of each other. The capacitor 38 is a variable capacitance capacitor, such as a trimmer capacitor and a varactor diode. The capacitance of the capacitor 38 of the first LPF 28 and that of the capacitor 38 of the second LPF 34 are variable independently of each other. When parameters of the resistors 36 and the capacitors 38 of the LPFs 28 and 34 are varied, the cutoff frequencies of the LPFs 28 and 34 are varied as shown in FIG. 3B, so that the characteristics of the LPFs are variable. [0023] The parameters of the resistors 36 and the capacitors 38 of the first and second LPFs 28 and 34 are set so that each of the cutoff frequencies fl and f2 is within a preset, predetermined range and that the cutoff frequency £2 of the second LPF 34 is always higher than the cutoff frequency fl of the first LPF 28 as described above.

[0024] Next, referring to FIG. 4, operation of the BPF 10 of this embodiment will be described. FIG. 4 shows frequency- gain characteristics at several points of the BPF 10 of this embodiment. In this embodiment, the signal received by the receiving antenna 14 of the receiver 12 is subjected to a frequency conversion in the mixer 20, and then supplied to the distributor 24 of the BPF 10.

[0025] In the BPF 10 of this embodiment, the signal supplied to the distributor 24 is distributed to the first line 26a and the second line 26b. The signal caused to flow from the distributor 24 through the first line 26a is blocked by the first LPF 28 when the signal has a frequency higher than the cutoff frequency fl of the first LPF 28. On the other hand, when the signal has a frequency lower than the cutoff frequency fl of the first LPF 28, the signal passes the first LPF 28 and is supplied to the phase shifter 30. The signal supplied to the phase shifter 30 is subjected to a phase shift so that the phase thereof is shifted by 180°.

[0026] The signal subjected to a phase shift in the phase shifter 30, and the signal caused to flow from the distributor 24 through the second line 26b are supplied to the mixer 32 and combined by the mixer 32. The signals that are combined by the mixer 32 are combined in opposite phase to each other. When such combination is performed, the level of the components of the signal after the combination by the mixer 32 that have frequencies lower than the cutoff frequency fl . of the first LPF 28 becomes very low, and, on the other hand, the level of the components of the signal after the combination by the mixer 32 that have frequencies higher than the cutoff frequency fl of the first LPF 28 is kept high.

[0027] The output signal from the mixer 32 is supplied to the second LPF 34. The signal supplied to the second LPF 34 is blocked by the second LPF 34 when the frequency of the signal is higher than the cutoff frequency f2 of the second LPF 34. On the other hand, the signal passes the second LPF 34 when the frequency of the signal is lower than the cutoff frequency f2 of the second LPF 34.

[0028] The cutoff frequency f2 of the second LPF 34 is always set so as to be higher than the cutoff frequency fl of the first LPF 28. Thus, in the frequency region lower than the cutoff frequency fl of the first LPF 28 and the frequency region higher than the cutoff frequency f2 of the second LPF 34, the level of the signal supplied to the BPF 10 becomes very low, and the signal is thus blocked by the BPF 10. On the other hand, in the frequency region higher than the cutoff frequency fl of the first LPF 28 and lower than the cutoff frequency f2 of the second LPF 34, the level of the signal is maintained, and the signal thus passes the BPF 10. [0029] According to the BPF 10 of this embodiment, by using the distributor 24, the first LPF 28, the phase shifter 30, the mixer 32, and the second LPF 34, it is made possible to pass and output the components of a signal having frequencies within a range from the cutoff frequency fl of the first LPF 28 to the cutoff frequency f2 of the second LPF 34. The signal output from the BPF 10 is converted into a digital signal for quantization by the A/D converter 22, and subjected to reception processing.

[0030] In this embodiment, the cutoff frequencies fl and f2 of the first and second LPFs 28 and 34 are set so that each of the cutoff frequencies fl and f2 is individually within a predetermined range, and the cutoff frequency f2 of the second LPF 34 is always higher than the cutoff frequency fl of the first LPF 28. The resistance and capacitance as parameters of the resistor 36 and the capacitor 38 are variable independently, so that the cutoff frequencies fl and f2 are variable independently of each other.

[0031] Thus, in this embodiment, it is made possible to realize the BPF 10 of which the cutoff frequencies fl and f2, and the passband width can be continuously varied, by making the cutoff frequencies fl and £2 of the first and second LPFs 28 and.34 variable independently of each other. Accordingly, it is possible to continuously vary the cutoff frequencies and the passband width of the BPF 10, using a simple configuration.

[0032] In the above first embodiment, the distributor 24 may be regarded as the

"distribution means" of the invention; the first LPF 28 may be regarded as the "first low-pass filter", or one of the "two low-pass filters" of the invention; the mixer 32 may be regarded as the "mixer" of the invention; and the second LPF 34 may be regarded as the "second low-pass filter", or one of the "two low-pass filters" of the invention.

[0033] FIG. 5 shows a configuration diagram of a band-pass filter (hereinafter referred to as the "BPF") 100, which is a second embodiment of the invention. It should be noted that, in the following description of this embodiment, the same components as those of the configuration shown in FIGS. 1, 2, and 3 A and 3B of the first embodiment described above are designated by the same reference numerals, and description thereof is omitted or simplified. The BPF 100 of this embodiment is incorporated into a receiver 102 that is mounted in a vehicle, for example. [0034] The receiver 102 includes a receiving antenna 14, an amplifier 16, a local oscillator 18, and a mixer 20. The BPF 100 includes a distributor 104 connected to the output of the mixer 20. The distributor 104 distributes the input signal, received from the mixer 20, to two lines 106a and 106b.

[0035] A first low-pass filter (hereinafter referred to as the first LPF) 108 is provided on the first line 106a. The first LPF 108 has a predetermined cutoff frequency fl as described later and passes the components of the input signal caused to flow from the distributor 104 through the line 106a that have frequencies lower than the cutoff frequency fl . A second low-pass filter (hereinafter referred to as the second LPF) 110 is provided on the second line 106b. The second LPF 110 has a predetermined cutoff frequency fl as described later and passes the components of the input signal caused to flow from the distributor 104 through the line 106b that have frequencies lower than the cutoff frequency f2. It should be noted that the cutoff frequency £2 is always set so as to be higher than the cutoff frequency fl of the first LPF 108 described above. [0036] A first A/D converter 112 is connected to the first LPF 108. The first A/D converter 112 performs an analogue-to-digital conversion of the frequency components of a signal that have passed the first LPF 108, in accordance with a predetermined sampling clock, for quantization. A second A/D converter 114 is connected to the second LPF 110. The second A/D converter 114 performs an analogue-to-digital conversion of the frequency components of a signal that have passed the second LPF 110, in accordance with the predetermined sampling clock, for quantization.

[0037] A subtracter 116 is connected to the first and second A/D converters 112 and 114. The subtracter 116 performs processing of subtracting the digital signal that is the output from the first A/D converter 112 from the digital signal that is the output from the second A/D converter 114. The signal obtained through the subtraction processing by the subtracter 116 is used in the following reception process.

[0038] In this embodiment, each of the first and second LPFs 108 and 110 includes a resistor 36 connected between the input and the output in series, and a capacitor 38 connected between the output and ground. The resistor 36 is a variable resistance resistor, such as a trimmer potentiometer and a pin diode. The resistance of the resistor 36 of the first LPF 108 and that of the resistor 36 of the second LPF 110 are variable independently of each other. The capacitor 38 is a variable capacitance capacitor, such as a trimmer capacitor and a varactor diode. The capacitance of the capacitor 38 of the first LPF 108 and that of the capacitor 38 of the second LPF 110 are variable independently of each other. When parameters of the resistors 36 and the capacitors 38 of the LPFs 108 and 110 are varied, the cutoff frequencies of the LPFs 108 and 110 are varied, so that the characteristics of the LPFs are variable.

[0039] The parameters of the resistors 36 and the capacitors 38 of the first and second LPFs 108 and 110 are set so that each of the cutoff frequencies fl and f2 is within a preset, predetermined range and that the cutoff frequency f2 of the second LPF 110 is always higher than the cutoff frequency fl of the first LPF 108 as described above.

[0040] Next, referring to FIGS. 6A and 6B, operation of the BPF 100 of this embodiment will be described. FIGS. 6 A and 6B are diagrams for explaining the operation of the BPF 100 of this embodiment. In this embodiment, the signal received by the receiving antenna 14 of the receiver 12 is subjected to a frequency conversion in the mixer 20, and then supplied to the distributor 104 of the BPF 100.

[0041] In the BPF 100 of this embodiment, the signal supplied to the distributor 104 is distributed to the first line 106a and the second line 106b. The signal caused to flow from the distributor 104 through the first line 106a is blocked by the first LPF 108 when the signal has a frequency higher than the cutoff frequency fl of the first LPF 108. On the other hand, when the signal has a frequency lower than the cutoff frequency fl of the first LPF 108, the signal passes the first LPF 108 and is supplied to the first A/D converter 112. The signal supplied to the first A/D converter 112 is converted into a digital signal by the first A/D converter 112, for quantization.

[0042] The signal caused to flow from the distributor 104 through the second line 106b is blocked by the second LPF 110 when the signal has a frequency higher than the cutoff frequency f2 of the second LPF 110. On the other hand, when the signal has a frequency lower than the cutoff frequency f2 of the second LPF 110, the signal passes the second LPF 110 and is supplied to the second A/D converter 114. The signal supplied to the second LPF 110 is converted into a digital signal by the second A/D converter 114, for quantization.

[0043] The digital signal resulting from quantization in the first A/D converter 112 and the digital signal resulting from quantization in the second A/D converter 114 are supplied to the subtracter 116. The subtraction in the subtracter 116 is performed by subtracting the digital signal that is the output from the first A/D converter 112 from the digital signal that is the output from the second A/D converter 114.

[0044] The cutoff frequency f2 of the second LPF 110 is always set so as to be higher than the cutoff frequency fl of the first LPF 108. Thus, in the frequency region lower than the cutoff frequency fl of the first LPF 108 and the frequency region higher than the cutoff frequency £2 of the second LPF 110, the components of the signal supplied to the BPF 100 that have frequencies in these frequency regions are cancelled and are thus blocked by the BPF 100. On the other hand, in the frequency region higher than the cutoff frequency fl of the first LPF 108 and lower than the cutoff frequency £2 of the second LPF 110, the signal components that have frequencies in the frequency region are preserved and thus pass the BPF 100 (see FIGS. 6A and 6B).

[0045] According to the BPF 100 of this embodiment, by using the first LPF 108, the second LPF 110, the first A/D converter 112, the second A/D converter 114, and the subtracter 116, it is made possible to pass and extract the components of a signal having frequencies within a range from the cutoff frequency fl of the first LPF 108 and the cutoff frequency f2 of the second LPF 110. The quantized signal extracted by the BPF 100 is subjected to reception processing.

[0046] In this embodiment, the cutoff frequencies fl and £2 of the first and second LPFs 108 and 110 are set so that each of the cutoff frequencies fl and £2 is individually within a predetermined range, and the cutoff frequency £2 of the second LPF 110 is always higher than the cutoff frequency fl of the first LPF 108. The resistance and capacitance as parameters of the resistor 36 and the capacitor 38 are variable independently, so that the cutoff frequencies f 1 and £2 are variable independently of each other.

[0047] Thus, in this embodiment, it is made possible to realize the BPF 100 of which the cutoff frequencies fl and £2, and the passband width can be continuously varied, by making the cutoff frequencies fl and £2 of the first and second LPFs 108 and 110 variable independently of each other. Accordingly, it is possible to continuously vary the cutoff frequencies and the passband width of the BPF 100, using a simple configuration.

[0048] In addition, although, in this embodiment, it is necessary to provide two A/D converters 112 and 114 to realize the BPF 100 of which the cutoff frequencies fl and £2, and the passband width can be continuously varied, it is unnecessary to provide the phase shifter 30 for shifting the phase of the signal that has passed the first LPF 28 by 180° as in the case of the first embodiment, and the digital signal calculation (subtraction by the subtracter 116) will suffice. For this reason, according to the second embodiment, it is possible to achieve size reduction and simplification of the BPF, and consequently, the receiver. [0049] In the second embodiment, the first LPF 108 may be regarded as the "first low-pass filter", or one of the "two low-pass filters" of the invention; the first A/D converter 112 may be regarded as the "first A/D converter" of the invention; the second LPF 110 may be regarded as the "second low-pass filter", or one of the "two low-pass filters" of the invention; the second A/D converter 114 may be regarded as the "second A/D converter" of the invention; and the subtracter 116 may be regarded as the "subtraction means" of the invention.

[0050] While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.

Claims

1. A band-pass filter characterized by comprising: distribution means for distributing an input signal into two signals; a first low-pass filter that passes a component of one of the two signals into which the input signal is distributed by the distribution means, the component having a frequency lower than a first cutoff frequency; a mixer that combines the other of the two signals with a signal obtained by reversing a phase of a signal output from the first low-pass filter; and a second low-pass filter that passes a component of a signal output from the mixer, the component having a frequency lower than a second cutoff frequency that is higher than the first cutoff frequency.

2. A band-pass filter characterized by comprising: a first low-pass filter that passes a component of an input signal, the component having a frequency lower than a first cutoff frequency; a first A/D converter that quantizes a signal output from the first low-pass filter into a digital signal; a second low-pass filter that passes a component of an input signal, the component having a frequency lower than a second cutoff frequency that is higher than the first cutoff frequency; a second A/D converter that quantizes a signal output from the second low-pass filter into a digital signal; and subtraction means for subtracting an output of the first A/D converter from an output of the second A/D converter.

3. A band-pass filter according to claim 1 or 2, wherein the first cutoff frequency of the first low-pass filter and the second cutoff frequency of the second low-pass filter are independently variable.

4. A band-pass filter according to any one of claims 1 to 3, wherein each of the first and second low-pass filters includes: a resistor connected between an input and an output of the low-pass filter in series; and a capacitor connected between the output and ground.

5. A band-pass filter including two low-pass filters, in which cutoff frequencies of the two low-pass filters are used as an upper limit and a lower limit of a passband, characterized in that the cutoff frequencies of the two low-pass filters are independently variable.

6. A receiver mounted in a vehicle characterized by comprising the band-pass filter according to any one of claims 1 to 5.

7. A band-pass filter comprising: a distribution device that distributes an input signal into two signals; a first low-pass filter that passes a component of one of the two signals into which the input signal is distributed by the distribution device, the component having a frequency lower than a first cutoff frequency; a mixer that combines the other of the two signals with a signal obtained by reversing a phase of a signal output from the first low-pass filter; and a second low-pass filter that passes a component of a signal output from the mixer, the component having a frequency lower than a second cutoff frequency that is higher than the first cutoff frequency.