JP2003318737A - Band division a/d converting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a band division A/D converting device having a digital filter which can make a gain characteristic flat after synthesizing. <P>SOLUTION: In the band division A/D converting device which divides an inputted analog signal into a plurality of bands to be converted into digital signals, a plurality of A/D converters 11-13 whose input terminals are respectively connected to a common input terminal 10 to which the analog signal is inputted perform an A/D conversion in different conversion bands respectively. The bands corresponding to the conversion bands of the A/D converters 11-13 are made pass band ranges or transition band ranges. The output of the A/D converters 11-13 are respectively inputted to digital filters 14-16 composed so that the transition band range may have a gain characteristic symmetric with respect to a point of a frequency almost half the gain of the pass band range, the output of the digital filters 14-16 are synthesized by an adder 17, and digital signals of all the bands are outputted to an output terminal 18. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a band division A for dividing an analog signal into a plurality of bands and converting it into a digital signal.
The present invention relates to a / D conversion device, and more particularly to a digital filter for removing quantization noise generated in an A / D converter in each band.

[0002]

2. Description of the Related Art Generally, an A / D converter has a trade-off between conversion accuracy and conversion band. For example, the flash A / D converter has a wide conversion band, but the conversion accuracy is not so high. On the other hand, the ΔΣ type A / D converter shows the opposite tendency to the flash type A / D converter. With the recent development of digital communication, it is expected that both high precision and wide band are required for A / D converters for converting received signals into digital signals in wireless communication systems such as mobile communication devices. Is becoming.

In order to meet the demand for high precision and wide bandwidth for A / D conversion, a plurality of A's performing conversion in different bands from each other.
The / D converter is connected to a common input terminal, and the output side of the A / D converter is connected to a digital filter whose pass band is a band corresponding to the conversion band of each A / D converter. There has been proposed a band division A / D converter that outputs digital signals in all bands by combining outputs to perform band combination (for example, Japanese Patent Laid-Open No. 11-17549). In particular, if a ΔΣ modulator is used for each A / D converter (such an A / D converter is referred to as a ΔΣ type A / D converter), only a band with less quantization noise can be extracted and combined, A high-accuracy and wide-band A / D converter can be realized.

[0004]

In the band division A / D converter as described above, the characteristics of the digital filter provided on the output side of each A / D converter have not been concretely examined so far. For example, JP-A-11-17549
Japanese Patent Laid-Open Publication No. 2003-242242 describes the use of a digital filter having a substantially flat gain characteristic (gain-frequency characteristic) after band combination. There is no specific disclosure of how to set the specific filter characteristics.

In addition, an A / D used especially in a radio system
In the converter, there are strict requirements for ripples in the signal band and group delay characteristics, and the characteristics that the gain characteristics are substantially flat are not sufficient. Japanese Patent Laid-Open No. 11-17549 does not mention such a requirement.

An object of the present invention is to provide a band division A / D conversion device having a digital filter capable of keeping the gain characteristic after band combination flat.

Another object of the present invention is to provide a band division A / D converter which can flatten the gain characteristic after band combination and can make the group delay constant.

[0008]

In order to solve the above problems, in the band division A / D conversion device according to the present invention, a plurality of common input terminals to which analog signals are input are connected to respective input terminals. A / D conversion is performed by the A / D converters in different conversion bands, and a band corresponding to the conversion band of each A / D converter is used as a pass band or a transition band. The outputs of the A / D converters are input to digital filters that are configured to have point-symmetrical gain characteristics centered around a half frequency, the outputs of the digital filters are combined, and digital signals of the entire band are output terminals. Is output.

As the A / D converter, a ΔΣ type A /
When the D converter is used, the ΔΣ type A / D converter has a characteristic that the quantization noise becomes minimum at different frequencies,
It is configured to perform A / D conversion in different conversion bands having the frequency as the center frequency. On the other hand, the digital filter includes a frequency at which the quantization noise of each A / D converter is minimized in the pass band or the transition band, and the transition band is point-symmetrical about the frequency at which the gain is almost half the gain of the pass band. It is configured to have various gain characteristics.

In the present invention, each digital filter is preferably constructed so that the group delay is constant within a desired band, and more specifically, the impulse response values are corner-symmetrical. With such a configuration, the band division A
The gain characteristic after band combination in the / D converter can be kept flat and the group delay can be made constant.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a band division A / D conversion device according to an embodiment of the present invention. In the present embodiment, the case where the input signal is divided into three bands and A / D converted is described, but the present invention is also applicable to the case where the number of band divisions is 2 or 4 or more.

In FIG. 1, the common input terminal 10 has an A /
An analog signal to be D-converted is input. This input analog signal is given to the input terminals of the three A / D converters 11, 12, and 13. A / D converters 11, 12, 13
Is an A / D converter using a ΔΣ modulator in the present embodiment,
That is, it is a ΔΣ type A / D converter, and the A / D converter 11
Is a ΔΣ modulator and A / D converter 1 having low-pass characteristics
Reference numerals 2 and 13 are configured using ΔΣ modulators having bandpass characteristics. A / D converter 11, 12 and 13, after sampling the input analog signal at a predetermined sampling frequency (the _{f s),} different conversion zone respectively (Lowpass, Bandpass_L, and Bandpass_H) by the A / D
The conversion is performed and the converted output is output to each output terminal. The A / D converters 11, 12 and 13 each have a characteristic that the quantization noise becomes minimum at different frequencies in the case of the ΔΣ type A / D converter, and the frequency having the minimum quantization noise is converted into the conversion band Lowpass. , Bandpass_L, Bandpass_H center frequencies.

The converted outputs from the A / D converters 11, 12, 13 are input to digital filters 14, 15, 16 respectively. The digital filters 14, 15 and 16 are connected to the A / D converters 11, 12, 13 respectively.
The band corresponding to the conversion band of is the pass band or transition band,
The transition region is configured so as to have a point-symmetrical gain characteristic around a frequency at which the gain is half that of the pass region.
impulse response) filter. The transition band of the digital filter is a region between the pass band and the stop band and is also called a transition region. Here, it is assumed that the frequency with the minimum quantization noise is the center of the conversion band of the A / D converter.
Since the centers of the digital filters are offset from each other, the transition region of the digital filter may be located outside the conversion band of the A / D converter.

The A / D converters 11, 12, 13 are ΔΣ type A
In the case of a D / D converter, digital filters 14, 15, 16
Corresponds to the conversion band of the A / D converters 11, 12, and 13 by being configured to include the frequency of the minimum quantization noise of the A / D converters 11, 12, and 13 in the pass band or the transition band. The band to be set becomes the pass band or transition band. The digital filters 14, 15 and 16 are more preferably configured so that the group delay is constant within a desired band of the pass band or the transition band. The outputs of the digital filters 14, 15 and 16 are combined by the adder 17, and the digital signal of the entire band which is the output of the band division A / D converter is output to the output terminal 18.

Next, the digital filters 14, 15, 16
Will be described. FIG. 2 shows the digital filters 14 and 1
It is a graph illustrating the characteristics of the FIR filter used as 5 and 16, the horizontal axis represents the frequency normalized by the sampling frequency f _s, and the vertical axis represents the gain. In this embodiment, an FIR filter having a cosine roll-off characteristic is used as a reference low pass filter. The reference low pass filter is
It is a filter that serves as a reference when configuring the digital filters 14, 15 and 16, and in this example, the frequency at which the gain is 0.5 is 2/38 Hz, and the gain characteristics in the transition band are centered around this frequency. It is symmetrical.

By moving the gain characteristic of this reference low-pass filter on the frequency axis, a band-pass filter having an arbitrary characteristic can be realized. For example, the bandpass filters Bandpass1, Bandpass_L, Bandpass_ in FIG.
H is the gain characteristic of the reference low-pass filter, which is 4 /
It has a gain characteristic that is moved by 38 Hz, 8/38 Hz, and 12/38 Hz. Bandpass filter Bandpass
The gain characteristics of 1, Bandpass_L, Bandpass_H are 2 / 38Hz frequency at which the gain of the reference low-pass filter is 0.5.
Bandpass filters Bandpass1, Bandpass_L, B
The gain characteristics of andpass_H intersect at a point where the gain is 0.5. Therefore, the bandpass filter Bandpass1, Bandpas
The gain characteristic after combining the gain characteristics of s_L and Bandpass_H is 1 at any frequency, that is, a flat characteristic.

In this embodiment, the low-pass ΔΣ type A
The digital filter 14 connected to the D / D converter 11 includes a reference low pass filter and a band pass filter Bandpa.
A lowpass filter Lowpass having a frequency characteristic of the sum of the frequency characteristics of ss1 is used, and two bandpass ΔΣ
The digital filters 15 and 16 connected to the type A / D converters 12 and 13 include bandpass filters Bandpa and Bandpa, respectively.
ss_L and Bandpass_H are used. In this case, the bandwidth of the low-pass filter Lowpass that is the digital filter 14 is 6/38 Hz, and the bandwidth of the band-pass filters Bandpass_L and Bandpass_H that are the digital filters 15 and 16 are both 4/38 Hz, and the specific ΔΣ A / D conversion is performed. It is also possible to filter the output of the converter (low-pass ΔΣ type A / D converter 11 in this example) with a wide bandwidth.

Here, in order to create a filter having a frequency characteristic which is the sum of the frequency characteristics of a plurality of digital filters (in this example, the reference lowpass filter and the bandpass filter Bondpass1), such as the lowpass filter Lowpass used as the digital filter 14. For this, a filter having an impulse response that is the sum of the impulse responses of the respective filters, that is, a filter that has the sum of the impulse responses as a tap coefficient may be created. In addition, the lowpass filter Lowpass is newly used as a reference lowpass filter, and the gain characteristic of the lowpass filter Lowpass is moved on the frequency axis so that the bandpass filter Bandpass_L,
You can also make a Bandpass_H.

Next, the digital filters 14, 15, 16
The relationship between the number of taps (number of impulse responses) N and the center frequency of the bandpass filters used in the digital filters 14, 15 and 16 will be described. Set the impulse response value of the reference low-pass filter to h ₀ (n)
(N = 0, ..., N−1), a bandpass filter (for example, Bandpass_L, Bandpass_H) obtained by moving the reference lowpass filter by the angular frequency ω ₁ on the frequency axis.
) Impulse response value h ₁ (n) is calculated by the following equation. h ₁ (n) = 2h ₀ (n) cos (nω ₁ ) (1) (n = 0, 1, ..., N−1) As shown in Expression (1), cos (nω ₁ ) (n = 0) ，
, ..., N-1) is used as a conversion coefficient, so that the gain characteristic of the reference low-pass filter has the same shape as that of the reference low-pass filter.
It can be converted to gain characteristics of _L and Bandpass_H.

On the other hand, regarding the phase characteristic of the digital filter, it is known that the linear phase, that is, the group delay is constant, when the impulse response value is corner symmetric. A reference low-pass filter whose impulse response value is corner-symmetric is, for example, the Parks-McClellan algorithm (TW Parks,
JH McClellan: “Chebyshev Approximation for Non
recursive Digital Filters with Linear Phase ”, IEE
E Trans. Circuit Theory, CT-19, No.2, pp. 189-194,
It can be designed using the method described in 1972).

In order to make the group delay of the entire band-split A / D converter, that is, the group delay of the digital signal of the entire band seen at the output of the adder 17, constant not only the reference low-pass filter but also the reference low-pass filter. The impulse response value of the bandpass filter after conversion from must also be corner symmetric. To this end, as shown in FIG. 3, the conversion coefficient cos (nω ₁ ) (n = 0, 1, ..., N−1)
The value of must also be corner-symmetric. Conversion coefficient cos (nω
_{1) (n = 0,1, ...} , to the corner symmetrical value of N-1), as shown in the following formula (N-1) omega cosine waveform M cycles between ₁ (M is (Integer) should be included. (N-1) ω ₁ = 2πM (2) At this time, if the sampling frequency of the A / D converters 11, 12, and 13 is fs, the center frequency f ₁ = ω ₁ / 2π of the bandpass filter is f _{1 = Mf s / (N-} 1) and becomes (3). For example, since N = 39 in the present embodiment, the center frequency of the bandpass filter is selected to be a multiple of 1 / (N-1) = _1/38 of the sampling frequency fs. Thus, the center frequency f of the bandpass filter
By setting ₁ , if the group delay of the reference low-pass filter is constant, the group delay of the band-pass filter after conversion is also constant, which makes it possible to make the group delay of the entire band-split A / D conversion device constant. it can. In the case of this embodiment, the bandwidth of the reference low-pass filter must be a multiple of 1/38 in order to flatten the gain characteristic after synthesis.

In the above embodiment, the bandpass filter Bandpass_H having the highest frequency can be replaced with a highpass filter. This is the same when the number of band divisions is 2 or 4 or more. That is, the digital filter connected to each A / D converter may include at least one of (a) a reference low-pass filter, (b) a band-pass filter and a high-pass filter obtained by frequency-converting the gain characteristics of the reference low-pass filter.

[0023]

As described above, A / A of each conversion band
By configuring the digital filter provided on the output side of the D converter according to the present invention, it is possible to realize a band-division A / D conversion device having a flat gain characteristic after synthesis and to make the group delay constant. You can also

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a band division A / D conversion device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a gain characteristic of the digital filter according to the first embodiment.

FIG. 3 is a diagram explaining a conversion coefficient for realizing the bandpass filter in the same embodiment.

[Explanation of symbols]

10 ... Common input terminal 11, 12, 13 ... A / D converter 14, 15, 16 ... Digital filter 17 ... Adder (combiner) 18 ... Output terminal

Claims (8)

[Claims] 1. A band division A / D conversion device for dividing an input analog signal into a plurality of bands and converting it into a digital signal, wherein a common input terminal to which the analog signal is input and a common input terminal are respectively provided. To a plurality of A / D converters that are connected to the input terminals of the A / D converters and perform A / D conversion in different conversion bands to output converted outputs to the respective output terminals; and output terminals of the plurality of A / D converters. A band corresponding to a conversion band of each A / D converter is connected to each other and is defined as a pass band or a transition band, and the transition band has a point-symmetrical gain characteristic around a frequency at which the gain is approximately half the gain of the pass band. A band division A / D conversion device comprising: a plurality of digital filters configured to have; and a combiner that combines outputs of the digital filters and outputs the digital signal. 2. A band division A / D conversion device for dividing an input analog signal into a plurality of bands and converting it into a digital signal, wherein a common input terminal to which the analog signal is input and a common input terminal are respectively provided. Input terminals are connected to each other, and each has a characteristic that the quantization noise is minimized at different frequencies. A / D conversion is performed in different conversion bands having the frequency as the center frequency, and the converted output is output to each output terminal. A plurality of ΔΣ A / D converters for outputting and a plurality of ΔΣ A / D converters are respectively connected to output terminals of the plurality of ΔΣ A / D converters, and a frequency at which the quantization noise of each A / D converter is minimized A plurality of digital filters that are included in the transition band, and the transition band has a gain characteristic that is point-symmetrical about a frequency at which the gain is approximately half the gain of the pass band; Band dividing A / D converter the output of filter combined to and a synthesizer for outputting the digital signal. 3. The band conversion A / D conversion device according to claim 1, wherein each of the plurality of digital filters is configured so that a group delay is constant within a desired band. 4. The band conversion A / D conversion device according to claim 3, wherein each of the plurality of digital filters is configured such that impulse response values are corner symmetric. 5. The band division A / D conversion device according to claim 1, wherein at least one of the digital filters is an FIR filter having a gain characteristic in a transition region having a cosine roll-off characteristic. . 6. The plurality of digital filters are a reference low-pass filter having a cosine roll-off characteristic in a gain characteristic in a transition region, and at least one of a band-pass filter and a high-pass filter obtained by frequency-converting the gain characteristic of the reference low-pass filter. 6. Any one of claims 1 to 5 including and
A band division A / D converter according to the item. 7. At least one of the digital filters is a sum of impulse responses of a reference low-pass filter and a plurality of arbitrary filters of a band-pass filter or a high-pass filter obtained by frequency-converting a gain characteristic of the reference low-pass filter. The band division A / D conversion device according to claim 1, which has a response. 8. The center frequency of at least one of the plurality of digital filters is Mfs / (N-1) (where fs
Is a sampling frequency, M is an arbitrary integer, N is the number of taps), and the band division A / D conversion device according to claim 1.