JPH10285228A - Digital modulation circuit - Google Patents

Abstract

(57) [Problem] To provide a digital modulation circuit capable of reducing a processing amount and reducing a circuit scale without deteriorating performance. A digital modulation circuit having a function of modulating transmission data and outputting the data by a digital circuit,
A baseband modulator that converts transmission data into baseband I and Q signals; a baseband filter that limits the bands of the I and Q signals; and a modulated wave having a desired center frequency by the band-limited I and Q signals , A means for up-sampling the output signal of the quadrature modulator, a D / A converter for converting the up-sampled signal into an analog signal, and obtaining a modulated wave output of a desired frequency Therefore, a configuration is provided that includes a band-pass filter that limits the band of the output signal of the D / A converter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital modulation circuit used in a digital portable telephone or the like.
In particular, the baseband filter and the quadrature modulator are connected to a DSP (D
An object of the present invention is to provide a digital modulation circuit capable of reducing the circuit scale without deteriorating performance when realized by a digital circuit such as an digital signal processor.

[0002]

2. Description of the Related Art In recent years, with the development of digital circuit technology, examples of realizing functions realized by analog circuits by digital circuits are increasing. When the functions are realized by digital circuits, there is no variation in characteristics, no aging,
The advantage that no adjustment is required is obtained. Furthermore, when a programmable device such as a DSP is used as a digital circuit, the function is described by software, so that there is an advantage that correction and change are easy. In the field of communication devices such as digital mobile phones, the shift to digital circuits is progressing. As an ultimate example, a software defined radio (JoMetola, “The Software”) that realizes most of the communication functions by software.
are RadioArchitecture ”, IE
EE Communication Magazine
May 1995 Vol. 33 No. 5) has been proposed. FIG. 4 shows an example in which a digital modulation circuit used in a digital mobile phone or the like is realized by a digital circuit using a DSP. In FIG. 4, this digital modulation circuit includes a baseband modulator 1 to which transmission data is input, first and second baseband filters 2 and 3 connected to the baseband modulator 1, First and second multipliers 4 and 5 connected to the first and second baseband filters 2 and 3, respectively; a 90 ° phase shifter 6 connected to the second multiplier 5; A local oscillator 7 connected to the multiplier 4 and the 90 ° phase shifter 6, an adder 8 connected to the first and second multipliers 4 and 5, and a D / A connected to the adder 8 Converter 1
0 and a low-pass filter 13 connected to the D / A converter 10. And the first and second
, A 90 ° phase shifter 6, a local oscillator 7 and an adder 8, constitute a quadrature modulator 12. The baseband modulator 1, the first and second baseband filters 2, 3, and the quadrature modulator The portion surrounded by the dotted line of the modulator 12 is a portion that realizes the function by the software of the DSP.

Next, the operation will be described. First, transmission data is input to the baseband modulator 1. The baseband modulator 1 converts transmission data into baseband I and Q signals according to a modulation scheme. The I signal is a signal that is an in-phase component of the modulated wave output (output signal of the low-pass filter 13), and the Q signal is a signal that is an orthogonal component of the modulated wave output (output signal of the low-pass filter 13). For example, π / 4QP used in digital mobile phones
In the SK modulation method, I,
A Q signal is generated. Here, the symbol frequency of the modulation, that is, the frequency at which the I and Q signals change is fb. Then, the I and Q signals are converted into first and second signals having the same characteristics.
To the baseband filters 2 and 3, respectively, to perform band limitation. Since the first and second baseband filters 2 and 3 are digital filters, they operate at a predetermined sampling frequency fs and output the band-limited outputs to the first and second multipliers 4 and 5 and the local The signal is input to a quadrature modulator 12 including an oscillator 7, a 90 ° phase shifter 6, and an adder 8. This quadrature modulator 12 also operates at the sampling frequency fs. Further, the local oscillator 7 oscillates a cosine wave having a desired output frequency fc of the digital modulation circuit. In the quadrature modulator 12, the band-limited I
A digitally modulated wave output signal is obtained by multiplying the signal by a cosine wave of frequency fc, while multiplying the band-limited Q signal by a sine wave of frequency fc, and adding the outputs of both. Here, the spectrum of the digitally modulated wave output signal of the quadrature modulator 12 is shown in FIG. Since the digital modulation wave output signal is a discrete signal having the sampling frequency fs, the spectrum of the image signal appears at the center frequency (fs-fc) in addition to the original signal having the center frequency fc. The digital output signal is converted into an analog signal by the D / A converter 10,
By removing the image signal with the analog low-pass filter 13, a desired modulated wave output can be obtained. However,
In order that the image signal can be removed by the low-pass filter, the condition of fs> 2fc + Wd (1) is necessary, as is easily understood from FIG. Here, Wd is the modulation wave bandwidth.

[0004]

However, in the above-mentioned conventional digital modulation circuit, there is a problem that the processing amount of the baseband filters 2 and 3 becomes very large for the following reasons, so that the circuit scale becomes large. That is,
Since the baseband filters 2 and 3 are digital filters, the processing amount is determined by the sampling frequency fs and the number of taps of the digital filter. In order to obtain a characteristic with no performance degradation compared to the case of realizing with an analog filter, it is necessary to prepare filter taps corresponding to a time response of eight symbols. Therefore, the required number of taps is (fs / fb) × 8. At this time, the two baseband filters 2, 3
Can be expressed by the following equation. fs × (fs / fb) × 8 × 2 (2) Here, as an example, the symbol frequency fb = 21 kHz,
Modulation wave output frequency fc = 455 kHz, modulation wave bandwidth W
Consider the case where d = 32 kHz. First, from the above equation (1), the condition of the sampling frequency is fs> 942 kHz, and the processing amount of the two baseband filters 2 and 3 when fs is the minimum of 942 kHz is 676 MIPS (Million Instructi
onsper Second). The processing capability of current general-purpose DSPs is generally 40 to 50 MIPS even at high speeds, so that the circuit scale for realizing the baseband filter is very large. SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems of the conventional digital modulation circuit described above, and provides a digital modulation circuit capable of reducing the processing amount and reducing the circuit scale without deteriorating the performance. The purpose is to provide.

[0005]

In order to achieve the above object, the present invention provides a digital modulation circuit having a function of modulating transmission data and outputting the data by a digital circuit.
A baseband modulator that converts transmission data into baseband I and Q signals; a baseband filter that limits the bands of the I and Q signals; and a modulated wave having a desired center frequency by the band-limited I and Q signals , A means for up-sampling the output signal of the quadrature modulator, a D / A converter for converting the up-sampled signal into an analog signal, and obtaining a modulated wave output of a desired frequency For this purpose, a band pass filter for limiting the band of the output signal of the D / A converter is provided. Another feature of the present invention is that the quadrature modulator generates a cosine wave having a frequency fL, and a 90 ° phase shifter that changes a phase of an output signal of the local oscillator to output a sine wave having a frequency fL. A first multiplier for multiplying the band-limited I signal by a cosine wave of frequency fL, a second multiplier for multiplying the band-limited Q signal by a sine wave of frequency fL, And an adder for adding the output of the second multiplier. Another feature of the present invention is that the up-sampling means up-samples the frequency of the quadrature modulator output signal to an n-fold frequency. Another feature of the present invention is the baseband modulator,
The baseband filter, the quadrature modulator, and the up-sampling means are realized by a digital circuit using a DSP.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on illustrated embodiments. FIG. 1 is a block diagram showing one embodiment of a digital modulation circuit according to the present invention. In FIG. 1, the digital modulation circuit includes a baseband modulator 1 to which transmission data is input, first and second baseband filters 2 and 3 connected to the baseband modulator 1, And second multipliers 4 and 5 connected to the first and second baseband filters 2 and 3, respectively, and 90 ° connected to the second multiplier 5
Phaser 6, first multiplier 4 and 90 ° phaser 6
Connected to the local oscillator 7 and the first and second
Adder 8 connected to the multipliers 4 and 5 of the
, A D / A converter 10 connected to the upsampler 9, and a bandpass filter 11 connected to the D / A converter 10. The first and second multipliers 4, 5, and 90 ° phase shifter 6, local oscillator 7, and adder 8
A quadrature modulator 12 is constituted, and the portions surrounded by the dotted lines of the baseband modulator 1, the first and second baseband filters 2, 3 and the quadrature modulator 12 and the upsampler 9 function by software of the DSP. This is the part that has been realized.

Next, the operation of the digital modulation circuit will be described. First, transmission data is input to the baseband modulator 1, and the baseband modulator 1 converts the transmission data to baseband I, Q according to the modulation method. Convert to a signal. Here, the I signal is a signal that is an in-phase component of the modulated wave output (the output signal of the band pass filter 11),
The signal is a signal that becomes a quadrature component of the modulated wave output. For example, π / 4QP used in digital mobile phones
In the SK modulation method, I and Q signals are generated according to a rule as shown in FIG.

Here, the symbol frequency of the modulation, that is, the frequency at which the I and Q signals change is fb. Then, the I and Q signals are input to first and second baseband filters 2 and 3 having the same characteristics, respectively, to perform band limitation. The first and second baseband filters 2,
Reference numeral 3 denotes a digital filter which operates at a predetermined first sampling frequency fs1, and outputs band-limited outputs to first and second multipliers 4 and 5, local oscillators 7 and 90.
° Input to quadrature modulator 12 comprising phase shifter 6 and adder 8. This quadrature modulator 12 also operates at the first sampling frequency fs1. Further, the local oscillator 7 oscillates a cosine wave having a frequency fL. fL is different from the desired output frequency fc of the digital modulation circuit, and is a frequency determined by conditions described later. In the quadrature modulator 12, a cosine wave having a frequency fL is applied to the band-limited I signal in the first direction.
, And a sine wave of frequency fL is multiplied by a second multiplier 5, and the outputs of both are added by an adder 8, whereby digital modulation of the center frequency fL is performed. A wave output signal is obtained. FIG. 2 shows the spectrum of the digitally modulated wave output signal of the quadrature modulator 12.

Since the digital modulated wave output signal is a discrete signal having the first sampling frequency fs1, the spectrum of the image signal appears at the center frequency (fs1-fL) in addition to the original signal having the center frequency fL. This output signal is converted by the upsampler 9 into a second sample frequency fs which is n times fs1.
Upsample to 2. There are various interpolation methods for up-sampling. Here, zero interpolation is performed in which all the values of the increased sample points are set to 0. As an example, n = 8
FIG. 3 shows the spectrum of the output signal of the upsampler 9 in the case of (1). As can be seen from FIG. 3, an image of the spectrum of FIG. 2 appears every fs1.

That is, when fs2 = n × fs1, the center frequency of the image signal is (k−1) × fs1 + fL, k × fs1-fL: k = 1, 2,..., N (3). These image signals are basically the same as the spectrum of the modulated wave signal, and any of them may be used. Therefore, fs1, fL, and k are set so that the center frequency of a certain image signal matches the desired output frequency fc, and these image signal groups are converted into analog signals by the D / A converter 10, and the analog band is set. Pass filter 11
A desired modulated wave output can be obtained by performing band limitation and removing unnecessary image signals.

However, in order for the unnecessary image signal to be removed by the band-pass filter 11, the condition fs1> 2fL + Wd (4) is necessary, as can be easily understood from FIGS. However, as can be imagined from FIG. 3, since fL can set a low frequency with respect to fc,
fs1 can use a value lower than the conventional sampling frequency fs, and as a result, the processing amount of the first and second baseband filters 2 and 3 can be reduced. As an example, the symbol frequency fb = 21 kHz, the modulation wave output frequency fc = 455 kHz, the modulation wave bandwidth Wd =
Consider the case of 32 kHz.

First, a combination for setting the output frequency fc to 455 kHz while satisfying the equations (3) and (4) is selected. First sampling frequency fs1 = 168k
Hz, local oscillation frequency fL = 49 kHz, n = 8,
If k = 3, the output frequency is fc = k × fs1−fL = 455 kHz from Expression (3). From the equation (4), the condition of the sampling frequency is fs1> 2fL + Wd = 130 kHz, which satisfies the condition. At this time, the processing amount of the two baseband filters 2 and 3 is fs1 × (fs1 / fb) × 8 × 2 = 21.5 MIPS from Expression (2), which is about 1/30 of the conventional case (676 MIPS).
Is the processing amount. This processing amount can be sufficiently handled by one general-purpose DSP.

[0013]

As described above, according to the present invention, an image signal is obtained by up-sampling the output signal of the quadrature modulator, and the unnecessary image signal is removed after D / A conversion. In addition, the processing amount of the baseband filter can be remarkably reduced as compared with the conventional system, and this has a great effect in reducing the circuit size of the digital modulation circuit.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of a digital modulation circuit according to the present invention.

FIG. 2 is a diagram illustrating a spectrum of an output signal of the quadrature modulator according to the embodiment illustrated in FIG. 1;

FIG. 3 is a diagram showing a spectrum of an output signal of an upsampler in the embodiment shown in FIG. 1;

FIG. 4 is a configuration diagram of a conventional digital modulation circuit.

5 is a diagram showing a spectrum of an output signal of a quadrature modulator of the conventional digital modulator shown in FIG.

FIG. 6 is a diagram showing rules for generating I and Q signals in a π / 4 QPSK modulation scheme.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Baseband modulator, 2, 3 ... 1st and 2nd baseband filter, 4, 5 ... 1st and 2nd multiplier, 6 ... 90 degree phase shifter, 7 ... Local oscillator, 8 ... Adder , 9 ... upsampler, 10 ... D / A converter, 11 ...
Band-pass filter, 12: quadrature modulator, 13: low-pass filter,

Continued on the front page (72) Inventor Makoto Ito 3-7-1 Ichibancho, Aoba-ku, Sendai City, Miyagi Prefecture Tohoku Electric Power Co., Inc. (72) Inventor Atsuya Yokoi 2-1-1 Kotani, Samukawa-cho, Koza-gun, Kanagawa Prefecture No. Toyo Tsushinki Co., Ltd.

Claims (4)

[Claims] 1. A digital modulation circuit in which a function of modulating and outputting transmission data is realized by a digital circuit,
A baseband modulator that converts transmission data into baseband I and Q signals; a baseband filter that limits the bands of the I and Q signals; and a modulated wave having a desired center frequency by the band-limited I and Q signals , A means for up-sampling the output signal of the quadrature modulator, a D / A converter for converting the up-sampled signal into an analog signal, and obtaining a modulated wave output of a desired frequency A digital modulation circuit, comprising: a band-pass filter for limiting a band of an output signal of the D / A converter. 2. A quadrature modulator for generating a cosine wave having a frequency fL, a 90 ° phaser for changing a phase of an output signal of the local oscillator to output a sine wave having a frequency fL, The frequency fL is applied to the restricted I signal.
Multiplier that multiplies the cosine wave of
A second multiplier for multiplying the signal by a sine wave of frequency fL;
2. The digital modulation circuit according to claim 1, further comprising an adder for adding outputs of said first and second multipliers. 3. The digital modulation circuit according to claim 1, wherein said up-sampling means up-samples the frequency of said quadrature modulator output signal to an n-fold frequency. 4. The apparatus according to claim 1, wherein said baseband modulator, baseband filter, quadrature modulator, and up-sampling means comprise
2. The digital modulation circuit according to claim 1, wherein the digital modulation circuit is realized by a digital circuit using an SP.