JP2008193209A - Quadrature modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable accurate quadrature modulation with less error without increasing a hardware scale. <P>SOLUTION: A quadrature modulator provided with an integrator 4 for integrating a digital composite signal and outputting the integrated result as a value with a code and a CORDIC 6 for generating a digital quadrature modulation signal by calculating the amplitude values of a sine wave and a cosine wave which correspond to the value outputted from the integrator 4 converts the digital composite signal into an operable value by the CORDIC 6 through integration and generates a quadrature modulation signal by the digital operation of a trigonometric function in the CORDIC 6 so that a lookup table can be made unnecessary, the resolution of an integration output value can be improved by providing the integrator 4 with a comparatively large bit width and an extremely accurate quadrature modulation signal with less error can be obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a quadrature modulator that generates an I signal (in-phase signal) and a Q signal (quadrature signal).

  In recent years, digitization of wireless communication has progressed. In digital wireless communication, digital modulation that directly modulates a carrier wave with a digital signal is used instead of modulation with a conventional analog signal. Examples of digital modulation schemes include quadrature modulation (IQ modulation) schemes that generate I and Q signals, such as QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation).

Conventionally, many quadrature modulators have generated I and Q signals using a PLL (Phase Locked Loop) circuit. On the other hand, there is a technique for digitally generating an I signal and a Q signal by using a lookup table of trigonometric functions (sin, cos) stored in a memory such as a ROM (see, for example, Patent Documents 1 and 2). ).
JP 11-74732 A JP-T-2004-515970

  However, in the conventional technique using a lookup table, there is a problem that if the accuracy of the generated quadrature modulation signal is increased, the amount of data to be stored in the ROM increases and the hardware scale increases. . On the other hand, if the number of table values to be stored is reduced in order to reduce the ROM capacity, the resolution of the generated I and Q signals deteriorates and the error increases. For this reason, there has been a problem that the linearity of the conversion characteristics at the time of demodulation cannot be secured.

  The present invention has been made to solve such problems, and it is an object of the present invention to enable accurate quadrature modulation with less error and without increasing the hardware scale. To do.

  In order to solve the above-described problems, the quadrature modulator of the present invention integrates a digital composite signal and outputs an integration result as a signed value, and a sine wave corresponding to the value output from the integrator. And a trigonometric function operation unit that generates a digital quadrature modulation signal by calculating the amplitude value of the cosine wave.

  According to the present invention configured as described above, the digital composite signal is converted into a value that can be calculated by the trigonometric function calculation unit through integration, and a quadrature modulation signal is generated by digital calculation of the trigonometric function in the trigonometric function calculation unit. Is done. For this reason, a look-up table is unnecessary, and a ROM for this purpose can be omitted, so that the hardware scale can be reduced. Also, by providing the integrator with a relatively large bit width, the resolution of the integrated output value can be increased, and the quadrature function calculation using the high-resolution integrated output value reduces errors and provides high-precision quadrature modulation. A signal can be obtained.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of a quadrature modulator according to the present embodiment. FIG. 2 is a diagram illustrating an output waveform of each unit illustrated in FIG. Hereinafter, the configuration and operation of the quadrature modulator according to the present embodiment will be described with reference to FIGS.

  In FIG. 1, reference numerals 1L and 1R denote A / D converters (ADC), which respectively convert an L channel audio signal and an R channel audio signal input as analog signals into digital signals.

  A digital pilot signal generator 2 generates a 38 kHz digital subcarrier signal and a 19 kHz digital pilot signal. Reference numeral 3 denotes a stereo modulator, which uses the subcarrier signal and pilot signal input from the digital pilot signal generator 3 to output the L channel and R channel digital signals output from the A / D converters 1L and 1R, or A digital composite signal is generated from L channel and R channel digital signals directly input from the outside.

  Reference numeral 4 denotes an integrator that integrates the digital composite signal generated by the stereo modulator 3 and outputs the integration result as a signed value. FIG. 2A shows the output waveform of the integrator 4. The possible range of the integral value is determined by the bus width of the digital data input to the integrator 4. For example, when the data bus width of the composite signal is 16 bits, the possible range of the integral value is a signed integer of −32768 to 32767. This corresponds to an angle of −180 ° to + 180 °.

In the integrator 4 having a 16-bit width, when 1 is added to the maximum value that can be represented by 16 bits, the overflow occurs when there is no carry output and the value becomes 0. Expressing this in hexadecimal,
FFFFh + 0001h = 0000h
It becomes. That is, when 1 is added to an integral value corresponding to + 180 ° (strictly, a value of 180 ° -1 bit), an integral value corresponding to −180 ° is obtained.

  Therefore, the output waveform of the integrator 4 is as shown in FIG. That is, the integral value gradually increases at a constant rate. When the integral value reaches a value corresponding to + 180 °, the integral value falls to a value corresponding to −180 ° at a stroke. Thereafter, the integral value gradually increases again at a constant rate. When the integral value reaches a value corresponding to + 180 °, the integral value again falls to a value corresponding to −180 °. A waveform in which such a movement is repeated becomes an output waveform of the integrator 4.

  Here, by providing the integrator 4 with a relatively large bit width, the resolution of the integrated output value can be increased. For example, if the data bus width of the integrator 4 is 21 bits, the range from −180 ° to + 180 ° is expressed by 21 bits, so the angle per bit is 0.00017 °. On the other hand, when using a look-up table as in the prior art, for example, if a 128-gradation ROM is used and the range of 0 ° to 90 ° is represented by 128 gradations, the angle per gradation is 0. It becomes rough with 703 °. Of course, increasing the number of gradations increases the angular resolution, but increases the ROM capacity. As can be seen, using the integrator 4 to provide a relatively large bit width effectively increases the angular resolution on a smaller hardware scale compared to the conventional example using a lookup table. Can do.

  A coefficient unit 5 multiplies the integral value output from the integrator 4 by a coefficient of “2π”. Reference numeral 6 denotes a CORDIC (Coordinate Rotation Digital Computer) corresponding to the trigonometric function calculation unit of the present invention, which calculates the amplitude values of the sine wave and the cosine wave corresponding to the values output from the coefficient unit 5, thereby performing digital orthogonal modulation. Generate a signal. Since the CORDIC 6 calculates the value of the trigonometric function by the arc degree method, the coefficient unit 5 is arranged in the preceding stage of the CORDIC 6 and the output value of the integrator 4 is converted into radians.

  The CORDIC 6 includes a preprocessing unit, an amplitude calculation unit, and a postprocessing unit as functional configurations realized by calculation. The preprocessing unit converts the value output from the coefficient unit 5 into an angular vibration numerical value within a range that can be calculated by the amplitude calculation unit. FIG. 2B shows an output waveform of the preprocessing unit. That is, since the range of values that can be handled by the CORDIC 6 is −90 ° to + 90 ° (−π / 2 to + π / 2), the range of −180 ° to −90 ° and the range of + 90 ° to + 180 °. The values are converted into values in the range of 0 ° to −90 ° and in the range of 0 ° to + 90 °, respectively. At that time, a flag indicating that is set in the converted value.

  Specifically, for a value in the range of −180 ° to −90 °, a value twice as much as −90 ° is subtracted from the value. For example, in the case of −100 °, the value corresponding to twice the amount below −90 ° (−10 °) is subtracted from the value corresponding to −100 ° (= (− 100 °) − (− 20 °)) ), A value corresponding to −80 °. Further, for a value in the range of + 90 ° to + 180 °, a value twice as much as + 90 ° is subtracted from the value. For example, in the case of + 150 °, a value corresponding to twice the amount exceeding + 90 ° (+ 60 °) is subtracted from a value corresponding to + 150 ° (= 150 ° -120 °) to obtain a value corresponding to + 30 °. .

The amplitude calculation unit calculates the amplitude values of the sine wave and the cosine wave corresponding to the angular vibration values obtained by the preprocessing unit. This amplitude calculation unit is an algorithm that realizes plane rotation of a two-dimensional vector by elementary function operations such as trigonometric functions, multiplications, and divisions. Can be obtained. For example, if the coordinates of a point P (x, y) rotated by a certain angular frequency with the point P ₀ (1,0) on the x axis as a reference point are obtained, the x coordinate corresponds to the angular frequency. The value of the cos function to be obtained, that is, the amplitude of the cosine wave signal to be obtained can be obtained. Further, the y coordinate can be obtained as the value of the sin function corresponding to the angular frequency, that is, the amplitude of the sine wave signal to be obtained.

  The calculation by the amplitude calculator corresponds to generating a sine wave signal y or cosine wave signal x having a waveform as shown in FIG. 2C from data having a waveform as shown in FIG. As described above, since the amplitude calculation unit calculates in the range of −90 ° to + 90 ° (−π / 2 to + π / 2), the sine wave signal y has a waveform in which the amplitude takes positive and negative values. Thus, the cosine wave signal x has a waveform with a positive amplitude only.

  The post-processing unit appropriately performs code conversion on the amplitude value of the cosine wave signal x obtained by the amplitude calculation unit. As described above, since the amplitude value of the sine wave signal y is output with a sign from the beginning of the amplitude calculation unit, there is no need for code conversion. On the other hand, regarding the amplitude value of the cosine wave signal x, since the amplitude values are all positive as a result of the preprocessing as shown in FIG. The amplitude value of the corresponding part is sign-converted.

By this post-processing, a sine wave signal and a cosine wave signal as shown in FIG. 2D can be obtained. This cosine wave signal corresponds to the I signal, and the sine wave signal corresponds to the Q signal. That is, in this embodiment, the I signal and the Q signal are defined as in the following (Expression 1) and (Expression 2). Here, τ is the sampling frequency of the integrator 4, m (τ) is a composite signal, and Kd is a conversion coefficient for converting the integration result into a range of 0 to ± 180 °.
I (t) = cos {Kd∫m (τ) dτ} (Formula 1)
Q (t) = sin {Kd∫m (τ) dτ} (Formula 2)

  Returning to FIG. 1, reference numerals 7I and 7Q denote D / A converters (DACs), which convert the I signal (cosine wave signal) and Q signal (sine wave signal) output from the CORDIC 6 from a digital signal to an analog signal, respectively. . 8 is a mixer, and 9 is a carrier wave oscillator. The mixer 8 mixes the I and Q signals converted into analog signals by the D / A converters 7I and 7Q and the carrier signal output from the carrier oscillator 9 to perform frequency conversion.

  The mixer 8 includes multipliers 8a and 8b and an adder 8c. The first multiplier 8a modulates the I signal output from the D / A converter 7I with an in-phase carrier wave (cosine wave signal), and outputs the result to the adder 8c. The second multiplier 8b modulates the Q signal output from the D / A converter 7Q with an orthogonal carrier wave (sine wave signal), and outputs the result to the adder 8c. The adder 8 c adds the signals output from the first and second multipliers 8 a and 8 b and outputs the result to the high frequency amplifier 10. The high frequency amplifier 10 amplifies the signal orthogonally modulated by the mixer 8 and outputs the amplified signal to the antenna 11. Thereby, the modulation signal is radiated from the antenna 11 as, for example, FM radio waves.

  As described above in detail, according to the present embodiment, the composite signal is integrated by the integrator 4 so that the integrated output falls within the range (−π / 2 to π / 2) that can be calculated by the CORDIC 6. The digital quadrature modulation signal is generated by calculating the amplitude values of the sine wave and the cosine wave in the CORDIC 6. As a result, the quadrature modulation signal is generated by the digital calculation of the trigonometric function in the CORDIC 6, so that a lookup table is unnecessary and the hardware scale can be reduced correspondingly. Further, by providing the integrator 4 with a relatively large bit width, it is possible to increase the resolution of the integrated output value and perform orthogonal modulation with little error and high accuracy.

  In the above embodiment, an example in which a stereo composite signal is FM-modulated has been described. However, a signal to be modulated is not limited to a stereo signal. For example, a monaural signal may be used. The signal modulation method is not limited to FM modulation. For example, AM modulation or PM modulation may be used.

  In addition, each of the above-described embodiments is merely an example of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. In other words, the present invention can be implemented in various forms without departing from the spirit or main features thereof.

  The present invention relates to a quadrature modulator that generates an I signal and a Q signal, and is useful for a radio transmitter such as an FM transmitter.

It is a figure which shows the structural example of the orthogonal modulator by this embodiment. It is a wave form diagram which shows the operation example of the quadrature modulator by this embodiment.

Explanation of symbols

3 Stereo Modulator 4 Integrator 5 Coefficient Unit 6 CORDIC (Trigonometric Function Operation Unit)

Claims (2)

A stereo modulator that generates a digital composite signal from the L-channel and R-channel digital signals;
An integrator that integrates the digital composite signal generated by the stereo modulator and outputs the integration result as a signed value;
A quadrature modulator comprising: a trigonometric function operation unit that generates a digital quadrature modulation signal by calculating amplitude values of a sine wave and a cosine wave corresponding to a value output from the integrator. The trigonometric function calculation unit converts a value output from the integrator into an angular vibration value within a range that can be calculated by the trigonometric function calculation unit;
An amplitude calculation unit that calculates amplitude values of sine waves and cosine waves corresponding to the angular vibration numerical values obtained by the preprocessing unit;
The quadrature modulator according to claim 1, further comprising: a post-processing unit that appropriately performs code conversion on the amplitude value of the cosine wave obtained by the amplitude calculation unit.