JP4013958B2 - Signal conversion apparatus and signal conversion method - Google Patents

Description

  The present invention relates to a signal conversion apparatus and method for converting a digital signal into an analog signal, and more particularly to a technique for suppressing spurious due to field through from a digital signal bus.

  In a D / A converter that converts a digital signal to an analog signal, if the signal to be converted is a highly periodic signal such as a sine wave, the periodic fluctuation component of the digital signal flowing on the digital signal bus is directly analog. In some cases, it is mixed with the signal and observed as unnecessary spurious. This is known as field-through from a digital signal, and is one of the obstacles in realizing a signal conversion device having a high SFDR (Supplied-Free Dynamic Range).

  Hereinafter, field-through will be described. FIG. 3 shows a bit value on the digital signal bus and its digital value (expressed in decimal number) when a sine wave having a period of 8T (T is a sampling clock period) is output on a 4-bit wide digital signal bus. FIG.

  As shown in FIG. 3, the 0th bit of the digital signal bus is 0 every 4T, and it can be seen that 1 / (4T) frequency components are included. As described above, each digital signal flowing on the digital signal bus often includes a different frequency component from the output signal of the D / A converter. Since the digital signal bus and the analog output of the D / A converter are coupled to each other though they are extremely weak due to electromagnetic induction or the like, spurious due to field through cannot be ignored particularly when the output power of the analog signal is small.

  In order to suppress spurious due to field through, a method of scrambling a digital signal is known (for example, see Non-Patent Document 1).

  FIG. 4 is a configuration diagram for spurious suppression described in Non-Patent Document 1. According to FIG. 4, the 4-bit digital signal input is input to one input terminal of each of the exclusive OR circuits 21 to 24 provided for each bit. A pseudo-noise sequence having a 1-bit width is input from the pseudo-noise sequence generation circuit 25 driven by a sampling clock to the other input terminals of the exclusive OR circuits 21 to 24. The outputs of the exclusive OR circuits 21 to 24 are input to one input terminal of the exclusive OR circuits 26 to 29, and the other input terminal of the exclusive OR circuits 26 to 29 is connected to the pseudo noise sequence generation circuit. A pseudo-noise sequence from 25 is input. The outputs of the exclusive OR circuits 26 to 29 and the sampling clock synchronized with the digital signal input are input to the D / A converter 1, and the converted analog signal is output from the D / A converter 1. Is done.

  In the configuration of FIG. 4, the digital signal input is scrambled by the pseudo-noise sequence in the exclusive OR circuits 21 to 24, and therefore the periodicity of the signal representing each bit in the wiring section of the digital signal bus indicated by reference numeral 5. Disappears and spurious contamination can be suppressed. The scrambled signal is descrambled by the exclusive OR circuits 26 to 29 immediately before the D / A converter 1 and reconverted into the original digital signal input.

“MAX5884, 3.3V, 14-Bit 200Msps High Dynamic Performance DAC with CMOS Inputs, Datasheet”, p.10., [Online], Maxim Integrated Products, [November 18, 2004 search], Internet <URL: http: //pdfserv.maxim-ic.com/en/ds/MAX5884.pdf>

  However, in the configuration described in Non-Patent Document 1, since the original digital signal is reconverted immediately before the D / A converter 1, the exclusive OR circuits 26 to 29 and the D / A The influence of coupling on the analog output of the D / A converter 1 from the digital signal bus between the converters 1 and the internal wiring of the D / A converter 1 cannot be suppressed.

  SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a signal conversion apparatus and method that solves the above-described problems and has low spurious due to field-through from a digital signal bus.

According to the signal conversion device of the present invention,
A digital signal obtained by sampling a band-limited required signal at a sampling clock speed and digitizing the digital signal into an analog signal, and a colored noise generating means for generating noise outside the band of the required signal; Noise, adding means for adding the digital signal, D / A converting means for converting the output signal of the adding means into an analog signal, and filter means for filtering the required signal from the output signal of the D / A converting means The colored noise generating means includes a frequency dividing means for dividing a sampling clock, a pseudo noise sequence generating means for generating pseudo noise driven by an output signal of the frequency dividing means, and a predetermined frequency. Random oscillating means for randomly generating a sine wave of a range along a time series, and an output signal of the pseudo noise sequence generating means as an output signal of the random oscillating means Characterized in that it comprises a frequency converter for more frequency conversion.

According to the signal conversion method of the present invention,
A method of converting a digital signal obtained by sampling a band-limited required signal at a sampling clock speed and digitizing the analog signal into an analog signal, generating noise outside the band of the required signal, and the noise and the digital Adding a signal to convert the signal into an analog signal, and filtering the required signal from the analog signal by a filter. Noise outside the band of the required signal is a sine wave in a predetermined frequency range. Are generated by randomly converting the output signal of the pseudo noise sequence generation circuit driven by the divided sampling clock and frequency-converted by the sine wave .

  By adding colored noise outside the required signal band to the digital signal input, the periodicity of each bit of the digital signal bus can be lost, thereby suppressing spurious due to field-through. Furthermore, since the added colored noise is removed after being converted into an analog signal, there is no influence of induction from the internal wiring of the D / A converter, which is a problem in the prior art, and it is good. A signal converter having spurious characteristics can be realized.

  The pseudo-noise sequence generating means is driven by the divided low-speed clock to generate colored noise. The pseudo-noise sequence generating means can be composed of a shift register and an exclusive OR circuit, Since such a complicated circuit is not required, a signal converter having a good spurious characteristic can be realized with a simple circuit configuration.

  In addition, the center frequency to which the pseudo noise is converted is randomly hopped along the time series by the random oscillating means and the frequency converting means. Accordingly, the total power is the same, but the average power density is small. It is possible to generate colored noise. As a result, the characteristics required for a filter that removes colored noise can be relaxed.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings.

  FIG. 1 is a block diagram of a signal conversion apparatus according to the present invention. As shown in FIG. 1, the signal conversion apparatus includes a D / A converter 1, an addition circuit 2, a colored noise generation circuit 3, and a filter 4. In the following description, it is assumed that the digital signal input is a digital digitized signal after the band-limited required signal is sampled at the sampling clock speed. The required signal means an analog signal to be output by the signal conversion device.

  The colored noise generating circuit 3 has a function of generating and outputting colored noise including only frequency components outside the required signal band, which are digitalized.

  The adder circuit 2 adds the digital signal input and the colored noise from the colored noise generating circuit 3 and outputs the result, and the D / A converter 1 receives the digital signal from the adder circuit 2 and converts it into an analog signal. And output.

  The filter 4 is a filter having a required signal band as a pass band, and filters the analog signal output from the D / A converter 1 to remove the colored noise added to the required signal in the digital domain. Retrieve the limited required signal.

  FIG. 2 is a diagram for explaining the operation of the signal conversion apparatus. In the adder circuit 2, the sampled required signal and the sampled colored noise are added, but the frequency band of the colored noise is outside the band of the required signal and therefore does not overlap on the frequency axis. Therefore, it is possible to remove the colored noise by the filter having the required signal as a pass band and extract only the required signal, and on the time axis, since the colored noise is added to the output of the adder circuit, The periodicity of each bit on the digital signal bus is lost, and as a result, spurious due to field-through is suppressed. Furthermore, since the added colored noise is removed after being converted into an analog signal, there is no influence of induction from the internal wiring of the D / A converter, which is a problem in the prior art, and it is good. A signal converter having spurious characteristics can be realized.

  Next, a configuration example of the colored noise generation circuit 3 will be described with reference to FIG. According to FIG. 1, the colored noise generating circuit 3 includes a frequency dividing circuit 31, a pseudo noise sequence generating circuit 32, a variable frequency oscillating circuit 33, and a multiplying circuit 34.

  The frequency dividing circuit 31 divides the sampling clock and inputs the divided clock to the pseudo noise sequence generating circuit 32. The pseudo noise sequence generation circuit 32 is driven by the input clock from the frequency dividing circuit 31, generates pseudo noise for each cycle of the input clock, and outputs it as a digital value.

  The variable frequency oscillation circuit 33 generates a sine wave having a frequency depending on the pseudo noise from the pseudo noise sequence generation circuit 32. For example, when the pseudo noise from the pseudo noise sequence generation circuit 32 is a 4-bit digital value m (m = 0 to 15), the variable frequency oscillation circuit 33 has f (m) = f0 · m (where, A sine wave having an oscillation frequency (f0 is a constant) is generated. However, the frequency of the sine wave is adjusted to be outside the required signal band. Thereby, the variable frequency oscillating circuit 33 randomly generates a sine wave in a predetermined frequency range along a time series.

  The multiplication circuit 34 multiplies the pseudo noise from the pseudo noise sequence generation circuit 32 and the sine wave from the variable frequency oscillation circuit 33 and outputs the result. As a result, the frequency band of the pseudo noise having a center frequency of 0 from the pseudo noise sequence generation circuit 32 is converted into a frequency band centered on the frequency of the sine wave from the variable frequency oscillation circuit 33. Since the frequency of the sine wave from the variable frequency oscillation circuit 33 changes randomly along the time series, the frequency converted output becomes the colored noise signal of FIG.

  As described above, the pseudo noise sequence generation circuit 32 is driven by the divided low-speed clock to generate colored noise. The pseudo noise sequence generation circuit 32 can be configured by a shift register and an exclusive OR circuit. Since a complicated circuit such as a narrow band filter is not required, a signal converter having a good spurious characteristic can be realized with a simple circuit configuration.

  Further, the variable frequency oscillation circuit 33 and the multiplication circuit 34 randomly hop the center frequency to which the pseudo noise is converted, so that the colored noise with a small average power density is generated while the total power is the same. Is possible. Thereby, the characteristic requested | required of the filter 4 which removes colored noise can be relieved.

  In the above example, the output of the pseudo noise sequence generation circuit 32 is used to change the oscillation frequency of the variable frequency oscillation circuit 33 at random. However, the oscillation frequency is randomly changed outside the required signal band. The configuration is not limited to the above.

It is a block diagram of the signal converter by this invention. It is a figure explaining operation | movement of a signal converter. It is a figure which shows each bit value of a digital signal bus in the case of outputting the sine wave of a period 8T by a 4-bit width digital signal bus, and its digital value. It is a block diagram for the spurious suppression by a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 D / A converter 2 Adder circuit 3 Colored noise generator circuit 4 Filter 5 Wiring section of digital signal bus 21-24, 26-29 Exclusive OR circuit 25, 32 Pseudo-noise series generator circuit 31 Divider circuit 33 Variable frequency Transmission circuit 34 Multiplication circuit

Claims (2)

A device that converts a digital signal obtained by sampling a band-limited required signal at a sampling clock speed and converting it into an analog signal,
Colored noise generating means for generating noise outside the band of the required signal;
Adding means for adding the noise and the digital signal;
D / A conversion means for converting the output signal of the addition means into an analog signal;
Filter means for filtering the required signal from the output signal of the D / A conversion means;
Equipped with a,
The colored noise generating means is
A frequency dividing means for dividing the sampling clock;
A pseudo noise sequence generating means that is driven by the output signal of the frequency dividing means and generates pseudo noise;
Random oscillation means for randomly generating a sine wave in a predetermined frequency range along a time series;
A frequency conversion means for converting the frequency of the output signal of the pseudo noise sequence generation means by the output signal of the random oscillation means;
Signal converting apparatus characterized by comprising a. A method for converting a digital signal obtained by sampling a band-limited required signal at a sampling clock speed and converting it into an analog signal,
Generate noise outside the band of the required signal,
Add the noise and the digital signal to convert to an analog signal,
Filtering the required signal from the analog signal by a filter ;
Noise outside the band of the required signal is
A sine wave in a predetermined frequency range is randomly generated along a time series,
A signal conversion method , wherein an output signal of a pseudo noise sequence generation circuit driven by a divided sampling clock is frequency-converted by the sine wave .

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