JPH08204558A - Da converter - Google Patents

Abstract

PURPOSE: To provide a DA converter, which effectively uses higher harmonics generated by sampling and lessens unnecessary spurious and noise, and the signal generator which uses this DA converter to generate a signal having a required frequency and lessens unnecessary spurious and noise. CONSTITUTION: The DA converter where the frequency characteristic is corrected and harmonic components generated by sampling are effectively used by extracting the fundamental wave component of a clock signal from this clock signal given to a DA converter 2 by a tank circuit 6 and multiplying the output signal of the DA converter 2 by this extracted signal is realized. The signal generator which has less unnecessary spurious and noise and generates a signal having a required frequency by providing this DA converter and a DDS (direct digital synthesizer) 7 and changing the clock frequency given to the DA converter 2 and the frequency setting code of the DDS 7 is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DA converter for converting a digital signal into an analog signal, a signal generator using the DA converter, and an FM modulator including the signal generator. .

[0002]

2. Description of the Related Art A conventional DA converter for correcting aperture distortion by multiplying an output signal of a DA converter by a pulse wave is disclosed, for example, in Japanese Patent Application No. 6-103149. . Hereinafter, this conventional technique will be described with reference to FIG. FIG. 3 shows a block configuration of this conventional technique. An n-bit (n is a natural number) digital signal is supplied from the digital signal input terminal 1 to the DA converter 2. In the DA converter 2, on the other hand, the n-bit digital signal is converted into an analog signal in accordance with the cycle of the clock signal supplied from the clock generator 5. The frequency characteristic of the output signal of the DA converter 2 is, for example, the characteristic shown in FIG. That is, in the figure, the frequency characteristic of the output signal of the DA converter 2 is that the spectrum a of the original analog signal is the sampling frequency (f
The signal spectrum c (dotted line) repeated for each s) has a frequency characteristic as shown by a solid line in which the characteristic b of the aperture effect (dotted line) is multiplied.

Here, when a frequency component of an analog signal is used up to near the Nyquist frequency (fs / 2) or a harmonic component by sampling is positively used,
Conventionally, a waveform shaping circuit 8 for performing a required waveform shaping process on a clock signal generated from a clock generator 5 is provided, and a signal shaped into a required pulse waveform and an output signal of the DA converter 2 are provided. Amplitude distortion due to the aperture effect is corrected by configuring the multiplier 3 to perform multiplication. In addition, the frequency component f of the output signal of the DA converter 2
As a configuration for converting the frequency into a required frequency band fcv, a signal fLO having a frequency fcv-f is generated from the local oscillator 9 as shown in FIG.
A configuration is generally used in which the output signal of the DA converter 2 and the output signal of the DA converter 2 are multiplied by the multiplier 3 to convert into a required frequency band fcv.

[0004]

Of the above-mentioned conventional examples,
Since the configuration shown in FIG. 3 is a configuration in which a pulse waveform signal containing many unnecessary harmonic components is applied to one input of the multiplier 3, the unnecessary harmonic components contained in this pulse wave signal, There is a drawback that the output signal of the DA converter 2 is multiplied and the output signal of the multiplier 3 also significantly increases unnecessary frequency components (spurious components) to be suppressed. In addition, there is a drawback that the harmonic component included in the pulse wave is also superimposed on the output signal of the filter 10 or the like provided in the subsequent stage of the multiplier 3 via the power supply line and the ground line of the circuit. In the frequency conversion circuit having the configuration shown in FIG. 8, spurious components are generated in the vicinity of the frequency-converted signal fcv depending on how the output signal fLO from the local oscillator 9 is selected.
There is a drawback that the filter in the latter stage cannot suppress it.

It is a first object of the present invention to provide a DA converter capable of removing a harmonic component contained in a pulse wave and obtaining an output signal having a remarkably small noise component including a spurious component. A second object of the present invention is to provide a signal generation device including a frequency conversion circuit capable of frequency conversion into a required frequency band and realizing a spurious component suppression filter in a subsequent stage with a relatively simple configuration. Especially.

[0006]

In order to achieve the first object, the present invention inputs a clock signal of a DA converting means and outputs a frequency component n times (n is a natural number) the fundamental wave component thereof. A tank circuit for extraction, and an output signal of the tank circuit and an output signal of the DA conversion means are multiplied to obtain a required output signal.

In order to achieve the above second object,
A digital signal generating means for generating a digital code signal of a predetermined frequency corresponding to a given output frequency setting code, a DA converting means for converting the digital code signal into an analog signal, the digital signal generating means and the DA converting means. A clock generating means for supplying a clock signal to the clock signal, and n times the fundamental wave component (n
Is a natural number) and a tank circuit for extracting a frequency component of the clock signal, and a multiplication means for multiplying an output signal of the tank circuit by an output signal of the DA conversion means to obtain a required output signal. Means for changing each of the frequency and the output frequency setting code in the digital signal generating means are provided.

[0008]

As a result, as for the input signal of the multiplier, the output signal of the tank circuit has less noise components such as spurious components as compared with the conventional pulse waveform signal, so that the noise component of the output signal of the multiplier is significantly reduced. As a result, it is possible to obtain a DA conversion output signal with a relatively simple configuration as described above, with low noise, and in which amplitude deterioration due to the aperture effect is corrected. Further, by using this DA converter, it is possible to realize a signal generator which is low in noise and can generate a required frequency signal.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the DA converter according to the present invention will be described below with reference to FIG. In FIG. 1, a digital signal input terminal 1 to which an n-bit digital signal is supplied
Is connected to the analog signal output terminal 4 via the DA converter 2, the multiplier 3, and the band pass filter 10. The output stage of the clock generator 5 is connected to the DA converter 2 and the tank circuit 6, and the tank circuit 6 is
It is connected to the other input of the multiplier 3.

This operation will be described below. From the digital signal input terminal 1 to the DA converter 2, n bits (n
Is a natural number). In the DA converter 2, on the other hand, in accordance with the cycle of the clock signal (frequency fs) supplied from the clock generator 5, the n
The bit digital signal is converted to an analog signal.
The frequency characteristic of the output signal of the DA converter 2 is, for example,
The characteristics are as shown in FIG. That is, in the figure, the frequency characteristic of the output signal of the DA converter 2 shows that the spectrum a of the original analog signal is an isolated rectangular pulse with respect to the signal spectrum c (broken line) repeated for each sampling frequency (fs). Fourier transform, that is, a frequency characteristic shown by a solid line multiplied by the characteristic b of the aperture effect (dotted line).

On the other hand, the clock signal output from the clock generator 5 is applied to the tank circuit 6, and a sine wave which is a fundamental wave component of the input clock signal is output. When the sine wave from the tank circuit 6 and the output signal of the DA converter 2 are multiplied by the multiplier 3, the frequency characteristic of the aperture effect is frequency-converted, and as shown in FIG. 5, the amplitude due to the frequency characteristic of the aperture effect is changed. The DA conversion output with the deterioration corrected is obtained. In the figure, a'represents the spectrum of the output signal at the analog signal output terminal 4 of the DA converter shown in FIG. 1, b'is the frequency characteristic of the frequency-converted aperture effect, and d is the bandpass filter 10 of FIG. Shows the frequency characteristics of. Here, if the tuning frequency f of the tank circuit 6 is tuned to the n-th harmonic (n is a natural number) harmonic of the clock frequency fs, similarly with respect to the n-fold sampling frequency fs, the amplitude distortion due to the aperture effect can be reduced. , Can be corrected. As a specific example of the circuit configuration of the tank circuit 6, for example, the circuit configuration as shown in FIG. 9 can be realized. In the figure, the tuning frequency in the tank circuit can be easily changed by changing the value of L or C in the circuit, and the required frequency (n times the clock frequency fs (n is a natural number) can be calculated from the input clock signal. It is possible to extract the signal component tuned to the frequency).

Next, an embodiment of an embodiment of the signal generator according to the present invention will be described with reference to FIG. A digital signal input terminal 1 to which a k-bit (k is a natural number) digital signal is supplied is a direct digital synthesizer.
It is connected to the analog signal output terminal 4 via the (DDS) 7, the DA converter 2, the multiplier 3, and the band limiting filter 10. On the other hand, the control signal input terminal 12 is connected to the variable frequency oscillator 11. The output stage of the frequency variable oscillator 11 is connected to the DDS 7, the DA converter 2, and the tank circuit 6, and the tank circuit 6 is connected to the other input of the multiplier 3. Hereinafter, this operation will be described. DD
S7 is a k-bit digital code signal supplied from the digital signal input terminal 1 and the frequency variable oscillator 11
It is an element for generating an n-bit digital code signal corresponding to the frequency (fs) of the clock signal supplied from the device. The n-bit digital code signal output from the DDS 7 is input to the DA converter 2. The subsequent operation of this embodiment is the same as that of the above-described embodiment of the DA converter. Here, it is considered that the DDS 7 is used to generate a frequency extremely lower than the sampling frequency (clock frequency). At this time, the oscillation frequency of the variable frequency oscillator 11 is assumed to be constant. As a specific example, when the clock frequency is 20 MHz and the frequency generated using the DDS 7 is 100 kHz, the spectrum of the output signal of the multiplier 3 is as shown in FIG. Among them, the signal actually required is the desired wave A (19.9M
Hz), it is necessary for the band limiting filter 10 to suppress frequency components other than the desired wave A. However, in order to sufficiently suppress the spurious component B that is closest to the desired wave A, a bandpass filter having a very high Q is required, which is difficult to realize. For example, when the spectrum of the output signal of the multiplier 3 has the characteristic shown in FIG. 6A, in order to obtain the spurious characteristic −70 dBc in the output signal of the band limiting filter 10, the band limiting filter 10 is , Q; 2000 6th order BPF
Must be used, which is virtually impossible to implement.

Here, when the oscillation frequency of the variable frequency oscillator 11 is controlled to be 30 MHz and the frequency generated by the DDS 7 is set to 10.1 MHz, the multiplier 3 thereof is set.
The output spectrum of has the characteristic shown in FIG. 6 (b), the frequency interval between the desired wave A and the spurious component is widened, the filter for suppressing the closest spurious component B is easily realized, and spurious and noise are generated. A small DA conversion output can be obtained. For example, when the spectrum of the output signal of the multiplier 3 has the characteristic shown in FIG. 6 (b),
In order to obtain a spurious characteristic of −70 dBc in the output signal of the band limiting filter 10, the band limiting filter 10 has a Q; 20 as shown in FIG.
It can be easily realized by using the BPF of the sixth order. In this way, the k-bit digital code given to the DDS 7 and the oscillation frequency (fs) of the frequency variable oscillator 11
By changing the, it becomes possible to frequency-convert the output of the DA converter 2 into a desired frequency region. The frequency variable oscillator 11 in the present embodiment can be specifically configured by using another DDS as well as a VCO (voltage controlled oscillator) or a VCXO (voltage controlled crystal oscillator).

Next, an application example of the present invention using this embodiment will be described with reference to FIG. FIG. 7 is a block diagram of an FM modulator configured by using the signal generator shown in FIG. The modulated wave input terminal 13 is provided with an adder 14 and a DDS.
7, DA converter 2, multiplier 3, and bandpass filter 1
It is connected to the analog signal output terminal 4 via 0. On the other hand, the control signal input terminal 12 is connected to the DDS 7, the DA converter 2, and the tank circuit 6 via the variable frequency oscillator 11, and the tank circuit 6 is connected to the other input of the multiplier 3. Hereinafter, this operation will be described. As described above, the DDS 7 can determine the clock frequency and the frequency of the signal generated according to the input digital code. Here, modulated wave input terminal 1
If the data input from 3 is 0, the circuit in FIG. 7 is equivalent to that in FIG. 2, and a single frequency component is output, which is a constant Kw that determines the center frequency of the FM modulated wave. An FM modulated wave is generated by adding a modulated wave component to this center frequency, changing the code input to the DDS 7 and changing the frequency. Subsequent operations are the same as those of the embodiment shown in FIG. 2, and it is possible to generate an FM modulated wave with less spurious components and noise in the required frequency band.

[0015]

As described above, according to the present invention, the amplitude frequency characteristic is corrected to effectively use the harmonic component of the DA converter output, and the DA component having a small noise component including the spurious component is small. It is possible to realize a DA converter that can obtain a converted output signal. Further, by using this DA converter, it is possible to realize a signal generator capable of generating a desired frequency signal with low noise, and an FM modulator.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a DA converter according to the present invention.

FIG. 2 is a block diagram showing an embodiment of a signal generator according to the present invention.

FIG. 3 is a block diagram showing a configuration example of a conventional DA converter.

FIG. 4 is a diagram showing an example of a frequency spectrum in an output signal of a DA converter.

FIG. 5 is a diagram showing an example of a frequency spectrum in an output signal of the DA converter according to the present invention.

FIG. 6 is a diagram showing an example of a frequency spectrum in an output signal of the signal generator according to the present invention.

FIG. 7 is a block diagram showing an application example of the present invention.

FIG. 8 is a block diagram showing a configuration example of a frequency conversion circuit including a conventional DA converter.

FIG. 9 is a circuit diagram showing an internal configuration example of a tank circuit according to the present invention.

FIG. 10 is a diagram showing an example of characteristics of a bandpass filter according to the present invention.

[Explanation of symbols]

1 ... Digital signal input terminal 2 ... DA converter 3 ... Multiplier 4 ... Analog signal output terminal 5 ... Clock generator 6 ... Tank circuit 7 ... Direct digital synthesizer 8 ... Pulse waveform shaping circuit 9 ... Fixed oscillator 10 ... Band Pass filter 11 ... Frequency variable oscillator 12 ... Control signal input terminal 13 ... Modulated wave input terminal 14 ... Adder a ... Original analog signal b ... Frequency characteristic of aperture effect c ... DA converter output when not affected by aperture effect Signal d ... Frequency characteristic of band-pass filter a '... Output signal of DA converter shown in FIG. 1 b' ... Frequency characteristic of aperture-converted aperture effect A ... Desired wave B ... Spurious component

Claims (4)

[Claims] 1. A DA converting means for converting an input digital signal into an analog signal, a clock generating means for supplying a clock signal to the DA converting means, and n times the fundamental wave component of the clock signal (n is a natural number). ), A tank circuit for extracting the frequency component of), and a multiplication means for multiplying the output signal of the tank circuit by the output signal of the DA conversion means to obtain a required output signal.
Conversion device. 2. A digital signal generating means for generating a digital code signal having a predetermined frequency corresponding to a given output frequency setting code, a DA converting means for converting the digital code signal into an analog signal, and the digital signal generating means. Generating means for supplying a clock signal to the means and the DA converting means, a tank circuit for extracting a frequency component of n times (n is a natural number) the fundamental wave component from the clock signal, and an output signal of the tank circuit And a multiplication means for multiplying the output signal of the DA conversion means to obtain a desired output signal. 3. The signal generator according to claim 2, further comprising means for changing each of the frequency of the clock signal and the output frequency setting code in the digital signal generating means. . 4. An F including the signal generator according to claim 3.
M modulator.