JPH066596Y2 - Modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a digital signal processing type modulator, and particularly in a communication device or the like, a frequency (FM) modulated wave or an amplitude (A)
M) A modulator that generates an analog modulated wave such as a modulated wave.

(Prior Art) As one conventional method for generating a modulated wave by a digital signal processing method, for example, as shown in FIG. 3, an input signal is supplied to a low-pass filter 1 to apply a certain band limitation to perform A / D conversion. It is supplied to the device 2 and converted into digital data. Here, for example, when an AM modulated wave with a carrier frequency of _C and an occupied bandwidth of Δ is extracted, ( _C + Δ /
The signal wave is sampled in the A / D converter 2 at a frequency _{S that} is at least twice that in 2). The digital data output from the A / D converter 2 is supplied to the digital signal processing circuit 3, multiplied by the discrete carrier signal of the frequency _{C by} the multiplier, and the multiplication output is converted into an analog signal by the D / A converter 4. After returning, the analog AM modulated wave is taken out via the low-pass filter 5 having the frequency _S / 2 as the cutoff frequency.

As another method, as shown in FIG. 4, a signal wave is sampled at a frequency _S ′ lower than the intermediate frequency ₂ desired to be taken out, converted into digital data by the A / D converter 2, and then converted into a digital signal. The processing circuit 3 multiplies the input signal by the discrete carrier signal of frequency ₁ and then the analog signal is converted by the D / A converter 4, and the modulated wave of the intermediate frequency ₁ is once passed through the low-pass of the cut-off frequency _S '/ 2. This modulated wave and frequency ( ₁ +
₂ ) The sine wave of ₂ ) is multiplied by the analog multiplier 7, that is, mixing processing is performed, and the output of the multiplier 7 is set to the center frequency ₂
The modulated wave of the intermediate frequency ₂ is extracted by supplying it to the band pass filter 8 of FIG.

(Problems to be solved by the invention) In the former case of the above-mentioned conventional example, the carrier frequency _C
As _S becomes higher, the sampling frequency _S must be increased accordingly. Therefore, there is a problem that a high-speed A / D converter is required.

Also, in the latter case of the conventional example, a bandpass filter and a mixing circuit must be separately provided, which results in a problem that the circuit scale becomes large, and spurious generated by the mixing process does not enter the band. In addition, the sampling frequencies _{S 1} , ₁ and ₂ must be properly selected, and there is a problem that the application range is limited due to the degree of freedom in frequency selection.

The present invention solves the above-mentioned problems and provides a modulator of digital signal processing system which uses a low-speed A / D converter and can extract a modulated wave of a desired intermediate frequency by adding a small number of circuits. The purpose is to

(Means and Actions for Solving Problems) The present invention has the following structure in order to solve the above problems.

The input analog signal wave is converted to digital data, the converted digital data is subjected to a predetermined digital operation for modulation processing, and the modulated digital data is converted to an analog signal by a digital / analog converter to convert the analog modulation wave. In a digital signal processing type modulator to be taken out, the output of the digital / analog converter is used as an input, and the output of the digital / analog converter is converted into a desired harmonic component at a sampling period during digital conversion of the input analog signal wave. An analog switch for taking out only a time width determined so that the level becomes a peak value on a frequency spectrum, and a bandpass filter having a frequency of the desired harmonic component as a passband are provided, and through the bandpass filter It was taken out as an analog modulated wave.

Originally, the signal obtained by sampling an analog signal contains not only the frequency component of the original analog signal but also harmonic components related by an integral multiple of the sampling frequency.The spectrum of these harmonic components is the original spectrum. Has been shifted in frequency. Further, this relationship is retained even if arbitrary digital signal processing is performed after quantizing the signal obtained by sampling. The present invention utilizes this property,
Sampling is performed at a relatively low frequency, digital modulation processing is performed, and then digital / analog conversion into an analog modulated wave is taken out again for a predetermined time with an analog switch to obtain a digital modulated wave. The harmonic component of the desired band of is to be extracted. However, since the level of the harmonic component is closely related to the pulse width of the sampled signal, in order to efficiently extract the desired harmonic component in the level, D / A
It is necessary to extract the output at a sample interval for a certain period and properly select the extraction time width. Usually, the time width is selected so as to maximize the power level near the carrier frequency to be extracted.

As is well known (for example, Shin OHM Bunko "Digital Signal Processing" Shigeo Tsujii, Hajime Kubota, published by Ohmsha, Sep. 25, 1986, pp. 27-31), pulse width τ, period 1 /
The amplitude spectrum of the pulse train with different _S is shown by Cn = E · _S · τ · (sinπnτ / πnτ). Here, _S is the sampling frequency, τ is the extraction time width by the analog switch, and E is the level of the pulses forming the pulse train. This is illustrated in FIG. The envelope curve of the amplitude spectrum follows (sinx / x), which is a sampling function. The pulse width (time width) τ of each pulse in the pulse train is the sampling period (1 /
Fig. 5 (a ₁ ) shows the pulse train when it is set to 1/2 of _S ).
Fig. 5 (a ₂ ) shows the amplitude spectrum.
The case where the time width τ is constant and the sampling period is 2 / _S is shown in FIGS. 5 (b ₁ ) and (b ₂ ) respectively. In this case, since the time width τ does not change, the envelope of the amplitude spectrum does not change. FIG. 5 (c ₁ ) shows the case where the time width τ is τ / 2 and the sampling period is 2 / _S , respectively.
It is shown in (c ₂ ). In this case, the shape of the envelope of the amplitude spectrum has changed.

In this way, by changing the time width when the signal is taken out by the analog switch, it becomes possible to bring the frequency component having the desired harmonic component to the peak point of the power level on the frequency spectrum. Finally, the analog modulated wave of the intermediate frequency _C is obtained by taking out the output from the analog switch through the bandpass filter having the carrier frequency as the center frequency.

(Example) Hereinafter, the present invention will be described with reference to an example.

FIG. 1 is a block diagram showing the construction of an embodiment of the present invention.

A signal wave such as an audio signal is supplied to the low-pass filter 1 to apply a certain band limitation, and the output of the low-pass filter 1 is A / D.
It is supplied to the converter 2 and converted into digital data. A / D
The digital data converted at the sampling frequency _S ″ by the converter 2 is subjected to a predetermined modulation algorithm operation using the carrier signal _DC digitized by the digital signal processing circuit 3. The output from the digital signal processing circuit 3 is D /
The analog signal is supplied to the A converter 4 and converted into an analog signal, and the analog signal converted by the D / A converter 4 is the analog switch 1
0, and the converted analog signal is taken out for a period of a predetermined time width in the sampling cycle of the A / D converter 2. The signal cut out by the analog switch 10 is supplied to a bandpass filter 11 having a center frequency _C and a bandwidth corresponding to the intermediate frequency component to be extracted.

Note that the sampling frequency _S ″, the carrier frequency
_DC and the intermediate frequency _C to be taken out are n _S ″ ±
_It shall be selected so that _DC = _C (n: natural number) holds.

The operation of the present embodiment configured as described above will be described by taking the case of DSB modulation as an example.

An input analog signal such as a voice signal is band-limited to a predetermined bandwidth, for example, 3 KHz or less by the low-pass filter 1 and input to the A / D converter 2 as a signal A. The frequency spectrum and waveform of the signal A are as shown in FIGS. 2 (a ₁ ) and (a ₂ ).

The signal A is sampled at the sampling frequency _S ″ by the A / D converter 2.
Are sampled and converted into digital data B. The frequency spectrum and the waveform of the converted digital data B are as shown in FIGS. 2 (b ₁ ) and (b ₂ ) and show the characteristic that the spectrum is arranged at intervals of the frequency _S ″. It is shown schematically.

The digital data B is subjected to DSB modulation processing in digital format in the digital signal processing circuit 3. D
For the SB modulation process, if the sine wave data sequence of the carrier frequency _DC is _DC = _S ″ / 4, +1,0,
, -1, 0, +1, 0, ..., This data series is multiplied by the digital data B, and digital DSB modulation is performed. The spectrum of the modulation output from the digital signal processing circuit 3 is (n ± 1/4) as shown in FIG. 2 (c ₁ ).
_The waveform generated at the position of _S ″ and shown schematically is shown in FIG. 2 (c ₂ ).
As shown in.

The output of the digital signal processing circuit 3 is supplied to the D / A converter 4 and converted into an analog signal.
Thus, the converted analog signal is taken out by the D / A converter 4 for each sampling period (1 / _S ″) of the input in the A / D converter 2 with the time width τ. As a result, the spectrum of the output from the analog switch 10 is The waveform is as shown in Fig. 2 (d ₁ ) and the waveform is as shown in Fig. 2 (d ₂ ) The spectrum at this time is the same as the output spectrum of the digital signal processing circuit 3 and S () = τ | sin ( πτ) / πτ |, and the time width τ is the intermediate frequency to be extracted.
The power level in the vicinity of _C (in this case _C = 5/4 _S ″) is selected to be maximum. That is, the power spectrum corresponds to the square of the amplitude spectrum Cn in FIG. This is the same as S () shown in Fig. 2 (d ₁ ) If the desired intermediate frequency _C is the frequency _C at which the power spectrum shown in Fig. 5 (b ₂ ) is 0. , If the extraction time width τ by the analog switch 10 is changed to τ / 2 as shown in FIG. 5 (c ₁ ), the second peak value of the power spectrum in FIG. 5 (c ₂ ) becomes the intermediate frequency. It is possible to bring in _{C. In} general, τ at the nth peak value of the power spectrum
= 1 / ( _C · 2 / (2n−1)).

The signal taken out from the analog switch 10 is taken out through a band pass filter 11 having a center frequency _{C 2} , which in this example is slightly over a bandwidth of 6 kHz. The spectrum of the DSB modulated wave extracted through the bandpass filter 11 is as shown in FIG. 2 (e ₁ ) and the waveform is as shown in FIG. ₂ (e ₂ ).

As described above, the output data from the A / D converter 2 includes not only the frequency component of the original signal but also the harmonic component related by n _S ″ ±, and is subjected to digital signal processing and analog conversion. After that, the period τ is selected so as to maximize the power level in the vicinity of the carrier frequency _C in the analog switch 10, and is taken out at each sampling cycle.Therefore, the D / A converter 4 and the analog switch 10 are just mixing operations of the conventional example. It can be said that the operation is equivalent to that of the multiplier 7 that performs

In addition, since the above harmonic output originally included in the output from the A / D converter 2 is extracted and used,
The sampling frequency _S ″ of the D converter 2 is n _S ″ ±
_As long as a selection is made within a range satisfying the relational expression _DC = _C , it is possible in principle to slightly exceed 12 kHz (twice the occupied bandwidth of the DSB signal). On the other hand, for example, when a modulated wave of an intermediate frequency having a center frequency _C of 455 kHz is desired, the sampling frequency _S needs to be approximately 1 MHz in the former example of the related art (example shown in FIG. 3). Comparing the two, it can be seen that an extremely low sampling frequency _S ″ is sufficient in this embodiment.

(Effect of the Invention) As described above, according to the present invention, it is possible to use, for example, an A / D converter for the audio band without requiring a high-speed A / D converter.

In addition, as compared with the former example of the conventional example, the low-pass filter 5
Is required to be replaced with the bandpass filter 11, and only an additional analog switch is required, which does not increase the circuit configuration scale.

Furthermore, even when it is desired to extract an arbitrary intermediate frequency modulated wave, the sampling frequency can be appropriately selected, and the degree of freedom is increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a spectrum distribution diagram and a waveform diagram for explaining the operation of the present invention. 3 and 4 are block diagrams showing a configuration of a conventional example. FIG. 5 is a diagram showing a pulse train and its amplitude spectrum. 1 ... Low pass filter, 2 ... A / D converter, 3 ... Digital signal processing circuit, 4 ... D / A converter, 10 ... Analog switch, 11 ... Band pass filter.

Claims (1)

[Scope of utility model registration request] 1. An input analog signal wave is converted into digital data, a digital operation is performed on the converted digital data to perform modulation processing, and the modulated digital data is converted into an analog signal by a digital / analog converter. In a modulator of a digital signal processing system for extracting a modulated wave, the output of the digital / analog converter is input, and the output of the digital / analog converter is set to a desired value at a sampling period during digital conversion of the input analog signal wave. An analog switch for taking out only a time width determined so that the level of the harmonic component becomes a peak value on the frequency spectrum, and a bandpass filter having a frequency of the desired harmonic component as a pass band, the bandpass filter It was taken out as an analog modulated wave via Modulator characterized and.