JPH05292133A - Digital demodulation circuit - Google Patents

Abstract

PURPOSE:To obtain a digital demodulation circuit suitable for the demodulation of a broad band modulation wave signal. CONSTITUTION:An analog LPF 1 implements band limit by using fS1/2 as a cut-off frequency of an input signal to eliminate the effect of aliasing. An A/D converter 2 quantizes the signal subject to band limit by using a sampling frequency fS1 being twice the highest effective frequency of a broad band modulation wave signal. A demodulation computing element 4 implements demodulation arithmetic operation by digital signal processing based on an output signal of a converter 2 at the same sampling rate and an output signal of a modulation signal generator 3. A comb-line filter 5, a decimeter 6 and an FIR filter 7 form entirely a digital LPF of a low-degree number, applies band limit processing as to an output of the computing element 4 thereby improving the S/N, that is, expanding the dynamic range. A decimater 8 applies interleave processing to the output of the filter 7 to decrease a sampling rate without deteriorating the S/N and to facilitate the processing at the post stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital demodulation circuit, and more particularly to a digital demodulation circuit suitable for demodulating a wideband modulated wave signal such as a spread spectrum signal.

[0002]

2. Description of the Related Art As is well known, as a demodulation method of a modulated wave signal, an analog demodulation method in which a mixer or the like is used to demodulate in a video band or an intermediate frequency band, and a signal in the video band or the intermediate frequency band is digitized There is a digital demodulation method that performs demodulation by digital calculation. In demodulation of a modulated wave signal, it is desirable to perform a highly accurate demodulation process and secure an appropriate SN ratio in all of the band (that is, secure a sufficient dynamic range). Be done.

[0003]

However, when these methods are applied to wideband modulated wave signals, there are advantages and disadvantages. That is, in the analog demodulation method, it is not so difficult to secure an appropriate SN ratio, but it is difficult to perform highly accurate demodulation processing due to the non-linear characteristic due to the use of the analog element and the characteristic change due to the environmental change. On the other hand, in the digital demodulation method, although highly accurate demodulation processing can be performed without any problem in principle, there is a difficulty in securing an appropriate SN ratio depending on the resolution of the A / D converter.

An object of the present invention is to enable a highly accurate demodulation process in the demodulation of a wideband modulated wave signal and to secure an appropriate SN ratio in all of the band, and at the same time, to facilitate the signal process in the subsequent stage. Another object of the present invention is to provide a digital demodulation circuit.

[0005]

In order to achieve the above object, the digital demodulation circuit of the present invention has the following configuration. That is, the digital demodulation circuit of the first invention is an analog LPF that performs band limiting processing on a video signal or an intermediate frequency band signal that is a wideband modulated wave signal; a sampling frequency that is at least twice the highest effective frequency of the wideband modulated wave signal. An A / D converter that digitizes the output of the analog LPF; a digital demodulator that performs a demodulation operation on the output of the A / D converter at the same sampling rate as the sampling frequency; and a band limitation on the output of the digital demodulator. Digital L processing
A PF; a first decimator that performs a sampling rate thinning process on the output of the digital LPF;
It is characterized by having.

A second aspect of the present invention is the digital demodulator circuit of the first aspect of the present invention;
F is a filter for filtering the output of the digital demodulator, and has a frequency f _S2 (f _S2 = f _S1 / m:
Here, m is an integer, and f _S1 is a comb filter having an attenuation range at an integer multiple of the sampling frequency of the A / D converter; and a second decimator for performing m: 1 decimation processing on the output of the comb filter. The cut-off frequency f _S3 (f _S3 = f _S2 // of the output signal of the second decimator
n: where n is an integer) and a FIR filter for limiting the band to half, and the first decimator is the F
The output of the IR filter is thinned by n: 1;

[0007]

Next, the operation of the digital demodulation circuit of the present invention configured as described above will be described. The digital demodulation circuit of the present invention is a system that combines an oversampling A / D conversion technique, a digital demodulation technique, and a decimation technique for improving quantization noise in A / D conversion.

In the oversampling A / D conversion, the SN ratio can be improved as follows. That is, in normal A / D conversion, as shown in FIG. 1A, the lower the sampling frequency f _{S, the} greater the influence of aliasing and the larger the noise density in the signal band.
As shown in FIG. 1B, the sampling frequency is k times k.
If f _S , the in-band noise density is reduced to 1 / k, and L
When the band is limited by the PF, the noise energy can be significantly reduced as shown in FIG.

Therefore, in the present invention, the sampling frequency of the A / D converter is set to be at least twice the maximum effective frequency of the wideband modulated wave signal, and demodulation calculation is performed at the same sampling rate after A / D conversion (digital demodulator). ), To improve the accuracy of the demodulation process. Then, based on the idea shown in FIG. 1C, band limitation is applied to the output signal of the digital demodulator (digital LPF) to obtain a narrow band signal with an improved SN ratio. Further, decimation is performed on the narrow band signal (first decimator) to reduce the sampling rate without deterioration of the SN ratio. This facilitates signal processing in the subsequent stage.

As is well known, the digital filter is designed on the basis of the transfer function. For example, as in the second invention, a digital LPF includes a comb filter and a second filter.
When the decimator and the FIR filter are used, the order of the transfer function can be reduced as compared with the case where the filter is configured as one independent filter, and the design can be facilitated.

According to the second invention, the noise level is
It becomes 1 / (m × n), and the SN ratio is improved by m × n times.
The sample rate of the output of the first decimator is (m ×
n): 1 is the thinned out.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a digital demodulation circuit according to an embodiment of the present invention. In FIG. 2, the input of the analog LPF 1 is a video band signal or an intermediate frequency band signal of a wideband modulated wave signal, but the analog LPF ₁ uses f _S1 / 2 (f _S1 is the sampling frequency of the A / D converter 2 for this input signal. ) Is performed as the cutoff frequency to limit the band, and the effect of aliasing is removed.

The A / D converter 2 quantizes the band-limited signal at the sampling frequency f _S1 and converts it into a digital signal. At this time, the sampling frequency f _S1
Is more than twice the maximum effective frequency of the broadband modulated wave signal.

The modulation signal generator 3 also generates a modulation signal in synchronization with the output signal of the A / D converter 2.

The demodulation arithmetic unit 4 uses the same sampling rate as the sampling frequency of the A / D converter 2 for A / D conversion.
Based on the output signal of the converter 2 and the output signal of the modulation signal generator 3, demodulation calculation by digital signal processing is performed.

Next, the comb filter 5, the (second) decimator 6 and the FIR filter 7 constitute a digital LPF as a whole.

First, as shown in FIG. 3A, the comb filter 5 has a frequency characteristic having an attenuation range at an integral multiple of the frequency f _S2 where f _S2 = f _S1 / m (m is an integer). Then, the output of the demodulation calculator 4 is filtered.

Next, the (second) decimator 6 decimates the output of the comb filter 5 by m: 1,
A digital signal of sampling frequency f _S2 is output.

The FIR filter 7 is shown in FIG.
The cut-off frequency f _S3 (f _S3 = f _S2 / n:
Is a whole number).

In summary, the band limiting process described above can be performed by omitting 5 and 6 and by using the FIR filter 7 alone, but if this is done, the order becomes considerably large. Therefore, as shown in FIG. , 7) and the decimator 6 are combined, as shown in FIG. 3C, a digital L having frequency characteristics with steep characteristics and a small ratio band.
The PF can be constructed with a low order. This digital L
The noise level becomes 1 / (m × n) due to PF, and SN
The ratio is improved by (m × n) times, and the dynamic range can be expanded.

Finally, the (first) decimator 8 is
The output of the R filter 7 is thinned by n: 1,
A digital signal of sampling frequency f _S3 is output.
As a result, the sample rate of the output of the decimator 8 is
Since (m × n): 1 is thinned out, the processing in the subsequent stage can be facilitated.

As described above, this digital demodulation circuit can handle a modulated signal with a frequency up to f _S1 / 2, and the maximum frequency of the demodulated signal is f _S3 / 2, but a wideband modulated wave signal is used. It can be used as a demodulation circuit of a communication device or a radar device to be handled.

[0023]

As described above, according to the digital demodulation circuit of the present invention, the oversampling A / D conversion technique, the digital demodulation technique, and the decimation technique for improving the quantization noise in the A / D conversion are provided. The combined configuration enables highly accurate demodulation processing in demodulation of a wideband modulated wave signal and secures an appropriate SN ratio in all of the band, and also simplifies signal processing in the subsequent stage. There is an effect that a digital demodulation circuit can be provided. According to the second invention, there is an effect that the digital LPF can be configured with a low order.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an operation principle of the present invention, in which (a) is a power spectrum diagram when a normal A / D conversion is performed,
(B) is a power spectrum diagram when oversampling A / D conversion is performed, and (c) is a power spectrum diagram when (b) is band-limited.

FIG. 2 is a configuration block diagram of a digital demodulation circuit according to an embodiment of the present invention.

3A and 3B are operation explanatory diagrams of the digital LPF in the embodiment circuit, FIG. 3A is a frequency characteristic diagram of the comb filter 5, FIG. 3B is a frequency characteristic diagram of the FIR filter 7, and FIG.
Comb filter 5, decimator 6, FIR filter 7
FIG. 5 is a frequency characteristic diagram in the case of connecting in series.

[Explanation of symbols]

 1 analog LPF 2 A / D converter 3 modulation signal generator 4 demodulation calculator 5 comb filter 6 decimator 7 FIR filter 8 decimator

Claims (2)

[Claims] 1. An analog LPF that performs band limiting processing on a video signal or an intermediate frequency band signal that is a wideband modulated wave signal; and the analog LPF at a sampling frequency that is at least twice the highest effective frequency of the wideband modulated wave signal.
An A / D converter for digitizing the output of the digital demodulator; a digital demodulator for demodulating the output of the A / D converter at the same sampling rate as the sampling frequency; and a digital for performing band limiting processing on the output of the digital demodulator. LPF; and the digital LPF
A first decimator for thinning the sampling rate of the output of the digital demodulator; 2. The digital demodulation circuit according to claim 1, wherein the digital LPF is a filter that performs a filtering process on the output of the digital demodulator,
A comb filter having an attenuation range at an integer multiple of the frequency f _S2 (f _S2 = f _S1 / m: where m is an integer and f _S1 is the sampling frequency of the A / D converter); and about the output of the comb filter a second decimator that performs a thinning process of m: 1; and an output signal of the second decimator is limited to a half band of a cutoff frequency f _S3 (f _S3 = f _S2 / n: where n is an integer). And an FIR filter;
The first decimator performs an n: 1 decimation process on the output of the FIR filter;