JPH11127087A - Digital demodulating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate sensitivity suppression by providing a band-pass filter right before a variable gain amplifier. SOLUTION: A 1st mixer (MIX) 5 mixes a received 1F signal and the output of a 1st local oscillator 4 and performs conversion to 70 MHz as a 1st intermediate frequency and the band-pass filter 6 passes only a necessary band signal. The 1st intermediate frequency IF1 is inputted to a 2nd MIX9, which mixes the 1st intermediate frequency IF1 and the output of a 2nd local oscillator B and performs conversion to 455 kHz as a 2nd intermediate frequency. After the 2nd intermediate frequency IF2 is inputted to the band-pass filter 17 to remove an unnecessary wave such as a disturbing wave, its output is inputted to the variable gain amplifier 18 having its gain controlled with an AGC voltage to set the level of the 2nd intermediate frequency IF2. The 2nd intermediate frequency IF2 is converted by an A/D converter 11 into a digital signal. A digital signal processor 12 digitally processes the digital IF signal, performs channel filtering, and imposes amplitude modulation and demodulation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital demodulator, and more particularly to a digital demodulator in a demodulator using amplitude modulation for wireless communication.

[0002]

2. Description of the Related Art Since the reception level fluctuation of a radio device is large,
Usually, an amplifier having a large gain is used to compensate for the level fluctuation, and a variable gain amplifier whose gain is controlled by an AGC voltage is used. The AGC system also has various circuit configurations. FIG. 2 is a schematic diagram showing a configuration example of a conventional digital demodulation device. In this figure, a digital demodulation device includes an antenna 1 for receiving a radio wave, a band-pass filter 2, a variable attenuator 3 that varies with an AGC voltage,
First local oscillator 4, received signal and first local oscillator 4
A first MIX 5 for mixing the output of the first mixer, a band-pass filter 6, and a variable gain amplifier 7 that varies with the AGC voltage.
, A second local oscillator 8, and a second MI that mixes the output signal of the first MIX 5 and the output of the second local oscillator 8.
X9, an amplifier 10, an A / D converter 11 for digitizing an analog signal, and a digital signal processor 12 for digitally processing the digitized signal.
A D / A converter 13 for converting a digital signal of the processed AGC voltage into an analog signal, and a low-pass filter 14
And a D / A converter 15 for converting the processed digital signal of the demodulated output into an analog signal, and a low-pass filter 16.

[0003] The digital demodulation device thus configured operates as follows. This demodulation device is generally called a double super system, and includes a first intermediate frequency IF1 and a second intermediate frequency IF2. An amplitude-modulated radio wave in the 2 to 30 MHz band is received by the antenna 1, and the received RF signal is passed through a band-pass filter 2 to extract only a necessary band. The passed RF signal is supplied to an AG (described later) in order to prepare for a sudden level increase.
The signal is input to the variable attenuator 3 controlled by the C voltage, the level is adjusted, and the output is input to the first MIX 5. In the first MIX 5, the received RF signal and the output of the first local oscillator 4 are mixed, and converted to 70 MHz as a first intermediate frequency IF1. The converted first intermediate frequency IF1 is input to the band-pass filter 6, passes only the required band signal, and is input to a variable gain amplifier 7 whose gain is controlled by an AGC voltage described later. The variable gain amplifier 7 keeps the level of the first intermediate frequency IF1 constant even when the reception level of the radio wave fluctuates, and normally guarantees the level fluctuation of about 100 dB. Next, the first intermediate frequency IF1 that has become a constant level is input to the second MIX9,
The first intermediate frequency IF1 and the output of the second local oscillator 8 are mixed and converted to 455 kHz as a second intermediate frequency IF2. Next, the amplifier 10 amplifies the second intermediate frequency IF2 to a level suitable for analog / digital conversion performed in the subsequent stage. The amplified second intermediate frequency IF2 is A /
The digital signal is converted into a digital signal of a required number of bits by the D converter 11 and input to the digital signal processor 12.
The digital signal processor 12 digitally processes the input digital IF2 signal to perform channel filtering and demodulation of amplitude modulation. The digital signal processor 12 is provided with the variable attenuator 3 described above.
In addition, digital arithmetic processing for controlling the AGC voltage applied to the variable gain amplifier 7 is also performed, and the level of the intermediate frequency IF is kept constant so that demodulation can be reliably performed regardless of the level of the radio wave reception level. The digital signal of the AGC voltage subjected to the digital operation processing is input to the D / A converter 13 and converted into an analog AGC voltage. Further, this AGC voltage is input to a low-pass filter 14 for removing unnecessary high-frequency components, and the output is input to the variable attenuator 3 and the variable gain amplifier 7 described above. On the other hand, an audio signal obtained by performing digital arithmetic processing on channel filtering and demodulation of amplitude modulation is converted into an analog signal by a D / A converter 15, and further a low-pass filter 16 is used to remove unnecessary high-frequency components. Through to the final output.

[0004]

However, in the above-described conventional digital demodulator, the control of the attenuation and the gain of the variable attenuator of the RF section and the variable gain amplifier of the first intermediate frequency IF section are performed by digital control. After the channel filtering of the input signal is performed inside the signal processor, the filtering is performed with the AGC voltage determined by the arithmetic processing for only the signal level of the desired demodulation band, and a large signal level existing outside the desired demodulation band, for example, an interference wave Is not controlled by something like Therefore, when a large-level interference wave is present, the level of the interference wave is amplified to a greater extent as in the case of amplifying the signal level of the desired demodulation band. For this reason, an excessive level is input to the second MIX or the amplifier at the subsequent stage of the variable gain amplifier, and the operation thereof is performed in a non-linear region, and the A / D converter exceeds the allowable input level. Above the saturation level.

FIG. 3 is an explanatory diagram when the A / D converter reaches a saturation level. In the figure, (a) is to raise the signal level of a desired demodulation band to a desired level before the variable gain amplifier is controlled by the AGC voltage. At this time, an interference wave exists outside the desired demodulation band as shown in the figure. Next, in (b), the variable gain amplifier is controlled by the AGC voltage to raise the signal level of the desired demodulation band to a desired level. At this time, the interference wave existing outside the desired demodulation band is amplified in the same manner as the signal in the desired demodulation band, and exceeds the saturation level of the A / D converter. As a result, in the digital demodulation device according to the prior art, sensitivity suppression due to an interference wave occurs remarkably, and especially when an A / D converter is saturated, a signal in a desired demodulation band is completely blocked. There is a problem that the quality of the signal in the desired demodulation band is deteriorated by the waves.

[0006]

The received RF signal is converted to an IF signal.
In a digital demodulation device which converts the signal into a signal and demodulates the signal by digital processing to obtain an audio signal, the demodulation means includes a band-pass filter for filtering an IF signal, an output of the band-pass filter, and A variable gain amplifier whose gain is controlled by an AGC control voltage; an A / D converter for performing analog / digital conversion on the output of the variable gain amplifier; filtering and demodulating the digitally converted IF signal; A digital signal processor for generating the AGC;
It comprises a D / A converter for digital / analog conversion of the control voltage, and a D / A converter for digital / analog conversion of the demodulated and output digital signal.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 1 is a schematic diagram showing a configuration example of a digital demodulation device according to the present invention. In the figure, a digital demodulation device includes an antenna 1 for receiving a radio wave, a band-pass filter 2, a first local oscillator 4, and a first MIX 5 for mixing a received signal and an output of the first local oscillator 4. , A band-pass filter 6, a second local oscillator 8, an output signal of the first MIX 5, and a second local oscillator 8.
MIX 9, which mixes the output of the first MIX, the band-pass filter 17, and the variable gain amplifier 1 which varies with the AGC voltage.
8 and an A / D converter 1 for digitizing an analog signal
1, a digital signal processor 12 for digitally processing the digitized signal, and a processed AGC
D / A for converting voltage digital signals to analog signals
It comprises a converter 13, a low-pass filter 14, a D / A converter 15 for converting a processed demodulated output digital signal into an analog signal, and a low-pass filter 16.

[0008] The digital demodulation device thus constructed operates as follows. The amplitude-modulated radio wave of the 2 to 30 MHz band is received by the antenna 1 and the received RF
The signal passes through a band-pass filter 2 to extract only the required band. In the first MIX5, the received RF
The signal and the output of the first local oscillator 4 are mixed and converted to 70 MHz as a first intermediate frequency IF1. The converted first intermediate frequency IF1 is input to the band-pass filter 6, and passes only the necessary band signal. Next, the first intermediate frequency IF1 is input to a second MIX 9, where the first intermediate frequency IF1 and the output of the second local oscillator 8 are mixed and converted to 455 kHz as a second intermediate frequency IF2. . Here, the second intermediate frequency IF2 is input to a band-pass filter 17 for removing unnecessary waves such as interfering waves, and then its output is input to a variable gain amplifier 18 for controlling the gain by an AGC voltage described later. The level of the second intermediate frequency IF2 is set to a value suitable for the input level of the A / D converter 11 at the next stage. The variable gain amplifier 18 keeps the level of the second intermediate frequency IF2 constant even if the reception level of the radio wave fluctuates, and normally guarantees the level fluctuation of about 100 dB. The level of the second intermediate frequency IF2 is set to A /
The digital signal is converted into a digital signal of a required number of bits by the D converter 11 and input to the digital signal processor 12.
The digital signal processor 12 digitally processes the input digital IF signal to perform channel filtering and demodulation of amplitude modulation. The digital signal processor 12 also performs digital arithmetic processing for controlling the AGC voltage applied to the above-mentioned variable gain amplifier 18, and adjusts the level of the intermediate frequency IF2 so that demodulation can be performed reliably regardless of the level of radio wave reception. Constant. The digital signal of the AGC voltage subjected to the digital operation processing is input to the D / A converter 13 and converted into an analog AGC voltage. Further, the AGC voltage is input to the low-pass filter 14 for removing unnecessary high-frequency components, and the output thereof is input to the variable gain amplifier 18 described above. On the other hand, an audio signal obtained by digital processing of channel filtering and demodulation of amplitude modulation is converted into an analog signal by a D / A converter 15 and further passed through a low-pass filter 16 to remove unnecessary high-frequency components. To the final output.

As described above, the digital demodulator according to the present invention eliminates the variable attenuator used in the conventional system,
The reception fluctuation level can be handled only by the variable gain amplifier. Next, the arrangement of the variable gain amplifier controlled by the AGC voltage was changed immediately before the A / D converter to simplify the AGC system. In addition, a band-pass filter is arranged immediately before the variable gain amplifier so that an interfering wave existing outside the desired demodulation band is not added to the AGC loop. However, since this band-pass filter is only for removing an interfering wave, an expensive large-sized steep filter required for channel filtering is not required. For example, a cheap filter such as a ceramic filter is used.

[0010]

As described above, according to the present invention, since the band-pass filter is disposed immediately before the variable gain amplifier, even if the variable gain amplifier has a large gain, the interference wave is not amplified.
Further, since the interfering wave is irrelevant to the generation of the AGC voltage, the problem of sensitivity suppression due to the interfering wave is eliminated. Further, since the AGC system is simplified, a highly reliable and inexpensive digital demodulator can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration example of a digital demodulation device according to the present invention.

FIG. 2 is a schematic diagram showing a configuration example of a conventional digital demodulation device.

FIG. 3 is an explanatory diagram when the A / D converter is at a saturation level.

[Explanation of symbols]

1 ... antenna, 2 ... band-pass filter,
3 ... variable attenuator, 4 ... first local oscillator,
5: first MIX, 6: band-pass filter,
7: variable gain amplifier, 8: second local oscillator, 9: second MIX, 10: amplifier, 11: A / D converter, 12: digital signal processor , 13 ... D / A converter,
14 low-pass filter, 15 D / A converter, 16 low-pass filter, 17 band-pass filter, 18 variable gain amplifier

Claims (1)

[Claims] A digital demodulator for converting a received RF signal into an IF signal and demodulating the IF signal by digital processing to obtain an audio signal, comprising a band-pass filter for filtering an IF signal, and a band-pass filter. A variable gain amplifier that amplifies the output and whose gain is controlled by an AGC control voltage, an A / D converter that converts the output of the variable gain amplifier from analog to digital, and filters and demodulates the digitally converted IF signal; And A
It comprises a digital signal processor for generating a GC control voltage, a D / A converter for digital / analog conversion of the AGC control voltage, and a D / A converter for digital / analog conversion of the demodulated and output digital signal. A digital demodulation device.