JP3637770B2 - Digital demodulator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a digital demodulator used for CATV and the like.
[0002]
[Prior art]
A conventional digital demodulator will be described below.
[0003]
As shown in FIG. 6, the conventional digital demodulator is composed of a tuner unit 1 connected to the input terminal 2 and a demodulator unit 8 connected to the tuner unit 1. The demodulation unit 8 includes a SAW filter 3 to which the output of the tuner unit 1 is connected, a detection circuit 4 connected to the output of the SAW filter 3, and a baseband filter 5 connected to the output of the detection circuit 4 The comparator 6 is connected to the output of the baseband filter 5, and the output terminal 7 is connected to the output of the comparator 6.
[0004]
In this case, the band of each digital modulation signal input to the input terminal 2 is very wide as shown in FIG. 7A, and it is normal to insert about 3 channels in the 6 MHz band 8a. It was. Therefore, the SAW filter 3 can sufficiently reduce the level of the adjacent channel, and the influence is eliminated. Therefore, the detection circuit 4 has no distortion.
[0005]
[Problems to be solved by the invention]
However, in such a conventional configuration, there is an increasing demand for transmitting a multi-channel signal in the same band as in recent years. As shown in FIG. When a multi-channel channel is transmitted, the adjacent digital signal cannot be dropped by the SAW filter 3 inevitably, and the signal of the adjacent channel that could not be dropped is distorted by the detection circuit 4, resulting in a demodulation circuit error. was there.
[0006]
An object of the present invention is to provide a digital demodulator capable of demodulating without causing an error even when multiple channels are transmitted in a narrow band.
[0007]
[Means for Solving the Problems]
In order to achieve this object, the digital demodulator of the present invention has an amplifier circuit inserted between the detector circuit and the baseband filter, and the input of the detector circuit is made small enough to complement the amplification degree of the amplifier circuit. This is a digital demodulator.
[0008]
As a result, even if multiple channels are transmitted in a narrow band, demodulation can be performed without causing an error.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The invention according to claim 1 of the present invention comprises a tuner unit connected to an input terminal to which a digital modulation signal of 1 channel or more per 1 MHz is input, and a demodulator unit to which an output of the tuner unit is connected, The demodulation unit includes a SAW filter to which the output of the tuner unit is connected, a detection circuit to which the output of the SAW filter is connected, a baseband filter to which an output of the detection circuit is supplied, and a baseband filter A comparator circuit to which an output is connected; and an output terminal to which an output of the comparator circuit is connected; an amplifier circuit is inserted between the detector circuit and the baseband filter; and an input of the detector circuit is the amplifier This is a digital demodulator that is small enough to complement the amplification level of the circuit. Thus, the input of the detection circuit complements the amplification level of the amplification circuit. By decreasing the extent that, even if have multi-channel is sent to even narrow band, because of the low input level, does not create distortion in the detector circuit. In addition, since an amplifier is provided after the detection circuit, the overall level does not drop, and demodulation can be performed without error.
[0010]
A baseband filter according to a second aspect of the present invention is the digital demodulator according to the first aspect in which the phase correction circuit is connected to the output of the low-pass filter. Since the phase correction circuit is included in this way, the baseband filter is demodulated. The phase of the received signal is corrected and can be demodulated without error.
[0011]
The low-pass filter according to the third aspect of the present invention is the low-pass filter according to the second aspect, which is a Bessel type or a Butterworth type. Thus, by using a Bessel type or Butterworth type filter, Phase fluctuation can be suppressed, and as a result, demodulation can be performed without error.
[0012]
According to a fourth aspect of the present invention, a second low-pass filter is connected to the phase correction circuit output, and the second low-pass filter is a Chebyshev type. By inserting a low-pass filter, a sufficient amount of attenuation can be secured, and adjacent signals can be sufficiently dropped.
[0013]
The invention described in claim 5 is the digital demodulator according to claim 1 in which an attenuator is inserted between the SAW filter and the detector circuit. By inserting the attenuator in this way, the input to the detector circuit is reduced. It will be smaller and will not be distorted by the detector circuit.
[0014]
The invention according to claim 6 is the digital demodulator according to claim 1 in which the detector circuit is arranged close to the SAW filter, and the detector circuit is arranged close to the SAW filter in this manner. Since noises are less likely to occur, errors are prevented.
[0015]
According to a seventh aspect of the present invention, there is provided a tuner unit comprising: an input terminal; an AGC amplifier circuit to which a signal input to the input terminal is supplied; an output of the AGC amplifier circuit is supplied to one input; And an output terminal to which the output of the mixing circuit is supplied, and an attenuator is inserted between the input terminal and the AGC amplifier circuit. 1 and the attenuator is inserted in this manner, the AGC amplifier circuit does not distort the adjacent signal.
[0016]
The invention according to claim 8 is the digital demodulator according to claim 7, wherein the low-pass filter for blocking the signal of the local oscillation circuit is provided between the output of the mixing circuit of the tuner section and the output terminal. Is prevented from leaking to the demodulator.
[0017]
The invention described in claim 9 is the digital demodulator according to claim 7, wherein the intermediate frequency amplifier circuit is provided between the mixer circuit of the tuner section and the low pass filter, and the intermediate frequency amplifier circuit is provided. The input level entering the mixing circuit can be reduced, and distortion in the mixing circuit can be reduced.
[0018]
According to a tenth aspect of the present invention, an AGC detection circuit is provided at the output of the baseband filter of the tuner section, and the amplification degree of the AGC amplification circuit of the tuner section is controlled by the output of the AGC detection circuit. Since it is a demodulator and an AGC detection circuit is provided at the output of the baseband filter, even if there is an adjacent interference, it is removed by the baseband filter, so that it is possible to control the AGC voltage without being affected by it. .
[0019]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a block diagram of a digital demodulator of the present invention. In FIG. 1, reference numeral 11 denotes a tuner connected to an input terminal 12, and a QPSK modulated signal of 18 MHz to 42 MHz is injected into this input terminal 12. The transmission rate corresponding to the information amount of this signal is 128 kbps, and the roll-off factor that determines the degree of band limitation is 0.3. Therefore, the band of this signal is about 83 kHz. In this system, as shown in FIG. 7B, 60-wave signals are transmitted in a 6 MHz band 8 at 100 kHz step intervals. Therefore, the tuner unit 11 needs to be able to select a channel at every 100 kHz step. In the case of the digital demodulator according to the present embodiment, the tuner circuit 11 is set by the PLL circuit 39 of FIG. 5 so that the channel can be selected at a 20 kHz step. The output 38 of the tuner unit 11 is connected to the SAW filter 13. The SAW filter 13 may pass adjacent channels above and below the desired signal, but it is important to drop the adjacent channel. This is to prevent third-order distortion between the adjacent channel and the adjacent adjacent channel from entering the band (desired signal). In order to improve the delay characteristics in the band, the input / output terminal of the SAW filter 13 has a 0.22 uH coil in series with the signal line, and 320 pF (220 pF and 100 pF between each input / output and the ground). The delay is corrected using a capacitor connected in parallel.
[0020]
An attenuator 14 is connected to the SAW filter 13. The attenuator 14 is configured by providing a capacitor and a resistor between the parallel outputs of the SAW filter 13, and has an amplification factor of about ¼. The output of the attenuator 14 is connected to the I and Q detector 15. The attenuator 14 also functions to improve the matching between the SAW filter 13 and the I / Q detector 15. Furthermore, in order to cope with the increase in the number of channels, I and Q detection in a narrow frequency range is necessary so that other channels are not received by mistake. Therefore, a crystal oscillator is provided in the I and Q detector 15. The local oscillation circuit 21 used is provided. The local oscillation circuit 21 can vary the frequency so as to follow the input frequency. By using this crystal oscillator, the frequency range that can be varied is narrowed, frequency fluctuations due to temperature and the like are suppressed, and erroneous reception of the adjacent channel is prevented.
[0021]
The output of the I and Q detector 15 is connected to an amplifier circuit 16 having an amplification factor of about 4 times, and its output is connected to a baseband filter 17. The amplifier circuit 16 is composed of an operational amplifier, and the amplification degree can be varied from 1/10 to 20 times by using a volume, and also serves to balance the level of the I and Q signals. Yes. The output of the baseband filter 17 is connected to a comparator 18, and the output of the comparator 18 is connected to an output terminal 19. Here, from the I and Q detector 15 to the output terminal 19, two systems, that is, the I system and the Q system are connected in series, respectively, but here they are described as one unit. .
[0022]
Reference numeral 20 denotes an AGC detection circuit connected to the output of the baseband filter 17, and the detection output is connected to the tuner unit 11. The AGC detection circuit 20 uses a thermistor resistor so that the output level of the AGC detection circuit 20 does not change with temperature. Here, what is important is to shorten the wiring between the SAW filter 13 and the I / Q detector 15. If this area is lengthened, noise is likely to be applied because the level here is low.
[0023]
Next, the baseband filter 17 will be described. As shown in FIG. 2, the baseband filter 17 includes a seven-stage operational amplifier between the input terminal 21 and the output terminal 22. Of these, the three stages 23a, 23b, and 23c constitute a low-pass filter. This low-pass filter uses a Bessel type of bi-cut a-type low-pass filter. This type is used in order to improve the phase characteristics at the expense of attenuation. Next, after this low-pass filter, a two-stage phase correction circuit of 24a and 24b is used. Thereby, the phase of the whole baseband filter is improved. Next, an amplification circuit 25 of about 10 times is provided, and then a second low-pass filter 26 is connected. The low-pass filter 26 is of a Chebyshev type and has a sufficient attenuation. However, since this type of filter has a poor delay characteristic, it is used with a slightly higher cut-off (less than the normal cut-off of 32 kHz, about 50 kHz) with less influence of the delay characteristic. Thereby, the attenuation amount of 70 kHz or more is ensured. The attenuation characteristic of this filter is shown in FIG. 3, and the phase characteristic is shown in FIG.
[0024]
In FIG. 3, the horizontal axis represents frequency and the vertical axis represents attenuation characteristics. The desired signal passes from 0 Hz to about 41 kHz. Actually, the amount of electric power is large up to around 30 kHz. Therefore, the desired signal is attenuated at 30 kHz or more, but the characteristic is not greatly affected. In addition, an attenuation of about 60 dB or more is secured in the vicinity of 70 kHz where the adjoining enters.
[0025]
4A shows the phase characteristics of the baseband filter after phase correction using the Bessel type, which is the delay characteristics of the present embodiment (including the amplifier circuit 25 and the low-pass filter 26). (B) shows the phase characteristics of the baseband filter using the Bessel type before phase correction (the amplifier circuit 25 and the low-pass filter 26 are not included). Further, (c) is a phase characteristic of the baseband filter using the Chebyshev type before phase correction (the amplifier circuit 25 and the low-pass filter 26 are not included).
[0026]
Hereinafter, the description of FIG. 4 will be given. In FIGS. 4A, 4B, and 4C, the horizontal axis represents frequency and the vertical axis represents delay. The delay on the vertical axis has no problem with the value itself, but must be designed so as not to fluctuate within the band. In this embodiment, since the transmission rate is 128 kbps, it is important to keep the delay constant up to about 36 kHz as shown by the frequency 50. The desired value of the delay is about 2.2 usec when the transmission rate is 128 kbps, and the actual characteristics are shown by the delay variation between the frequency 0 Hz and 36 kHz in FIG. It has become. This value is about 11 usec as shown by 52 in the filter (b) using the Bessel type before delay correction. In the filter (c) using the Chebyshev type before delay correction, as indicated by 53, it is about 52 usec. From the above results, delay correction is necessary, and it is necessary to use a Bessel type filter. As described above, in the low-pass filter 26 using the Chebyshev type, the cutoff is designed in accordance with a frequency 18 kHz higher than that of the low-pass filter 23 to suppress delay characteristics.
[0027]
Next, the tuner unit 11 will be described. As shown in FIG. 5, the tuner unit 11 includes an approximately 6 dB attenuator 30 connected to the input terminal 12, a tuning filter 31 connected to the output of the attenuator 30, and an AGC amplification connected to the output of the tuning filter 31. A circuit 32, a second tuning filter 33 connected to the output of the AGC amplifier circuit 32, and an output of the second tuning filter 33 is connected to one input, and the other input has a local oscillation A mixing circuit 35 to which the output of the circuit 34 is connected, an intermediate frequency amplifying circuit 36 to which the output of the mixing circuit 35 is connected, a low pass filter 37 connected to the output of the intermediate frequency amplifying circuit 36, and the low pass filter The output terminal 38 is connected to 37 outputs.
[0028]
The output terminal 38 is connected to the input of the SAW filter 13 shown in FIG. Further, the local oscillation circuit 34 is connected to the PLL circuit 39 in a loop, and the channel selection frequency is determined by data from the interface 40. The level control terminal 41 of the AGC amplifier circuit 32 is connected to the output of the AGC detection circuit 20 of FIG.
[0029]
Hereinafter, the operation of the tuner unit 11 will be described. As shown in FIG. 7B, a 60-wave signal (one wave is a desired signal and the remaining 59 waves are interference signals) that are QPSK modulated during the 6 MHz band 8 is input from the input terminal 12, and the attenuator 30. to go into. The attenuator 30 has an attenuation of about 6 dB, and the entire level including the interference wave is reduced by 6 dB. Thus, no distortion occurs in the AGC amplifier circuit 32 even when adjacent interference is input.
[0030]
The QPSK modulated signal that has passed through the attenuator 30 enters the tuning filter 31, where the interference signal that is about 4 MHz or more away from the desired wave is attenuated. The AGC amplifier circuit 32 has an amplification degree of about minus 40 dB to about plus 20 dB, and the output level is always maintained even when the input level changes from 45 dBuV to 65 dBuV due to the AGC voltage from the level control terminal 41 of the AGC amplifier circuit 32. It is kept constant.
[0031]
The second tuning filter 33 is composed of a double tuning circuit in order to ensure a narrow band characteristic. Here, the interference signal separated from the desired wave by about 2 MHz or more is attenuated. The tuning filter 33 has a loss of 10 dB or more. On the other hand, the tuning filter 33 serves to lower the input to the mixing circuit 35.
[0032]
The mixing circuit 35 serves to convert an input 18 MHz to 42 MHz QPSK modulated signal to an intermediate frequency of 45 MHz. The intermediate frequency amplifier 36 employs an element that is resistant to distortion and has a high degree of amplification. This amplification degree is about 30 dB, which is larger than the total amplification degree from the attenuator 30 to the mixing circuit 35. Thereby, since the AGC voltage from the level control terminal 41 of the AGC amplifier circuit 32 can be set to suppress the amplification degree of the AGC amplifier circuit 32, distortion in the mixing circuit 35 can be suppressed.
[0033]
The low-pass filter 37 functions to suppress a frequency component generated from the local oscillation circuit 34. This is to prevent the frequency component of the local oscillation circuit 34 from appearing as an unnecessary noise component in the necessary band by mixing with other unnecessary frequency components.
[0034]
As described above, according to the present embodiment, even if an interference signal is input, it is not distorted by the tuner unit and the detection circuit, so that it can be demodulated without causing an error.
[0035]
In this embodiment, the modulation method is the QPSK method, but the same effect can be obtained when demodulating a signal modulated by the QAM, QPR, or BPSK modulation method.
[0036]
【The invention's effect】
As described above, according to the present invention, a digital demodulator is provided in which an amplifier circuit is inserted between the detector circuit and the baseband filter, and the input of the detector circuit is made small enough to complement the amplification degree of the amplifier circuit. Therefore, by making the input of the detection circuit small enough to complement the amplification degree of the amplification circuit in this way, even if multiple channels are sent to a narrow band, the input level is low, so distortion occurs in the detection circuit. There is nothing. In addition, since an amplifier is provided after the detection circuit, it is possible to perform digital demodulation without causing an error and without reducing the overall level.
[Brief description of the drawings]
1 is a block diagram of a digital demodulator according to an embodiment of the present invention. FIG. 2 is a block diagram of a baseband filter used in the digital demodulator. FIG. 3 is an attenuation characteristic diagram of the baseband filter. 4A is a Bessel type delay characteristic diagram after phase correction (b) is a Bessel type delay characteristic diagram before phase correction (c) is a Chebyshev type delay characteristic diagram before phase correction. 5 is a block diagram of a tuner unit used in the digital demodulator. FIG. 6 is a block diagram of a conventional digital demodulator. FIG. 7A is a diagram of a digital demodulated signal when there are few channels in the band. Transmission diagram (b) is a diagram of digital demodulated signal transmission when there are many channels in the band.
DESCRIPTION OF SYMBOLS 11 Tuner part 12 Input terminal 13 SAW filter 14 Attenuator 15 I, Q detector 16 Amplifier circuit 17 Baseband filter 18 Comparator 19 Output terminal 20 AGC detector circuit

Claims (10)

A tuner unit connected to an input terminal to which a digital modulation signal of 1 channel or more per 1 MHz is input and a demodulator unit to which an output of the tuner unit is connected. The demodulator unit is connected to an output of the tuner unit. Surface acoustic wave (hereinafter referred to as SAW) filter, a detection circuit to which the output of the SAW filter is connected, a baseband filter to which the output of the detection circuit is supplied, and an output of the baseband filter are connected A comparator circuit and an output terminal to which the output of the comparator circuit is connected, and an amplifier circuit is inserted between the detector circuit and the baseband filter, and the input of the detector circuit determines the amplification factor of the amplifier circuit. Digital demodulator that is small enough to complement. The digital demodulator according to claim 1, wherein the baseband filter has a phase correction circuit connected to an output of the low-pass filter. The digital demodulator according to claim 2, wherein the low-pass filter is a Bessel type or a Butterworth type. 3. The digital demodulator according to claim 2, wherein a second low-pass filter is connected to the output of the phase correction circuit, and the second low-pass filter is a Chebyshev type. The digital demodulator according to claim 1, wherein an attenuator is inserted between the SAW filter and the detection circuit. The digital demodulator according to claim 1, wherein a detector circuit is arranged close to the SAW filter. The tuner unit has an input terminal, an AGC amplifier circuit to which a signal input to the input terminal is supplied, an output of the AGC amplifier circuit is supplied to one input, and the other input is an output of the local oscillation circuit. The digital demodulator according to claim 1, further comprising: a mixing circuit connected to each other; an output terminal to which an output of the mixing circuit is supplied; and an attenuator inserted between the input terminal and the AGC amplifier circuit. 8. The digital demodulator according to claim 7, wherein a low pass filter for blocking a signal of the local oscillation circuit is provided between the output of the mixing circuit of the tuner section and the output terminal. 8. The digital demodulator according to claim 7, wherein an intermediate frequency amplifier circuit is provided between the mixing circuit of the tuner section and the low pass filter. 8. The digital demodulator according to claim 7, wherein an AGC detection circuit is provided at the output of the baseband filter, and the amplification degree of the AGC amplifier circuit of the tuner unit is controlled by the output of the AGC detection circuit.

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