JP2004147160A - Receiving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a receiving device which allows a adjacent channel suppressing capability to be sufficiently obtained independently of the position of a reception segment with the degree of each filter left as it is. <P>SOLUTION: The reception device provided with a mixer circuit for frequency conversion of a reception signal and filters for removing unnecessary signals from the frequency converted reception signal is provided with a means which changes cut-off frequencies of filters in accordance with positions of partial segments selected and received of a signal which has a plurality of segments connected and is subjected OFDM modulation, so that the adjacent channel suppressing capability can be sufficiently obtained. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a receiving device for digital terrestrial broadcasting.
[0002]
[Prior art]
The configuration of a conventional receiving apparatus will be described with reference to FIG.
[0003]
The ISDB-T signal received by the antenna 1 is amplified by the RF amplifier 2 and input to the mixer 3. The PLL 4 generates an oscillation signal having a predetermined frequency, and the oscillation signal is supplied to the mixer 3 and frequency-converted to a first intermediate frequency (fIF1 = 1400 MHz). The output of the mixer 3 is input to the BPF 5, selects and passes only the segment desired to be received, suppresses the signals of other segments and adjacent channels, and is input to the IF amplifier 6 and amplified, and then the mixer 7 and the mixer 8 And a quadrature mixer constituted by the phase shifter 10. The output of the quadrature mixer is further suppressed by the LPF 51 and LPF 52 to signals of other segments and adjacent channels, and only the segment desired to be received is selected. And convert it to a digital signal. Then, the signals from the ADCs 32 and 33 and the numerically controlled oscillator NCO 35 are frequency-converted by the complex multiplier 34. After the image frequency is suppressed by the LPFs 36 and 37, the image is input to the OFDM demodulator 38, where the demodulation is performed in accordance with the modulation process at the time of ISDB-T transmission, and the TS is output to the TS output terminal.
[0004]
As prior art document information relating to the invention of this application, for example, Japanese Patent Application No. 2001-306121 of an unpublished in-house application can be mentioned.
[0005]
[Problems to be solved by the invention]
In terrestrial digital broadcasting, analog broadcasting may be present in an adjacent channel, and a high capability of suppressing an adjacent channel is required.
[0006]
In particular, in digital terrestrial audio broadcasting performed using the VHF band, analog broadcasting always exists in the upper channel and the lower channel, so that particularly high suppression capability is required.
[0007]
In digital terrestrial audio broadcasting, eight segments are connected and transmitted, and have a spectrum shown in FIG.
[0008]
When the desired reception segment is the third from the bottom, as shown in FIG. 10 (b), both the lower adjacent voice carrier and the upper adjacent voice are sufficiently separated from the desired segment, and the BPF 5 and the LPFs 51 and 52 can be sufficiently suppressed.
[0009]
When the first segment from the top is received, the frequency with the upper adjacent video carrier becomes closer as shown in FIG.
[0010]
When receiving the first segment from the bottom, as shown in FIG. 10D, the frequency of the lower adjacent voice carrier is closer to the upper side.
[0011]
As described above, the degree of suppression of the BPF5, LPF51, and LPF52 is lower than when the first segment from the top or the first segment from the bottom is received and when the segment near the center of the channel, for example, the third segment from the bottom is received. More needed. To realize this characteristic, it is necessary to increase the order in order to sharpen the characteristics of each filter. However, increasing the order has a problem that the shape of the filter becomes large. It is an object of the present invention to provide a receiving apparatus capable of sufficiently obtaining the adjacent suppression capability irrespective of the position of the receiving segment while keeping the order of each filter.
[0012]
[Means for Solving the Problems]
An invention according to claim 1 of the present invention is a receiving apparatus including a mixer circuit for converting a frequency of a received signal and a filter for removing an unnecessary signal from the frequency-converted received signal, wherein the plurality of segments are connected. Device for selecting a partial segment of an OFDM-modulated signal and changing the cutoff frequency of the filter in accordance with the position of the segment to be received, and capable of sufficiently obtaining adjacent suppression capability Can be obtained.
[0013]
The invention according to claim 2 of the present invention includes a first filter and a second filter for removing an unnecessary signal from a frequency-converted received signal, and includes a first filter and a second filter which are arranged in accordance with characteristics of the first filter. 2. The receiving apparatus according to claim 1, further comprising: means for changing a cutoff frequency, wherein the receiving apparatus is capable of sufficiently obtaining the adjacent suppression capability even when the characteristic variation of the first filter exists during manufacturing. it can.
[0014]
According to a third aspect of the present invention, there is provided a frequency adjustment circuit connected to a reference clock, and a DC voltage correction circuit connected to the frequency adjustment circuit, wherein the DC voltage correction circuit is a filter for different reference clock signal frequencies. 2. The receiving apparatus according to claim 1, further comprising means for making a cutoff frequency of the filter constant, wherein the receiving apparatus is capable of correcting a frequency of a filter corresponding to a plurality of reference clocks and sufficiently obtaining adjacent suppression capability. be able to.
[0015]
The invention according to claim 4 of the present invention is characterized in that a DC voltage correction circuit is provided between the second filter and the frequency adjustment circuit, and means for performing DC voltage correction for different reference clock signal frequencies is provided. And a receiving apparatus capable of performing a frequency correction of the filter in accordance with a manufacturing characteristic variation of the first filter corresponding to a plurality of reference clocks and sufficiently obtaining the adjacent suppression capability.
[0016]
The invention according to claim 5 of the present invention is the receiving apparatus according to claim 1, further comprising means for changing a cutoff frequency according to a bit error rate. It is possible to obtain a receiving device capable of sufficiently obtaining the capability.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
[0018]
FIG. 1 is a block diagram of a receiving device of the present invention.
[0019]
In FIG. 1, 1 is an antenna, 2 is an RF amplifier, 3 is a mixer, 4 is PLL, 5 is BPF, 6 is IF amplifier, 7 and 8 are mixers, 9 is PLL, 10 is a phase shifter, and 61 and 62 are variable. Cut-off low-pass filter (LPF), 30, 31 are baseband amplifiers (BB amplifiers), 32, 33 are AD converters (ADC), 34 is a complex multiplier, 35 is a numerically controlled oscillator (NCO), 36, 37 is a digital LPF, 38 is an OFDM demodulator, 21 is a transport stream output terminal (TS output terminal), and 63 is a microprocessor. The cut-off frequency of the filter is changed according to the position of the segment received by the microprocessor 63.
[0020]
The configuration of this receiving apparatus is a receiving apparatus that performs partial reception of one segment. The ISDB-T signal received by the antenna 1 is amplified by the RF amplifier 2 and input to the mixer 3. The PLL 4 generates an oscillation signal having a predetermined frequency, and the oscillation signal is supplied to the mixer 3 and frequency-converted to a first intermediate frequency (fIF1 = 1400 MHz). fIF1 is a fixed frequency.
[0021]
At this time, if the frequency of PLL2 is set higher than fIF1, the spectrum is inverted with respect to the antenna input in the first intermediate frequency band. Although the frequency of the PLL 2 can be set lower than the frequency fIF 1, this embodiment will be described on the higher side.
[0022]
The signal that has been frequency-converted to fIF1 is subjected to BPF5 to remove a first unnecessary signal. The fIF1 segment signal band-limited by the BPF 5 is amplified by the IF amplifier 6, is multiplied by a PLL 9 in the mixer 8, and a signal obtained by shifting the signal of the PLL 9 by 90 ° in the mixer 7 by the phase shifter 10, and a complex baseband signal is obtained. Is output.
[0023]
The output of the quadrature mixer is further suppressed by LPF61 and LPF62 in other segments and adjacent channels, and only the segment desired to be received is selected and amplified by the BB amplifiers 30 and 31 to a predetermined level of the ADC in the next stage and input to the ADCs 32 and 33. And convert it to a digital signal. When fOFS = 500 KHz, the signal of the NCO 35 and the ADCs 32 and 33 are subjected to complex multiplication by the complex multiplier 34 to be converted to baseband. After the image frequencies are suppressed by the LPFs 36 and 37, they are input to the OFDM demodulator 38, and demodulated such as complex Fourier transform, frequency interleaving, time interleaving, and error correction according to the modulation process at the time of ISDB-T transmission. The processing is performed, and the TS is output to the TS output terminal.
[0024]
Hereinafter, the suppression of the adjacent carrier by the BPF 5, the LPF 61 and the LPF 62 with the required value of the suppression level of the adjacent carrier according to the segment position to be received being 40 dB or more will be described with reference to FIGS.
[0025]
FIG. 6A shows the spectrum of the first intermediate frequency band when the third segment from the bottom is received. The hatched portion is the reception segment. The reception segment and the upper adjacent video carrier are separated by a certain distance or more, and the BPF 5 can suppress, for example, 40 dB or more, and the lower adjacent audio carrier can suppress 20 dB.
[0026]
FIG. 7A shows the characteristics of the baseband signal and the LPFs 61 and 62 when the third segment from the bottom is received. Since the cutoff of the LPFs 61 and 62 is lower than the frequency of the lower adjacent voice carrier, the level is sufficiently suppressed. The LPFs 61 and 62 can suppress 20 dB, and the required amount of suppression (40 dB) can be obtained together with the suppression amount 20 dB of the BPF 5.
[0027]
FIG. 6B shows the spectrum of the first intermediate frequency band when the first segment from the top is received. The hatched portion is the reception segment, and the reception segment and the lower adjacent voice carrier are separated by a certain distance or more, and the BPF 5 can suppress, for example, 40 dB, but the frequency is close to the upper adjacent video carrier, so the suppression is only 10 dB. Therefore, it is necessary to ensure greater suppression in the LPFs 61 and 62.
[0028]
FIG. 7B illustrates the characteristics of the baseband signal and the LPFs 61 and 62 when the first segment from the top is received. As described above, since the suppression of the upper adjacent video carrier wave of the BPF 5 is insufficient at 10 dB, it is necessary to compensate for the characteristics with a low-pass filter. In addition, the frequency of the received segment and that of the carrier wave are closer as compared with FIG. The level of the upper adjacent video carrier is suppressed by controlling the cutoff of the LPFs 61 and 62 from the microprocessor. The set point of the cutoff frequency in this case will be described. The lower the cut-off frequency is, the more the carrier wave level can be suppressed, but the received segment is deformed such as the spectrum is cut off. The performance can be improved by setting the cutoff frequency so that the performance improvement due to the suppression of the level of the upper adjacent video carrier is greater than the performance deterioration due to the spectrum deformation.
[0029]
FIG. 7B shows a case where the control is performed so that the LPFs 61 and 62 can suppress 30 dB or more. Although the spectrum of the received segment is slightly reduced, the required value 40 dB is obtained by combining the suppression amounts of the BPF 5 and the LPFs 61 and 62. Since it can be secured, the performance can be improved as compared with the case where the characteristics of the LPFs 61 and 62 in FIG. 7A are used.
[0030]
FIG. 6C shows the spectrum of the first intermediate frequency band when the first segment from the bottom is received. The hatched portion is the receiving segment, and the receiving segment and the upper adjacent video carrier are separated by a certain distance or more, and the BPF 5 can suppress, for example, 40 dB. However, the frequency is close to the lower adjacent voice carrier, and thus the suppression is only 10 dB. Therefore, it is necessary to ensure greater suppression in the LPFs 61 and 62.
[0031]
FIG. 7B illustrates the characteristics of the baseband signal and the LPFs 61 and 62 when the first segment from the bottom is received. As described above, since the suppression of the lower adjacent voice carrier of the BPF 5 is insufficient at 10 dB, it is necessary to supplement the characteristics with a low-pass filter, and the frequency of the received segment is closer to that of the carrier as compared with FIG. Therefore, the level of the lower adjacent voice carrier is suppressed by controlling the cutoff frequency of the LPFs 61 and 62 from the microprocessor.
[0032]
The set point of the cutoff frequency in this case will be described. The lower the cut-off frequency is, the more the carrier wave level can be suppressed, but the received segment is deformed such as the spectrum is cut off. The performance can be improved by setting the cutoff frequency so that the performance improvement due to the level suppression of the lower adjacent voice carrier is greater than the performance deterioration due to the spectrum deformation.
[0033]
FIG. 7C shows a case where the control is performed so that the LPFs 61 and 62 can suppress 30 dB or more. Although the spectrum of the received segment is slightly reduced, the required value 40 dB is obtained by combining the suppression amounts of the BPF 5 and the LPFs 61 and 62. Since it can be secured, the performance can be improved as compared with the case where the characteristics of the LPFs 61 and 62 in FIG. 7A are used.
[0034]
FIG. 2 shows a configuration in which the performance of the BPF 5 is corrected using an OFDM demodulator 38 having general-purpose digital input terminals 65 and 66 in addition to the contents described in FIG. As the BPF 5, a SAW (surface acoustic wave) filter or the like is used, but at a high frequency of 1400 MHz, the characteristic variation is large. Therefore, in the configuration of FIG. 1, there may be a case where sufficient suppression of adjacent carrier waves cannot be obtained or a case where a signal is cut.
[0035]
For example, as shown in FIG. 6B, when the first segment from the top is received, the BPF 5 can suppress only 10 dB with respect to the upper adjacent video carrier. Are set to Low and High, respectively. The microprocessor 63 reads the logic of the terminals 65 and 66 and detects that the BPF 5 can secure only 8 dB of suppression. The microprocessor 63 sets the cutoff frequency of the LPFs 61 and 62 to be low, and controls the LPFs 61 and 62 to obtain an attenuation of 32 dB, so that the BPF 5 and the LPFs 61 and 62 can secure the suppression of 40 dB.
[0036]
As shown in FIG. 6B, when the first segment from the top is received, the BPF 5 suppresses 10 dB for the upper adjacent video carrier, and the 15 dB suppression is performed due to the variation, and the spectrum is slightly reduced. In this case, the terminals 65 and 66 are set to High and High, respectively.
[0037]
The microprocessor 63 reads the logic of the terminals 65 and 66 and detects that the BPF 5 has secured an excessive suppression of 15 dB.
[0038]
The microprocessor 63 sets the cut-off frequency of the LPFs 61 and 62 to be higher and controls the LPFs 61 and 62 so that attenuation of 25 dB can be obtained, so that the BPFs 5 and the LPFs 61 and 62 can secure suppression of 40 dB.
[0039]
A desired suppression of 40 dB can be ensured without excessively lowering the cutoff frequency of the LPFs 61 and 62 without excessively reducing the spectrum of the reception segment.
[0040]
As described above, the optimum cutoff frequency of the LPFs 61 and 62 can be set even when the BPF 5 causes manufacturing variations, so that the spectrum of the receiving segment is not unnecessarily reduced while obtaining the desired carrier suppression.
[0041]
FIG. 3 shows a configuration in which the optimum cutoff frequency of the LPFs 61 and 62 according to the segment position can be obtained even when there is a manufacturing variation of the LPFs 61 and 62. The frequency of the LPFs 61 and 62 can be adjusted using the output signal of the frequency adjustment circuit 67.
[0042]
However, when this receiving device is incorporated in a small portable terminal, the frequency of the reference clock signal 68 varies from company to company. Therefore, it has been considered that the DC voltage correction circuit 64 corrects the control voltage of the LPFs 61 and 62 from the microprocessor 63 to the correction voltage V1 according to the frequency of the reference clock signal 68.
[0043]
As described with reference to FIG. 1, since the optimal cutoff frequency of the LPFs 61 and 62 differs depending on the segment position, the optimum cutoff frequency is obtained by changing the control voltage of the LPFs 61 and 62 to the converted voltage V2 according to the segment position. be able to.
[0044]
Since the frequency and the reception segment of the reference clock signal 68 are known, the microprocessor 63 controls the DC voltage correction circuit 64 to superimpose V1 + V2 on the output voltage of the frequency adjustment circuit 67, thereby obtaining the frequency and the segment of the reference clock signal 68. Since control can be performed according to the position, it is possible to obtain the optimum cutoff frequency according to the segment position while performing the variation correction of the LPFs 61 and 62 even for the different reference clock signals 68.
[0045]
FIG. 4 shows that the microprocessor 63 controls the DC voltage correction circuit 64 by setting the logic of the general-purpose digital input terminals 65 and 66 in accordance with the manufacturing variation of the BPF 5 in addition to the configuration of FIG. Even if there is a manufacturing variation of the LPF 61, the LPF 62 and the BPF 5 with respect to the signal 68, an optimal cut-off frequency can be obtained by controlling the cut-off frequency of the LPF 61 and the LPF 62 according to the position of the receiving segment.
[0046]
As described in FIG. 3, the addition voltage (V1 + V2) of the correction voltage V1 corresponding to the frequency of the reference clock signal 68 and the correction voltage V2 corresponding to the position of the segment is superimposed on the control voltage from the frequency adjustment circuit 67. Thus, cutoff frequency control of the LPF 61 and the LPF 62 is performed. In addition, when there is a manufacturing variation in the BPF 5, the microprocessor 63 further superimposes the correction voltage V3 according to the manufacturing variation by reading the logic of the general-purpose digital input terminals 65 and 66.
[0047]
By superimposing V1 + V2 + V3 on the control voltage from the frequency adjustment circuit 67 as described above, cutoff frequency control of the LPF 61 and the LPF 62 is performed according to the manufacturing variation of the BPF 5 and the LPFs 61 and 62, the reference clock signal 68, and the segment position. As a result, an optimal cutoff frequency can be obtained.
[0048]
In FIG. 5, a signal indicating the occurrence of a byte that could not be demodulated correctly by the OFDM demodulator 38 is connected to the error rate counting circuit 69, and the error rate counting circuit 69 is connected to the microprocessor 63.
[0049]
The BPF 5 is a filter that allows one segment to pass in the 1400 MHz band, and is affected by temperature characteristics. In addition, since the strength of the adjacent carrier and the strength of the reception segment differ depending on the receiving location, the cutoff frequency required for the LPFs 61 and 62 varies with the aging of the BPF 5 and the receiving location although there is a desired value for the worst condition.
[0050]
It is effective to count the error rate in order to adjust the cutoff of the LPFs 61 and 62 optimally with respect to a change with time and a change in the receiving place.
[0051]
First, the microprocessor 63 controls the LPF 61 and the LPF 62 so as to have a predetermined cutoff frequency according to the position of the segment. For example, when the frequency characteristic of the BPF 5 at the time of receiving the uppermost segment is 10 dB at the beginning due to a temporal change, and the amount of adjacent video carrier suppression is deteriorated to 5 dB, a desired adjacent frequency combining the BPF 5 and the LPFs 61 and 62 is obtained. If the image carrier suppression amount is 40 dB, the conventional LPFs 61 and 62 can only receive the attenuation amount of 30 dB as described with reference to FIG.
[0052]
The OFDM demodulator 38 performs demodulation processing such as complex Fourier transform, frequency interleaving, time interleaving, and error correction according to the modulation processing at the time of ISDB-T transmission, and outputs a TS to a TS output terminal.
[0053]
After Viterbi decoding, the OFDM demodulator 38 performs RS decoding (Reed-Solomon decoding). It is known that it is possible to detect an RS byte that cannot be error-corrected at the time of RS decoding. When the RS byte occurs, the OFDM demodulator 38 outputs a signal indicating that an error-correctable byte has occurred to an error rate counting circuit 69. Send to
[0054]
The error rate counting circuit 69 calculates the error rate by counting the number within a certain time, and notifies the microprocessor 63 of the result. When the calculated error rate is lower than the desired error rate, the microprocessor 63 does nothing, but when the calculated error rate is higher, controls the cutoff frequencies of the LPFs 61 and 62 in a stepwise manner sequentially from the initial value.
[0055]
The cutoff frequency is changed until the calculated error rate becomes lower than the desired error rate in the stepwise changing process. By performing such control, not only a low bit error can be obtained even when the BPF 5 changes over time, but also an aging of the LPF 61 and the LPF 62 can be simultaneously handled, and a low error rate can be obtained even at a receiving place where the adjacent carrier is stronger. be able to.
[0056]
An example of a step-like change will be described with reference to the cutoff frequency and error rate diagrams of FIG.
[0057]
When there is no aging, the cutoff frequency is at the point indicated by 105, and since it is lower than the desired error rate, the control from the microprocessor 63 is no longer necessary. When the initial value becomes 101 due to aging, for example, if control is performed in a stepwise manner from 101 → 103 → 102 → 104, the error rate becomes lower than the desired error rate at the time when the control becomes 104. Thus, the error rate can be made lower than the desired error rate.
[0058]
Analog broadcasting has been abolished in the VHF band, and the expansion of channels such as another digital audio broadcasting has been considered. Therefore, the required adjacent exclusion is expected to change.
[0059]
In the receiving apparatus having the configuration shown in FIGS. 1 to 4, it is possible to easily cope with a change in the situation by changing the software of the microprocessor 63.
[0060]
In the case of FIG. 5, since the cutoff frequency is further changed depending on the error rate, there is no need to change the software.
[0061]
Further, even when the level of the adjacent carrier is different from the standard depending on the area or channel, it can be flexibly changed by changing the software in the examples of FIGS.
[0062]
【The invention's effect】
As described above, according to the present invention, in a demodulation device that performs partial reception of an ISDB-T modulated signal, a reception device that can sufficiently obtain adjacent exclusion capability regardless of the position of a reception segment without increasing the order of a filter And the size of the receiver can be reduced.
[Brief description of the drawings]
1 is a block diagram of a receiving device according to an embodiment of the present invention; FIG. 2 is a block diagram of a receiving device of another example of the present invention; FIG. 3 is a block diagram of a receiving device of still another example of the present invention; FIG. 4 is a block diagram of a receiving device of the present invention. FIG. 5 is a block diagram of a receiving device of another example of the present invention. FIGS. 6A to 6C are spectrum diagrams of a first intermediate frequency band of the present invention. 7 (a) to 7 (c) are spectrum diagrams of a baseband frequency band of the present invention. FIG. 8 is a diagram of a cutoff frequency and an error rate of the present invention. FIG. 9 is a block diagram showing a configuration example of a conventional receiving apparatus. 10A to 10D are spectrum diagrams for explaining a conventional receiving apparatus.
1 Antenna 2 RF amplifier 3, 7, 8 Mixer 4, 9 PLL
5 BPF
6 IF amplifier 10 Phase shifter 21 Transport stream output terminal (TS output terminal)
30, 31 Baseband amplifier (BB amplifier)
32, 33 AD converter (ADC)
34 Complex multiplier 35 Numerically controlled oscillator (NCO)
36,37 Digital LPF
38 OFDM demodulators 61 and 62 Low pass filter (LPF)

Claims (5)

What is claimed is: 1. A receiving apparatus comprising: a mixer circuit that converts a frequency of a received signal; and a filter that removes an unnecessary signal from the frequency-converted received signal. A receiving device comprising means for changing a cutoff frequency of a filter according to a position of a segment to be selected and received. 2. The apparatus according to claim 1, further comprising: a first filter and a second filter for removing unnecessary signals from the frequency-converted received signal, and means for changing a cutoff frequency of the second filter according to characteristics of the first filter. The receiving device according to claim 1. A frequency adjustment circuit connected to the reference clock; and a DC voltage correction circuit connected to the frequency adjustment circuit, wherein the DC voltage correction circuit has means for making the cutoff frequency of the filter constant for different reference clock signal frequencies. The receiving device according to claim 1. The receiving device according to claim 1, further comprising a DC voltage correction circuit between the second filter and the frequency adjustment circuit, and a unit that performs DC voltage correction for different reference clock signal frequencies. The receiving apparatus according to claim 1, further comprising means for changing a cutoff frequency according to a bit error rate.

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