JP2003264523A - Direct conversion receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct conversion receiver capable of minimizing the influence of a DC offset. <P>SOLUTION: This direct conversion receiver is provided with: a downconverter for converting the frequency of an analog RF signal modulated by an OFDM system such that the predetermined frequency of an occupied frequency of a receiving target segment becomes frequency zero; an AD converting means for converting a down-converted signal obtained by the downconverter into a digital signal; an FFT (fast Fourier transform) means for performing fast Fourier transform of the signal obtained by the AD converting means and converting a time axis into a frequency axis; and an undesired element eliminating means for eliminating undesired elements except the receiving target segment from an output signal of the FFT means. The downconverter converts the frequency of the analog RF signal so as to arrange a particular carrier signal included in the receiving target segment at the position of the frequency zero. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct conversion receiver, and more particularly to a direct conversion receiver that can be used for partial reception of only one segment in terrestrial digital TV broadcasting.

[0002]

2. Description of the Related Art In recent years, in a system for transmitting a video signal or an audio signal, an OFDM (Orthogonal Frequency Division Multiplexing) system has been proposed as a system excellent in high quality transmission and improvement in frequency utilization efficiency. ing.

The OFDM system is a modulation system in which a large number of subcarriers are set up within the band of one channel. For example,
MPE after converting analog TV signal to digital signal
Data compression is performed by G (Moving Picture Experts Group). Byte interleaving and bit interleaving are performed to disperse the cause of error occurrence in the transmission line such as noise in this data signal, and QPSK (Quadrature
Phase Shift Keying), 16QAM (Quadrature Amplit)
ude Modulation) and other mapping methods.

The mapped data is subjected to IFFT (Inverse Fourier Transform) after quadrature modulation after time interleaving and frequency interleaving to disperse the cause of error occurrence in the transmission path such as fading.
It is converted into an RF frequency and transmitted.

FIG. 1 shows the configuration of a digital television receiver.

[0006] In the digital television receiver, the TV signal is demodulated by performing an operation which is completely opposite to that on the transmitting side.

The RF input input from the antenna is input to the mixer 21. The mixer 21 includes a local signal generator 2
The signal corresponding to the channel selection is also input from 2, and the desired frequency is BP.
A signal whose frequency has been converted so as to be within the band of the F (band pass filter) 23 is output. The BPF 23 extracts only the desired frequency component.

The output of the BPF 23 is input to the mixers 24 and 25, respectively. To the mixers 24 and 25, the cosine signal and the sine signal are input from the 90-degree phase shifter 27, which receives the signal from the local signal generator 26. The mixers 24 and 25 down-convert the output of the BPF 23, which is the IF frequency, to obtain the real axis (I axis) component and the imaginary axis (Q axis).
It is converted into a Low IF signal composed of (axis) components and output to the analog / digital converters 7 and 8.

The analog / digital converters 7 and 8 convert the analog signals (I-axis component and Q-axis component) into digital signals and output them to the FFT circuit 9. The FFT circuit 9 performs a fast Fourier transform on the input signal, converts the time axis data into frequency data, and outputs the frequency data to the frequency deinterleave circuit 12.

The frequency deinterleave circuit 12 restores the frequency interleave performed to compensate for the loss of the specific frequency signal due to the reflection of radio waves. The output of the frequency deinterleave circuit 12 is the time deinterleave circuit 13
Sent to. The time deinterleave circuit 13 restores the time interleave applied for anti-fading or the like.

The time-deinterleaved I-axis and Q-axis signals are sent to the demapping circuit 14 and converted into 2 bits (QPSK), 4 bits (16QAM) or 6 bits (64QAM). The demapped signal is sent to the bit deinterleave circuit 15. The bit deinterleave circuit 15 cancels the bit interleave performed for the purpose of increasing error resilience. The output of the bit deinterleave circuit 15 is sent to the Viterbi decoding circuit 16. The Viterbi decoding circuit 16 performs error correction using the convolutional code performed on the transmission side.

The signal subjected to the Viterbi decoding is sent to the byte deinterleave circuit 17. The byte deinterleave circuit 17 cancels the byte interleave performed for the purpose of increasing the error resilience like the bit interleave. The output of the byte deinterleave circuit 17 is sent to the RS decoding circuit 18. The RS decoding circuit 18 is an RS (Reed Solomon)
Decoding is performed and error correction is performed. The error-corrected signal is
It is sent to the MPEG decoding circuit 19. The MPEG decoding circuit 19 expands the error-corrected signal (compressed signal) and outputs it to the digital / analog converter 20. The digital / analog converter 20 converts the signal sent from the MPEG decoding circuit 19 into an analog video and analog audio signal and outputs it.

In the Japanese terrestrial digital broadcasting system,
A signal format divided into segments is adopted. In the case of television broadcasting, 13 segments are grouped together and transmitted within a band of 6 MHz. In addition, in the central one of the 13 segments, partial reception that allows data broadcasting and the like is possible with only one segment. In contrast to 13-segment reception, the bandwidth is only 1/13 in partial reception, and reception is possible with almost the same configuration.

As the reception system, the so-called superheterodyne system in which the IF frequency is once converted and then the IF frequency is converted to LowIF has been described in the above description. QPSK
In the single carrier transmission method such as modulation, a direct conversion method for directly converting an RF signal into a baseband signal is also used as another reception method. In the direct conversion method, the bandpass filter 23 or the like which is usually realized by the SAW filter is not necessary, so that the number of parts can be deleted.

FIG. 2 shows the configuration of a direct conversion type digital television receiver developed by the present applicant (see Japanese Patent Application No. 2001-332910). This direct conversion digital TV receiver
It is different from the conventional direct conversion digital television receiver in that the unnecessary component removing circuit is provided, and it is not known at the time of filing of the present application.

The difference from the digital television receiver of the super-heterodyne system of FIG. 1 is as follows.

(1) A circuit (mixer 2) for converting an RF signal into an IF signal and further into a Low IF signal.
1, the local signal generator 22, the BPF 23, the local signal generator 26, the mixers 24 and 25, and the 90-degree phase shifter 27) have been deleted.

(2) A down converter 50 (local signal generator 1, mixers 2, 3 and 90 degree phase shifter 4) and LPFs 5 and 6 for down converting an RF signal are added.

(3) The unnecessary component removing circuits 10 and 11 are added.

[0020]

In OFDM reception, R
Besides directly converting the F signal to the Low IF signal, a method of frequency converting the RF signal so that the center frequency of the occupied frequency of the partial reception target segment becomes zero (DC component) is also possible (Japanese Patent Application No. 2001-2001). 332912). In this case, the direct current component (zero frequency component) is also an effective signal component, so a signal without a DC offset is required. However, it is not easy to generate a signal having no DC offset by the DC offset of the down converter or the AD converter.

Therefore, the applicant of the present invention has found that the center frequency of the occupied frequencies of the partial reception target segment is frequency zero (DC component).
In the direct conversion system digital television receiver in which the RF signal is frequency-converted so as to achieve the following, a device in which the DC offset is compensated has been developed and applied for a patent (see Japanese Patent Application No. 2001-332911).

By the way, the OFDM signal includes the transmission system ARIB STD-B31 for the terrestrial digital television broadcasting of the Radio Industries Association.
An Auxiliary Channel signal (hereinafter referred to as an AC signal) defined by is placed at a predetermined carrier position. This AC signal is a carrier signal for extension used for transmitting additional information and is not used as a data signal in the receiver.

The present inventors have noticed that the AC signal is not used as a data signal in the receiver, and that some of the AC signals are arranged near the center of the occupied band of the segment. , It was discovered that the influence of the DC offset can be minimized by performing the direct conversion so that the AC signal is arranged at the position of the DC frequency after the direct conversion.

The present invention has been made based on the above findings, and an object thereof is to provide a direct conversion receiver capable of minimizing the influence of DC offset.

[0025]

According to a first aspect of the present invention, an analog RF signal modulated by the OFDM system is down-converted so that a predetermined frequency of occupied frequencies of a reception target segment becomes zero. A converter, an AD conversion means for converting a down-converted signal obtained by the down converter into a digital signal, and an FFT means for performing a fast Fourier transform on the signal obtained by the AD conversion means to convert a time axis into a frequency axis. In a direct conversion receiver including an unnecessary component removing unit that removes an unnecessary component other than the reception target segment from the output signal of the FFT unit, the down converter uses a specific carrier signal included in the reception target segment at a frequency of zero. Analog R so that it is placed in the position of
The F signal is frequency-converted.

According to the invention described in claim 2, in the direct conversion receiver according to claim 1, a specific carrier signal is used for transmitting additional information specified by Japanese terrestrial digital broadcasting. Auxiliary Ch
It is an annel signal.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIGS.

FIG. 3 shows the structure of a direct conversion type digital television receiver. FIG. 4 shows the spectrum of each part of FIG.

In FIG. 3, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. The receiver of FIG. 3 and FIG.
The difference from the conventional example is as follows.

(1) A circuit (mixer 2) for converting an RF signal into an IF signal and further into a Low IF signal.
1, the local signal generator 22, the BPF 23, the local signal generator 26, the mixers 24 and 25, and the 90-degree phase shifter 27) have been deleted. (2) A down converter 60 (local part) that frequency-converts the RF signal so that one Auxiliary Channel signal (AC signal) near the center of the occupied frequency of the partial reception target segment is arranged at the position of frequency zero (DC component). Signal generator 1, mixers 2, 3, 90-degree phaser 4) and LPFs 5, 6
Added that. (3) The unnecessary component removing circuits 10 and 11 are added.

In terrestrial digital broadcasting, the spectrum is transmitted as shown in FIG. In FIG. 4A, U
The spectra for the HF 14-16 channels are shown. One channel is 6 HMz and one channel is composed of 13 segments. The central segment of these is a signal for partial reception. U
A case where the HF 15 is partially received will be described.

FIG. 4B is an enlarged view of the UHF 15, in which the segment to be partially received is S15-0 and the preceding and following segments are S15-1 to S15-12, of which S15-
0, and S15-1 and S15-2 both before and after that are shown. These segments S15-0 to S15-2
The upward arrow indicates the Auxiliary Channel signal (AC
Signal).

The RF signal of the UHF 15 is frequency-converted by the down converter (local signal generator 1, 90-degree phase shifter 4 and mixers 2, 3) shown in FIG. In order to convert the frequency of the RF signal from the local signal generator 1, one AC signal near the center of the occupied frequency of the partial reception target segment S15-0 is arranged at the position of frequency zero (DC component). The signal of the frequency is output. Further, since the 90-degree phase shifter 4 outputs the real axis component and the imaginary axis component,
The cosine signal and the sine signal are output to the mixers 2 and 3.

The spectrum after frequency conversion by the down converter is as shown in FIG. 4 (c). The down converter frequency-converts the RF signal so that one AC signal near the center of the occupied frequency of the partial reception target segment S15-0 is arranged at the position of frequency zero (DC component). Folding occurs at the center, and the returned signals are multiplexed.

The LPFs 5 and 6 remove harmonic components from this signal. The output spectrum after the harmonic components are removed by the LPFs 5 and 6 is as shown in FIG. L
Since an analog filter having a relatively gentle characteristic is used as the PFs 5 and 6, the spectrum of the output signal after the harmonic components are removed by the LPFs 5 and 6 is adjacent to the partial reception target segment S15-0 as well as the partial reception target segment S15-0. Some of the segments S15-1 and S15-2 also remain.

With respect to this signal, the FFT 9 converts the time axis data into the frequency axis data. The output spectrum of the FFT 9 is as shown in FIG.

The output of the FFT 9 is supplied to the unnecessary component removing circuit 10,
At 11, the unnecessary components S15-1 and S15-2 other than the partial reception target segment S15-0 are removed and output to the frequency deinterleave circuit 12. Therefore, the spectrum of the output signals of the unnecessary component removing circuits 10 and 11 is only the partial reception target segment S15-0 as shown in FIG. 4 (f). The signal processing after the frequency deinterleave circuit 12 is the same as that of the receiver of FIG.

As shown in FIG. 4 (f), the DC frequency component of the output of the FFT 9 is assigned to the AC signal.
The AC signal is an extension signal for transmitting additional information and is a signal that does not need to be demodulated by the receiver. Therefore, since the DC frequency component (AC signal) of the output of the FFT 9 is not a data signal component that needs demodulation, there is no problem even if there is a DC offset. Therefore, it is possible to avoid a data error caused by a slight deviation of the DC offset.

The actual carrier arrangement of the AC signal will be described. FIG. 5 shows the carrier arrangement of AC signals per segment.

In Mode 1, the number of carriers per segment is 108 with carrier numbers 0 to 107. AC signals (AC1_1, AC1_2) are assigned to carrier numbers 35 and 79. Thus, in Mode 1, the center of the carrier (frequency) of one segment is 54, while the AC signal is 3
They are located at 5,79 and there is no AC signal near the center of the carrier of one segment.

Therefore, in Mode 1, one AC signal is RF so that it is placed at the position of frequency zero (DC component).
When the signal is frequency converted, at the FFT output,
Since the balance between the positive frequency component and the negative frequency component becomes large, the present invention can be applied, but it is not preferable in terms of circuit configuration.

In Mode 2, the number of carriers per segment is 216 with carrier numbers 0 to 215. AC signals (AC1_1 to AC1_4) on carrier numbers 98, 101, 118, 136
Are arranged. The center of the career of one segment is 10
8 is the carrier number 101 or
The RF signal may be frequency-converted so that the 118 AC signals (AC1_2 to AC1_3) are arranged at the position of frequency zero (DC component).

AC of carrier number 101 in Mode 2
The spectrum of the output signals of the unnecessary component removing circuits 10 and 11 when the RF signal is frequency-converted so that the signal (AC1_2) is arranged at the position of frequency zero (DC component) is as shown in FIG.

In Mode 3, the number of carriers per segment is 432 with carrier numbers 0 to 431. The carrier number 7,89,206,209,226,244,377,407, AC signal (AC1_
1 to AC1_8) are arranged. Since the center of the carrier of one segment is 216, the RF signal is frequency-converted so that the AC signals (AC1_4 to AC1_5) of the carrier number 209 or 226 near it are arranged at the position of frequency zero (DC component). do it.

As shown in FIG. 7, a circuit for removing the DC offset may be provided to improve the performance such as preventing the FFT 9 from overflowing. In addition, DC
The causes of the offset are mixers 2 and 3, LPFs 5 and 6
When is an active filter, DC offsets of active elements and analog / digital conversion circuits 7 and 8 are included.

That is, the direct conversion type digital television receiver shown in FIG. 7 is provided with a DC offset detection circuit 103 for detecting a DC offset based on the output of the FFT circuit 9 and is provided for each analog / analog unit.
DC offset correction circuits 101 and 102 for correcting the DC offset based on the DC offsets detected by the DC offset detection circuit 103 are provided between the digital conversion circuits 7 and 8 and the FFT circuit 9.

Each DC offset correction circuit 101, 102
Then, according to the DC offset amount of the DC offset detection circuit 103, the DC of each of the analog / digital conversion circuits 7 and 8 is
Correct the output value.

FIG. 8 shows the configuration of the DC offset detection circuit 103.

The DC offset detection circuit 103 has an FFT.
The DC component of the output of the circuit 9 is integrated for a certain period, and the integration result is used as the DC offset amount. This integration result contains the DC offset amount and the AC signal component. The AC signal is "1" when no data is transmitted, but since it is not used by the receiver, the A signal is corrected by the DC offset correction.
Even if the C signal cannot be demodulated accurately, there is no problem in operation.

The DC offset detection circuit 103 shown in FIG.
A first circuit (DC position detection circuit 201) for giving an offset amount to one of the DC offset correction circuits 101
And an integrator 202) and a second circuit (DC position detection circuit 301 and integrator 302) for giving an offset amount to the other DC offset correction circuit 102.

Since the operations of both circuits are the same, only the operation of the first circuit will be described. DC position detection circuit 201
Receives one of the signals output from the FFT circuit 9, and detects the level at the DC position. The integrator 202 is D
The level of the DC position detected by the C position detection circuit 201 is integrated for a certain period, and the integration result is output as an offset amount.

[0052]

According to the present invention, the effective data component is D
A direct conversion receiver in which C offset does not occur can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a conventional digital television receiver.

FIG. 2 is a block diagram showing a configuration of a direct conversion type digital television receiver developed by the present applicant.

FIG. 3 is a block diagram showing a configuration of a digital television receiver according to the embodiment of the present invention.

FIG. 4 is a schematic diagram showing a spectrum of each part of FIG.

FIG. 5 is a diagram showing an AC signal carrier arrangement per segment.

FIG. 6 is an AC signal of carrier number 101 in Mode 2
It is a schematic diagram which shows the spectrum of the output signal of the unnecessary component removal circuits 10 and 11 when frequency-converting RF signal so that (AC1_2) may be arrange | positioned at the position of frequency zero (DC component).

FIG. 7 is a block diagram showing a configuration of a digital television receiver according to another embodiment.

FIG. 8 is a block diagram showing a configuration of a DC offset detection circuit 103.

[Explanation of symbols]

1 Local signal generator A few mixers 4 90 degree phaser 5, 6 LPF 9 FFT circuit 10, 11 Unnecessary component removal circuit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kiyoo Hanafusa             2-5-3 Keihan Hondori, Moriguchi City, Osaka Prefecture             Within Yo Denki Co., Ltd. (72) Inventor Toshiya Iwasaki             2-5-3 Keihan Hondori, Moriguchi City, Osaka Prefecture             Within Yo Denki Co., Ltd. F-term (reference) 5C025 AA30 BA25 BA30 DA01                 5K022 DD01 DD33

Claims (2)

[Claims] 1. An analog RF modulated by the OFDM system
A signal is obtained by a down converter for performing frequency conversion so that a predetermined frequency of the occupied frequency of the reception target segment becomes zero, and AD conversion means and AD conversion means for converting the down conversion signal obtained by the down converter into a digital signal. Fast Fourier transform is applied to the obtained signal to transform the time axis into the frequency axis.
In the direct conversion receiver including an unnecessary component removing unit that removes an unnecessary component other than the reception target segment from the output signals of the FFT unit and the FFT unit, the down converter uses a frequency of a specific carrier signal included in the reception target segment. A direct conversion receiver characterized by performing frequency conversion of an analog RF signal so as to be arranged at a position of zero. 2. The direct conversion reception according to claim 1, wherein the specific carrier signal is an Auxiliary Channel signal used for transmitting additional information, which is regulated by Japanese terrestrial digital broadcasting. Machine.