JP2005151477A - Transmitter and digital quadrature modulation circuit used for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a digital quadrature modulation circuit with high stability of frequency, wherein output carrier frequency is not influenced by the variation of sampling frequency of a digital baseband signal with low stability of frequency. <P>SOLUTION: A sampling clock of a sampling frequency generator 16 for processing a digital baseband signal is used for a quadrature carrier wave of digital quadrature modulation, and at the same time, is used as a signal for local signal generation for up-converting the quadrature modulation output. By this method, in a mixer 15 for the up-converting, frequency variation components of the sampling clock are canceled, and an output carrier signal with high stability of frequency is obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a transmission apparatus and a digital quadrature modulation circuit used therefor, and more particularly to a digital quadrature modulation system with improved frequency stability.

  In a television broadcasting system, a voice, and a mobile phone communication system, an orthogonal modulation method is widely adopted as a signal modulation method (see Patent Documents 1 and 2). This orthogonal modulation method is a method for modulating the I and Q components (orthogonal components) of a baseband signal to be transmitted using a pair of orthogonal carriers having a frequency equal to the frequency of the output carry-out wave.

  A conventional quadrature modulation circuit analog-converts each of the I and Q components of a digital baseband signal digitally processed at a specified sampling frequency by a D / A converter, and performs analog modulation using a specified orthogonal carrier wave. And a signal having a prescribed output carrier frequency is obtained, which is called an analog quadrature modulation method. FIG. 3 shows a functional block of a transmission apparatus using this modulation method.

  In FIG. 3, reference numerals 1 and 2 denote I and Q baseband signal processing units, and reference numerals 17 and 18 denote D / A for converting the I and Q baseband signals into analog signals at the sampling frequency (fb) by the sampling frequency generation unit 16. It is a converter. Reference numerals 19 and 20 denote low-pass filters (LPF) for the purpose of suppressing high-frequency components in the outputs of the D / A converters 17 and 18, and reference numeral 21 denotes an analog quadrature modulation unit. In the quadrature modulation unit 21, a pair of orthogonal carriers is generated by the orthogonal carrier generation circuit 24 using the local signal (lf) from the local signal oscillator 26, and baseband signals from the LPFs 19 and 20 are output by the mixers 22 and 23. Are up-converted to the frequency (lf) of the pair of orthogonal carriers. Reference numeral 10 denotes a synthesizer.

Here, when the I signal is cos ωt, the Q signal is sin ωt, the pair of orthogonal carriers is cos ωct, and sin ωct, the quadrature modulation signal is
cosωct × cosωt = (1/2) {cos (ωct + ωt) +
cos (ωct−ωt)} (1)
sin ωct × sin ωt = (1/2) {cos (ωct + ωt) −
cos (ωct−ωt)} (2)
It becomes. Therefore, the output of the analog quadrature modulation unit 21 is
(1) Expression + (2) Expression = cos (ωct−ωt) (3)
It becomes.

  As can be seen from Equation (3), the frequency stability of the modulation output in the analog quadrature modulation method depends only on the frequency stability of the orthogonal carrier wave by the local signal. FIG. 4 shows the frequency spectrum of each part of the signal in the analog quadrature modulation circuit shown in FIG. 3. The I and Q components of the baseband signal are directly increased to the specified carrier frequency (FR = lf). It turns out that it is converting.

  In recent years, high accuracy has been required for the modulation accuracy of the quadrature modulation circuit, and a digital quadrature modulation method that uses digital signal processing to reduce the quadrature modulation error has been adopted. FIG. 5 shows a functional block diagram of a transmission apparatus using this digital quadrature modulation method. In FIG. 5, 1 and 2 are I and Q digital baseband signal processing units, and 3 and 4 perform speed conversion of the I and Q baseband signals to a rate of 4n times (n is a positive integer) by oversampling. Is an interpolation filter for filtering. Reference numeral 16 denotes a sampling frequency (fb) generator, and reference numeral 5 denotes a circuit for multiplying the sampling clock by four.

  Reference numeral 6 denotes a digital quadrature modulation unit that digitally quadrature-modulates the I and Q signals that are output signals of the interpolation filters 3 and 4, and reference numeral 11 denotes a D / D that converts the modulation output into an analog signal using the output of the quadruple circuit 5. An A converter, 15 is a mixer that mixes an analog signal from the D / A converter with a local frequency (lf + fb) from the local signal oscillator 17 and up-converts the analog signal.

  In the digital quadrature modulation unit 6, a pair of quadrature carriers is generated from the output of the quadrature circuit 5 by the quadrature carrier generation circuit 9, and frequency-mixed with the I and Q signals from the interpolation filters 3 and 4 in the mixers 7 and 8. Then, by combining by the combiner 10, an orthogonal modulation signal is output. The quadrature modulation signal thus obtained is converted into an analog signal by the D / A converter 11 and is up-converted by the mixer 15 to the target output carrier frequency.

  FIG. 6 shows the frequency spectrum of each part of the signal in the digital quadrature modulation circuit shown in FIG. 5. The I and Q baseband signals are frequencies lower than the specified output carrier frequency by the digital quadrature modulation unit 6 once. After that, it is up-converted to the specified output carrier frequency FR by the mixer 15, and it is understood that the two-stage frequency conversion is performed.

  FIG. 7 shows, for reference, that the digital quadrature modulation circuit shown in FIG. 5 can perform digital quadrature modulation at a rate of 1 / 4n using three values of 0, 1, and -1. Is. That is, after the I and Q digital baseband signal processing units 1 and 2 process the I and Q signals at the sampling rate fb, they are input to the interpolation filters 3 and 4 for rate conversion. In the interpolation filters 3 and 4, in order to make the sampling rate (fb) of the I and Q baseband signals and the output frequency of the digital quadrature modulation unit 6 the same, the rate of the I and Q baseband signals is increased four times, In the digital quadrature modulation unit 6, the I and Q signals maintain the relationship of n multiples of 4 (4nfb), so that the digital quadrature carrier signal uses three values of 0, 1, and -1, as shown in FIG. Digital quadrature modulation at a rate of 1 / 4n becomes possible.

JP-A-8-167918 JP 2001-217891 A

  The first problem with the above-described quadrature modulation method is signal quality degradation. The reason is that there are mainly two types of signal quality degradation in the above-described quadrature modulation method, one of which is degradation of modulation accuracy due to quadrature modulation error, and the other is degradation due to output frequency fluctuation. The signal degradation due to the quadrature modulation error is caused by a phase difference of 0, 90 degrees and an amplitude error of the quadrature carrier generation circuit. The output frequency fluctuation is caused by the frequency fluctuation of the local signal oscillator.

  In the conventional analog quadrature modulation circuit as shown in FIG. 3, the digital signal processing and quadrature modulation processing of the I and Q baseband signals are independent from each other, and are separated from each other. With regard to the performance, no special consideration is required for digital signal processing. Therefore, when obtaining a highly accurate stability for the output frequency, there is an advantage that it can be realized by synchronizing the local oscillator with a highly stable reference signal from the outside.

  However, in a system that requires a high C / N (Carrier to Noise Ratio), the analog quadrature modulation circuit is disadvantageous in terms of quality degradation peculiar to analog quadrature modulation, such as performance degradation and quadrature error due to leakage of a carrier signal. Therefore, digital quadrature modulation has become mainstream.

  The second problem is that in digital quadrature modulation, both the sampling clock and the local signal carrier affect the output carrier. The reason is that the digital quadrature modulation circuit as shown in FIG. 5 does not cause the carrier leak problem and the quadrature modulation error problem that have occurred in the analog quadrature modulation method because the digital quadrature modulation is performed. The stability issue is emerging. As described above, since the combination of the digital quadrature modulation output frequency and the frequency of the local signal for up-conversion to the specified carrier becomes the output carrier frequency stability, digital High stability is required for the sampling clock frequency used in signal processing.

  Furthermore, since the clock stability for processing the I and Q digital baseband signals does not have the high frequency accuracy required for the output carrier wave, the frequency stability by the digital quadrature modulation processing and the output carrier wave It is necessary to separate the frequency stability.

  In addition, since the output frequency (FR) has a radio wave regulation, it is a value that satisfies it. However, the sampling clock (fb) varies depending on the processing of each system. It is necessary to examine the response of the PLL (Phase Locked Loop) circuit of the local oscillator. Another problem is that the frequency of the local signal oscillator needs to be changed in response to the digital sampling frequency change accompanying the system change.

  The object of the present invention is to maintain a high output carrier accuracy as in the case of the analog quadrature modulation method even when the digital quadrature modulation is adopted in the quadrature modulation method that requires stable quadrature modulation performance, and the digital signal. To provide a transmitter with high frequency stability and a digital quadrature modulation circuit used therefor so that the stability of the sampling signal used for processing is not affected by the stability of the local signal to be up-converted. .

  Another object of the present invention is to provide a transmitter with high frequency stability and a digital quadrature modulation circuit used therefor that can very easily cope with a digital sampling frequency change accompanying a system change.

  A transmission apparatus according to the present invention includes a digital orthogonal modulation means for orthogonally modulating a digital baseband signal, and a frequency conversion means for converting the modulation output into a transmission frequency and outputting the transmission frequency, wherein the digital orthogonal modulation means A signal synchronized with the orthogonal carrier wave is used for generating a local signal of the frequency converting means.

  A digital quadrature modulation circuit according to the present invention is a digital quadrature modulation circuit in a transmission apparatus that quadrature modulates a digital baseband signal and converts the modulation output into a transmission frequency by a frequency conversion means, and outputs the transmission frequency. The conversion unit is configured to be synchronized with a signal used for generating a local signal.

  The operation of the invention will be described. A sampling clock for baseband signal processing is used as a quadrature carrier wave of the digital quadrature modulation circuit, and is also used to generate a local signal of a frequency converter for up-conversion of quadrature modulation output. As a result, the signal synchronized with the quadrature carrier wave for digital quadrature modulation is also used for local signal generation, and the frequency fluctuation of the sampling clock with poor frequency stability is canceled by the up-converter frequency converter. As a result, the frequency stability of the output carrier wave, which is a transmission signal, is maintained well.

  According to the present invention, by using a signal synchronized with an orthogonal carrier wave used for digital orthogonal modulation for generating a local signal for up-conversion for obtaining an output carrier wave, it is possible to solve the problem of frequency performance as well as orthogonal performance. effective.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, and the same parts as those in FIG. 5 are denoted by the same reference numerals. Referring to FIG. 1, reference numerals 1 and 2 denote I and Q baseband signal processing units. Reference numerals 3 and 4 convert the I and Q baseband signals to a rate of 4n times (n is a positive integer) by oversampling. It is an interpolation filter for performing and filtering. Reference numeral 5 denotes a multiplier that performs speed conversion (frequency conversion) of the clock signal generated from the sampling frequency (fb) generation unit 16 to a digital quadrature modulation rate (4fb).

  6 is a digital quadrature modulation unit, mixers 7 and 8 for frequency-mixing the I and Q signals and the digital local signal, a quadrature carrier generation circuit 9 for generating a pair of digital quadrature carriers that are digital local signals, And a combiner 10 that combines the outputs of the mixers 7 and 8. The digital local signal is a signal synchronized with a clock signal having a frequency of 4 fb which is an output of the multiplier 5.

  Reference numeral 11 denotes a D / A converter that converts the output of the digital quadrature modulation unit 6 into an analog signal, and operates according to the sampling clock of the sampling frequency generation unit 16. A local signal oscillator 12 generates a local signal (frequency lf), and a mixer 13 mixes the frequency of the local signal and a sampling clock (frequency fb). Reference numeral 14 denotes a BPF (bandpass filter) that extracts a local signal necessary for generating an output carrier signal (frequency: FR) from the output of the mixer 13, and 15 is extracted by the output of the D / A converter 11 and the BPF 14. It is a mixer that mixes the frequency of the local signal. This mixer output becomes an output carrier signal (FR).

  The I and Q signals processed at the sampling rate fb in the I and Q digital baseband signal processing units 1 and 2 are input to the interpolation filters 3 and 4 for rate conversion. In the interpolation filters 3 and 4, in order to make the sampling rate (fb) of the I and Q baseband signals the same as the digital quadrature modulation output frequency, the rate of the I and Q baseband signals is multiplied by 4n. In the digital quadrature modulation unit 6, the I and Q signals maintain the relationship of n multiples of 4 (4n × fb), so that the digital quadrature carrier signal has three values of 0, 1, and −1 as shown in FIG. The fact that digital quadrature modulation at a rate of ¼n using can be performed is the same as the conventional example of FIG.

  As shown in FIG. 7, the digital carrier wave generation circuit 9 generates signals of cos φ and sin φ, which are orthogonal to each other. Therefore, orthogonal modulation can be performed with a simple circuit.

FIG. 2 shows the frequency spectrum of each signal in the circuit of FIG. 1, and the operation and effect of the present invention will be described with reference to FIG. It is assumed that the frequency fb in the sampling frequency generation unit 16 has changed by A, for example, to the plus side. At this time, the center frequency of the digital quadrature modulation output is (nfb + A), and the output frequency of the mixer 13 that outputs the frequency of the local signal is lf + (nfb + A). In the mixer 15, a sum component and a difference between them are generated. Therefore,
Sum component = (lf + nfb + A) + (nfb + A)
= Lf + 2nfb + 2A
Difference component = (lf + nfb + A) − (nfb + A)
= Lf
It becomes.

  By using the difference component lf as the output carrier signal (FR) as the output of the mixer 15, even if the frequency fb in the sampling frequency generator 16 has a variation A, this variation A has no effect on the output carrier signal. It can be seen that the digital quadrature modulation system with high stability can be obtained. Needless to say, the sampling frequency fb is exactly the same even if there is a negative fluctuation.

  The conventional digital quadrature modulation circuit shown in FIG. 5 generates a frequency obtained by adding the specified output frequency lf (FR) and the digital quadrature modulation frequency fb in order to set the oscillation frequency of the local signal oscillator 17 to (lf + fb). Although two crystals are required (in FIG. 5, it is shown as one crystal oscillator 17 for simplification), this circuit generates a specified output frequency lf (FR). Only one crystal is required, and the structure is simple. This is because the mixer 13 that mixes the frequency of the digital sampling clock (fb) with the output (lf) of the local signal oscillator 12 is used.

It is a functional block diagram which shows embodiment of this invention. It is a frequency spectrum figure of each part signal in the block of FIG. It is a functional block diagram of a conventional analog quadrature modulation circuit. It is a frequency spectrum figure of each part signal in the block of FIG. It is a functional block diagram of a conventional digital quadrature modulation circuit. It is a frequency spectrum figure of each part signal in the block of FIG. It is a figure explaining the operation | movement in digital quadrature modulation.

Explanation of symbols

1 I digital baseband signal processor
2 Q digital baseband signal processor
3,4 interpolation filter
5 multiplier circuit
6 Digital quadrature modulator 7, 8, 13, 15 Mixer
9 Orthogonal carrier wave generation circuit
10 Synthesizer
11 D / A converter
12 Local signal oscillator
14 BPF
16 Sampling frequency generator

Claims (5)

  A transmission apparatus including digital orthogonal modulation means for orthogonally modulating a digital baseband signal and frequency conversion means for converting the modulation output to a transmission frequency and outputting the transmission frequency, wherein the signal is synchronized with an orthogonal carrier wave in the digital orthogonal modulation means Is used for generating a local signal of the frequency conversion means.   The frequency converting means includes means for generating a local signal having a frequency equal to the transmission frequency, a first mixer for mixing the frequency of the local signal and the frequency of a signal synchronized with the orthogonal carrier, and the first 2. The transmission apparatus according to claim 1, further comprising a second mixer for converting a modulation output frequency of the digital quadrature modulation means using an output frequency of the mixer.   The transmission apparatus according to claim 2, wherein the second mixer derives a signal component of a frequency difference.   4. The transmission apparatus according to claim 1, wherein the orthogonal carrier wave of the digital orthogonal modulation means is a signal synchronized with a sampling frequency signal for the digital baseband signal processing.   A digital quadrature modulation circuit in a transmitter for orthogonally modulating a digital baseband signal and converting the modulated output into a transmission frequency by a frequency conversion unit and outputting the transmission frequency, and is used to generate a quadrature carrier and a local signal of the frequency conversion unit A digital quadrature modulation circuit configured to be synchronized with a signal.

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