JP2006310940A - Signal generating circuit, frequency conversion circuit, frequency synthesizer, and communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a signal generating circuit capable of generating a sine wave with reduced spurious radiation with a small circuit scale. <P>SOLUTION: The signal generating circuit is characterized in to include: a rectangular wave output circuit 12 for outputting a rectangular wave signal; and a polyphase filter 14 for receiving a signal outputted from the rectangular wave output circuit 12 and associating a pole of its own frequency region with a third harmonic included in the received signal. Thus, the signal generating circuit can obtain the sine wave signal wherein the third harmonic being a most influencing factor of the spurious radiation is reduced with a small circuit area. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a signal generation circuit that generates a sine wave, a frequency conversion circuit using the same, a frequency synthesizer, and a communication system.

  In the communication system, a frequency conversion circuit is used, and a mixer can be used as the frequency conversion circuit (see, for example, Patent Document 1). In such a frequency conversion circuit such as a mixer, when a signal including an unnecessary tone, that is, spurious is input, spurious is also generated in the output signal.

For example, when a mixer is used in a frequency synthesizer or the like, a frequency divider is often connected in front of the mixer input terminal. In this case, the input signal of the mixer is a square wave. Here, when considering a square wave of a certain frequency ω ₀ , this square wave is synthesized by a sine wave of frequencies ω ₀ , 2ω ₀ , 3ω ₀ , 4ω ₀ , 5ω _0. It is represented by That is, when square waves are mixed, their harmonics are also mixed, and many spurious are generated. In order to avoid this, the signal input to the mixer is preferably sinusoidal.
JP 2002-135157 A

  However, conventionally, in order to obtain a nearly complete sine wave from a signal including spurious and a square wave, a low path filter (LPF), a high path filter (HPF), a band path filter (BPF), and a band rejection filter (BRF). However, in this case, a high-order BPF or LC resonator with high frequency selectivity is required, and these circuits require a large area, which has been a factor in raising the cost of the semiconductor chip. .

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a signal generation circuit capable of generating a sine wave with reduced spurious with a small circuit area.

  One embodiment of the present invention relates to a signal generation circuit. A square wave output circuit for outputting a square wave signal and a signal output from the square wave output circuit are input, and the Nth harmonic (N is an odd number of 3 or more) included in the input signal One or a plurality of polyphase filters each having a corresponding frequency domain pole are provided.

  According to this aspect, the polyphase filter can remove harmonics of a square-wave signal that causes spurious generation. Therefore, it is possible to obtain a sine wave from which spurious is removed or reduced by a square wave output circuit that can be realized with a simple circuit configuration and a polyphase filter having a small circuit area.

  In this aspect, at least one of the one-stage or multiple-stage polyphase filters may associate the pole of its own frequency domain with the third harmonic contained in the input signal. . As a result, it is possible to remove or reduce the third harmonic that causes spurious noise in the vicinity of the desired wave with a small circuit area.

  In this aspect, of the one-stage or plural-stage polyphase filters, at least one of the polyphase filters may include a variable element so that the “pole” can be controlled. Thereby, even when the frequency of the square-wave input signal fluctuates, the “pole” of the polyphase filter can always correspond to the third harmonic of the input signal by controlling the variable element.

  Another aspect of the present invention relates to a frequency conversion circuit. This circuit includes a mixer and a signal generation circuit according to the present invention connected to at least one of the input ports of the mixer. According to this aspect, since the signal from which unnecessary spurious is removed is input to the mixer, it is possible to obtain a signal without spurious even in the frequency-converted signal. In addition, since the polyphase filter is used, the circuit scale does not increase so much. Therefore, it is possible to achieve both improvement in the characteristics of the frequency conversion circuit and size reduction.

  Yet another aspect of the present invention relates to a frequency synthesizer. This frequency synthesizer is a frequency synthesizer including a plurality of frequency conversion circuits, and the plurality of frequency conversion circuits connected in parallel or in a column, and the frequency conversion circuit located at least in the final stage is the frequency conversion circuit according to the present invention. It is characterized by being. According to this aspect, since the frequency conversion circuit located at least in the final stage is a frequency conversion circuit using a polyphase filter, the output of the frequency synthesizer obtains a sine wave having a desired frequency in which unnecessary spurious is suppressed. In addition, it can be configured with a small circuit area. Therefore, both improvement in characteristics and downsizing can be achieved.

  Yet another embodiment of the present invention relates to a communication system. This communication system includes an oscillating unit that oscillates a local signal, and a frequency conversion circuit that generates a signal having a predetermined frequency by mixing the oscillated local signal and an externally received signal. It is characterized by comprising the frequency synthesizer according to the invention. According to this aspect, it is possible to construct a communication system having both characteristics and circuit scale.

  Note that any combination of the above-described constituent elements, and those in which the constituent elements and expressions of the present invention are mutually replaced between methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to the present invention, a sinusoidal signal with reduced spurious can be obtained with a small circuit area.

  Hereinafter, a description will be given based on a preferred embodiment of the present invention.

(Embodiment 1)
FIG. 1 is a diagram showing a configuration of a sine wave generation circuit 10 according to Embodiment 1 of the present invention. The sine wave generation circuit 10 includes a square wave output circuit 12 and a polyphase filter (PPF) 14.

  The square wave output circuit 12 outputs a multi-phase square wave. That is, four square waves I +, I−, Q +, and Q− having the same frequency F1 and having phases shifted from each other by 90 ° are output. When I + is used as a reference phase, a waveform advanced 90 ° from the reference phase is Q +, a waveform advanced 180 ° from the reference phase is I−, and a waveform advanced 270 ° from the reference phase is Q−. The square wave output circuit 12 that outputs such a square wave can be realized by a phase locked loop (PLL) circuit, a ring voltage controlled oscillators (VCO), a ring oscillator, or the like. A frequency divider may be used as the square wave output circuit 12. In this case, the square wave output circuit 12 has an input terminal to which a square wave signal is input.

  The four square waves I +, I−, Q +, Q− output by the square wave output circuit 12 are input to the polyphase filter 14. Although details will be described later, the polyphase filter 14 has a characteristic of removing or attenuating a frequency component corresponding to the pole of the frequency characteristic when operated with a negative frequency characteristic. There are four output terminals and four paths connecting them. Then, the square wave I + is input to the first path, the square wave Q + is input to the second path, the square wave I− is input to the third path, and the square wave Q− is input to the fourth path. Is input. Further, in the polyphase filter 14 according to the first embodiment, the frequency characteristic pole Fd is adjusted to be three times the square wave frequency F 1 output from the square wave output circuit 12.

  Details of the polyphase filter will be described below. FIG. 2 is a diagram illustrating a configuration of the polyphase filter. As described above, the polyphase filter 14 includes four input terminals and four output terminals, and resistors R2, R4, R6, and R8 are provided on the four paths that connect them. The input side of the first resistor R2 in the first path is connected to the output side of the second resistor R4 in the second path, and the first capacitor C2 is inserted in series in the path. Further, the output side of the first resistor R2 in the first path and the input side of the fourth resistor R8 in the fourth path are connected, and a fourth capacitor C8 is provided in series in the path. Similarly, the input side of the second resistor R4 in the second path is connected to the output side of the third resistor R6 in the third path, and the second capacitor C4 is provided in series in the path. Further, the input side of the third resistor R6 in the third path is connected to the output side of the fourth resistor R8 in the fourth path, and the third capacitor C6 is provided in series in the path. The four resistors R2, R4, R6, and R8 have the same resistance value R, and the four capacitors C2, C4, C6, and C8 have the same capacitance value C.

  In this configuration, a sine wave of the reference phase is input to the input terminal of the first path, a sine wave advanced by 90 ° from the reference phase is input to the input terminal of the second path, and a sine wave advanced by 180 ° from the reference phase is obtained. When a sine wave input to the input terminal of the third path and advanced by 270 ° from the reference phase is input to the input terminal of the fourth path, the polyphase filter 14 operates in the positive frequency region.

  In contrast, a sine wave of the reference phase is input to the input terminal of the second path, a sine wave advanced by 90 ° from the reference phase is input to the input terminal of the first path, and a sine wave advanced by 180 ° from the reference phase. When a sine wave input to the input terminal of the fourth path and advanced by 270 ° from the reference phase is input to the input terminal of the third path, the polyphase filter 14 operates in the negative frequency region.

  FIG. 3 is a diagram showing the positive and negative frequency characteristics of the polyphase filter connected together. As shown in FIG. 3, the polyphase filter has poles a and b in both positive and negative frequency regions. In particular, a pole a with a large attenuation, that is, a dip frequency exists in the negative frequency region. In the polyphase filter of FIG. 2, this dip frequency is expressed by 1 / (2πRC).

  In the present embodiment, as described above, the dip frequency of the polyphase filter 14 is adjusted to be three times the frequency of the input square wave. That is, the polyphase filter 14 can remove harmonics, particularly third harmonics, contained in the input square wave.

Here, the circumstances for removing the third harmonic from the harmonics included in the square wave will be described. As an example, consider a square wave with a duty ratio of 50%. Since this is an odd function, the included harmonics are only odd-order components. When the Fourier series is expanded, the following equation is obtained.
_{E = sinω 0 -1 / 3sin3ω 0} + 1 / 5sin5ω 0 -1 / 7sin7ω 0 + ···
Among the harmonics, the third-order harmonic has the largest amplitude, and the amplitude decreases as the order increases from the fifth order to the seventh order. Further, since the third harmonic has the frequency closest to the fundamental wave component, spurious is generated in the vicinity of the desired wave. That is, the third order effect is the largest, and the adverse effect is less likely to occur as the order increases from the fifth order to the seventh order. Therefore, if the third harmonic of the square wave can be removed, a waveform that can be regarded as a sine wave recently can be obtained.

  4A shows a square wave I + output from the square wave output circuit 12, FIG. 4B shows a square wave Q +, FIG. 4C shows a square wave I−, and FIG. 4D shows a square wave Q−. Each phasor diagram is shown. When the phases of the four square waves are shifted from each other by 90 °, the respective primary components (1st) are also shifted from each other by 90 °. On the other hand, the third harmonic (3rd) has a phase rotation amount three times that of the first order component, and therefore is shifted by 270 ° (−90 °). That is, the phase relationship of the third harmonic in the four square waves I +, I−, Q +, Q− is opposite to that of the primary component.

  As described above, the polyphase filter 14 has a square wave I + at the input terminal of the first path, a square wave Q + at the input terminal of the second path, and a square wave I- at the input terminal of the third path. The wave Q− is input to the input terminal of the fourth path. As a result, the first-order component of the square wave is filtered with a positive frequency characteristic, and the third-order harmonic is filtered with a negative frequency characteristic. Therefore, the primary component of the square wave is output through the polyphase filter 14 almost as it is. On the other hand, since the pole Fd of the frequency characteristic of the polyphase filter 14 is adjusted to be three times the frequency F1 of the square wave output from the square wave output circuit 12, the third harmonic of the square wave is polyphase It is removed or attenuated by the filter 14. That is, the polyphase filter 14 can obtain a sine wave in which the third harmonic is removed from the square wave.

  High-order bandpass filters and LC resonators require a sharp frequency selectivity between the first-order component and the third-order harmonic component, and therefore a steep frequency characteristic is required. Multi-stage is required. When the polyphase filter is used, it is not necessary to have a sharp frequency selectivity between the first-order component and the third-order harmonic component of the square wave. That is, no steep frequency characteristics are required in the negative frequency region shown in FIG. As described above, the polyphase filter has a steep frequency characteristic because the operation regions of the first-order component and the third-order harmonic of the square wave are divided into “positive” and “negative”, respectively. Even if not, the first-order component passes through the polyphase filter almost as it is, and only the third-order harmonic is removed or attenuated. Therefore, the polyphase filter does not need to be multistaged and can be realized with a small circuit.

  A high-order bandpass filter or LC resonator is a filter that passes the first-order component, and is configured with a time constant corresponding to the first-order component, whereas a polyphase filter is a third-order harmonic. Since the filter removes or reduces the wave, it may be configured with a time constant corresponding to the third harmonic. A circuit having a larger time constant corresponding to a low frequency is required, and this is also the reason why the polyphase filter can be made smaller than a high-order bandpass filter or LC resonator.

  As described above, according to the first embodiment, a multi-phase square wave is easily generated by a circuit such as a PLL, a VCO, and a frequency divider, and the frequency characteristic pole is three times that of the generated square wave. By filtering the square wave with a polyphase filter adjusted to the frequency, an almost complete sine wave can be obtained with a small circuit area compared to the case of using a high-order bandpass filter or an LC resonator. it can.

(Embodiment 2)
FIG. 5 is a diagram showing a configuration of the sine wave generation circuit 20 according to the second embodiment of the present invention. The configuration of the sine wave generation circuit 20 in the second embodiment is basically the same as the configuration in the first embodiment. The difference is that the polyphase filter 14 in the first embodiment is configured by a variable element.

  That is, the polyphase filter 22 according to the second embodiment uses variable elements such as variable resistors and variable capacitors as constituent elements. Specifically, the resistors R2, R4, R6, and R8 and the capacitors C2, C4, C6, and C8 of the polyphase filter of FIG. 2 are variable resistors and variable capacitors, respectively, and their resistance values and capacitance values are set by external signals. Can be controlled.

  As described above, the pole of the frequency characteristic of the polyphase filter is 1 / (2πRC). Therefore, by controlling the resistance value R of the resistors R2, R4, R6, and R8 and the capacitance value C of the capacitors C2, C4, C6, and C8 with an external signal, the pole of the frequency characteristic of the polyphase filter is controlled. Is possible.

  Thereby, in the sine wave generation circuit 20, even when the frequency of the square wave output from the square wave output circuit 12 changes, the value of the variable element of the polyphase filter 22 is externally adjusted in accordance with the frequency of the square wave. Can be controlled. That is, the pole of the frequency characteristic of the polyphase filter 22 can be easily set to three times the frequency of the square wave.

  As described above, the sine wave generation circuit 20 according to the second embodiment can be obtained by dynamically controlling the frequency of the square wave output from the square wave output circuit 12 and the pole of the frequency characteristic of the polyphase filter 22. The frequency of the sine wave can be made variable. In addition, since a polyphase filter is used, it can be configured with a small circuit area.

(Embodiment 3)
FIG. 6 is a diagram showing a configuration of a frequency conversion circuit 30 according to the third embodiment to which the sine wave generation circuit according to the present invention is applied. The frequency conversion circuit 30 includes two sine wave generation circuits 32 and 34 and a single side band (SSB) mixer (hereinafter referred to as an SSB mixer) 36. As the sine wave generation circuits 32 and 34, the sine wave generation circuits 10 and 20 of the first embodiment or the second embodiment can be applied.

  The frequency conversion circuit 30 generates four sine waves I +, I−, Q +, and Q−, which are generated by the sine wave generation circuit 32 and all have the frequency F1 and are out of phase with each other by 90 °. Is input to the input port. Also, the other input port of the SSB mixer 32 has four sine waves I +, I−, Q +, Q− generated by the sine wave generation circuit 34, all having the frequency F2 and having phases shifted from each other by 90 °. Is entered. The SSB mixer 36 has four sine waves I +, I−, and Q + having a difference frequency (F1−F2) from two sets of sine waves having frequencies F1 and F2, respectively, and having phases shifted from each other by 90 °. , Q-.

  In the two sets of sine waves input to the SSB mixer 36, since the harmonics included in the square wave are both reduced and the signal is input to the SSB mixer 36, unnecessary spurious can be efficiently suppressed. Can do. In addition, since a polyphase filter is used, it can be configured with a small circuit area.

  As described above, the frequency conversion circuit 30 according to the third embodiment can achieve both improvement in characteristics and miniaturization.

(Embodiment 4)
FIG. 7 is a diagram showing a configuration of a frequency synthesizer 40 according to the fourth embodiment to which the frequency conversion circuit according to the present invention is applied. The frequency synthesizer of FIG. 6 is composed of a plurality of frequency conversion circuits, and among these, the PPF-mounted frequency conversion circuit 30 provided at the output stage of the frequency synthesizer adapts the frequency conversion circuit 30 described in the third embodiment. Can do.

  In FIG. 6, a signal (frequency FIN) input from the outside is input to frequency conversion circuits 42 and 44, and is converted to a desired frequency. The signal frequency-converted by the frequency conversion circuit 42 is input to one input port of the PPF-mounted frequency conversion circuit 30. The signal frequency-converted by the frequency conversion circuit 44 is further frequency-converted by the frequency conversion circuits 46 and 48 and input to the other input port of the PPF-mounted frequency conversion circuit 30. The PPF frequency conversion circuit 30 performs frequency conversion based on the signals input from the frequency conversion circuits 42 and 48 as described in the third embodiment, and outputs a sine wave signal having the frequency FOUT.

  In the frequency synthesizer 40 according to the fourth embodiment, one stage of the frequency conversion circuit and one side of the frequency conversion circuit until the input signal is input to the two input ports of the PPF-mounted frequency conversion circuit 30. Although passing through three stages, it is not limited to this, and the number of stages of the frequency conversion circuit may be any number. Further, the input signal may be directly input to the PPF-mounted frequency conversion circuit 30 without going through the frequency conversion circuit. A PPF-mounted frequency conversion circuit may be used for the frequency conversion circuit other than the final stage.

  As described above, since the frequency synthesizer 40 according to the fourth embodiment uses the frequency conversion circuit 30 described in the third embodiment as the frequency conversion circuit located at the final stage, unnecessary spurious is suppressed. A sine wave having a desired frequency can be obtained. In addition, since a polyphase filter is used, it can be configured with a small circuit area. Therefore, the frequency conversion circuit 40 according to the fourth embodiment can achieve both improvement in characteristics and downsizing.

(Embodiment 5)
FIG. 8 is a diagram showing a communication system according to the fifth embodiment to which the frequency synthesizer according to the present invention is applied. The communication system 50 of FIG. 8 shows a direct conversion reception (DCR) system, but is not limited thereto, and can be applied to other reception systems such as a heterodyne reception system.

  In FIG. 8, the RF signal received from the antenna 52 is input to an LNA (Low Noise Amplifier) 56 via a band pass filter 54. The LNA 56 amplifies the RF signal with low noise and outputs the amplified RF signal to two frequency conversion circuits 57 for the I signal and the Q signal which are orthogonal baseband signals.

  The local oscillator 58 outputs a local signal having a local (Lo) frequency. The frequency synthesizer 40 described in the fourth embodiment can be applied to the local oscillator 58. The phase shifter 60 outputs the Lo signal without changing the phase of the Lo signal to the frequency converter circuit 10 of the I system, and the phase advances by 90 ° with respect to the Lo signal output to the frequency converter circuit 10 of the Q system. The Lo signal is also output to the Q system frequency conversion circuit 10. The signals output to these are square waves.

  The frequency conversion circuit 57 for the I system and the Q system mixes the RF signal and the Lo signal, and outputs a signal having a frequency difference between them to the low-pass filters 62 and 68, respectively. The output signals of the low-pass filters 62 and 68 are amplified by the amplifiers 64 and 70 of the respective systems and converted into digital signals by the analog-digital converters 66 and 72 of the respective systems.

  As described above, by using the local oscillator 58 in the present embodiment in the communication system 50, it is possible to achieve both improvement in characteristics and downsizing of the communication system. In particular, when all or part of the circuit elements described as the communication system 50 is formed as a semiconductor chip as a frequency synthesizer, the chip area can be reduced.

  The present invention has been described based on the embodiments. The embodiments are exemplifications, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. .

  For example, in the first and second embodiments, in order to reduce the third-order harmonics, a single-phase polyphase filter is used in which the poles of its own frequency region correspond to the third-order harmonics. In this regard, in order to reduce the third and fifth harmonics, one stage of the polyphase filter in which the poles of its own frequency region correspond to the third harmonic, and the polyphase filter corresponding to the fifth harmonic One stage, a total of two stages, may be used. Furthermore, in order to reduce the frequency components after the 7th harmonic, a configuration of three or more stages in which polyphase filters corresponding to the poles of its own frequency region are connected in cascade for each harmonic may be adopted. According to this, spurious can be reduced more accurately.

  In the second embodiment, the polyphase filter including a variable element has been described. In the second embodiment, both resistance and capacitance can be controlled as variable elements, but only one of them may be controlled as variable elements. In this regard, one or more of the plurality of polyphase filters constituting the frequency conversion circuit may include a variable element. Also by this, the frequency of the obtained sine wave can be made variable.

  In the third embodiment, the configuration in which the polyphase filters 32 and 34 are provided for both of two input signals having different frequencies to the SSB mixer 36 has been described. In this regard, a polyphase filter may be provided only on one side. This also exhibits a certain degree of spurious reduction effect.

  Furthermore, a polyphase filter may be provided on the output side of the SSB mixer 36. In this case, the spurious can be reduced by making the frequency domain pole in the polyphase filter correspond to the frequency domain of the spurious to be reduced generated in the output signal of the SSB mixer 36. In this case, the polyphase filter 32 may not be provided on the input side of the SSB mixer 36.

It is a block diagram of the sine wave generation circuit which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of a polyphase filter. It is a figure which connects and shows the positive and negative frequency characteristic of a polyphase filter. (A) It is a phasor figure of sine wave I +. (B) Phasor diagram of sine wave Q +. (C) Phasor diagram of sine wave I-. (D) It is a phasor figure of sine wave Q-. It is a block diagram of the sine wave generation circuit which concerns on Embodiment 2 of this invention. It is a block diagram of the frequency converter circuit which concerns on Embodiment 3 of this invention. It is a block diagram of the frequency synthesizer which concerns on Embodiment 4 of this invention. It is a block diagram of the communication system which concerns on Embodiment 5 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Sine wave generation circuit 12 Square wave output circuit 14 Polyphase filter 20 Sine wave generation circuit 22 Polyphase filter 30 Frequency conversion circuit 32 Sine wave generation circuit 34 Sine wave generation circuit 36 SSB mixer 40 Frequency synthesizer 42, 44, 46, 48 Frequency conversion circuit 50 Communication system 52 Antenna 54 Bandpass filter 56 LNA
57 Frequency conversion circuit 58 Local oscillator 60 Phase shifter 62 Low pass filter 64 Amplifier 66 Analog to digital converter

Claims (6)

A square wave output circuit for outputting a square wave signal;
A signal output from the square wave output circuit is input, and a polyphase filter is used in which the poles of its own frequency domain correspond to the Nth harmonic (N is an odd number of 3 or more) included in the input signal. A signal generation circuit comprising one or more stages.   The at least one polyphase filter of the one-stage or plural-stage polyphase filters corresponds to a third-order harmonic contained in an input signal with its own frequency domain pole. Item 2. The signal generation circuit according to Item 1.   3. The signal generation circuit according to claim 1, wherein at least one of the one-stage or multiple-stage polyphase filters includes a variable element, and makes the pole controllable. 4. A mixer,
The signal generation circuit according to any one of claims 1 to 3, connected to at least one of the input ports of the mixer;
A frequency conversion circuit comprising: A frequency synthesizer including a plurality of frequency conversion circuits, wherein the plurality of frequency conversion circuits are connected in parallel or in columns,
The frequency synthesizer according to claim 4, wherein the frequency conversion circuit located at least in the final stage is the frequency conversion circuit according to claim 4. An oscillator that oscillates a local signal;
A frequency conversion circuit that mixes an oscillated local signal and an externally received signal to generate a signal of a predetermined frequency, and
6. The communication system according to claim 5, wherein the oscillating unit comprises the frequency synthesizer according to claim 5.

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