JP2006121665A - Receiver if circuit including image rejection mixer and active band-pass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a receiver IF circuit which reduces costs of a receiver and a receiver substrate area by low-cost and high-performance integration of an image rejection filter and a channel selection filter. <P>SOLUTION: A receiver IF circuit includes: a variable gain amplifier 2 for amplifying an RF input signal; a frequency converter 3a, 3b for obtaining, from the RF signal amplified by the variable gain amplifier polyphase intermediate-frequency signals that are used for suppressing an image component; a polyphase filter 5 for receiving the polyphase intermediate-frequency signals and outputting an intermediate-frequency signal whose image component is suppressed; a frequency variable band-pass filter 6 for selecting a channel of the intermediate-frequency signal outputted from a polyphase filter while changing a frequency response in accordance with a supplied control signal; an IF detector 8 for detecting the intermediate-frequency signal; and an automatic gain controller 9 for detecting a level of an output signal of the IF detector and controlling a gain of the variable gain amplifier in accordance with the detected level. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a reception IF (intermediate frequency) circuit, and more particularly to a reception IF circuit in which a filter constituting an image rejection mixer and a filter having a channel selection function are integrated.

  FIG. 7 shows a conventional superheterodyne radio receiver. An unnecessary signal including an image signal is removed from the input RF signal by the filter 1, and a desired signal is extracted. The RF signal that has passed through the filter 1 is amplified by the variable gain amplifier 2, mixed with the signal of the local oscillation frequency from the oscillator 4 by the frequency mixer 3, and converted to an intermediate frequency (IF). An unnecessary signal is removed from the output signal of the frequency mixer 3 by the band filter (BPF) 6b, and only a desired IF signal is extracted. The band filter 6b is mainly composed of an external passive component such as a ceramic filter. The output signal of the band filter 6b is amplified by the IF amplifier 7 and then converted into a baseband signal by the detector 8.

  The amplitude of the signal after detection is detected by an AGC (automatic gain control circuit) 9, and its output is supplied to the variable gain amplifier 2 and the IF amplifier 7 as a gain control voltage for making the baseband signal amplitude constant. . Based on the gain control voltage, the gains of the variable gain amplifier 2 and the IF amplifier 7 are controlled so that a dynamic range appropriate for the amplifier and the filter is maintained. A range surrounded by a broken line indicates the integrated block 10. The same applies to the following description. The RF filter 1 and the band filter 6b are disposed outside the integrated block 10.

Next, image disturbance which is a problem in the heterodyne method will be described. FIG. 8 shows a conceptual diagram of image disturbance. FIG. 8A shows a frequency conversion unit by the mixer 3. This figure shows the down-conversion operation when the desired signal V _RF and the image wave V _IM are included in the RF signal input to the mixer 3 through the RF filter 1. As shown in (b), the desired wave V _RF is a frequency (f _LO + f _IF ) higher than the local signal frequency f _LO by the IF frequency f _IF , and the image wave V _IM is from the local frequency f _LO to the IF frequency f. _The frequency is lower by _IF (f _LO -f _IF ).

As shown in (c), in the receiving system, whether the desired wave V _RF of the frequency (f _LO + f _IF ) or the image wave V _IM of the frequency (f _LO -f _IF ) is input, the mixer circuit 3 is converted to the same intermediate frequency f _IF in the signal V _OUT after being down-converted by 3 and passing through the band-pass filter 6b. As a result, interference due to the image signal occurs and the reception quality deteriorates. Therefore, conventionally, it has been common to remove image waves in advance by the RF filter 1.

  However, the external RF filter 1 causes an increase in cost and it is difficult to reduce the mounting substrate density. For this reason, in recent years, an image rejection mixer that removes an image wave by circuit technology has been adopted as a countermeasure against this image interference (see, for example, Patent Documents 1 to 3). By using this image rejection mixer, the function of removing the image wave from the external RF filter 1 can be excluded. An example of an image rejection mixer is shown in FIG.

9 shows, as an RF input, if the desired wave A _RF cos .omega _RF t and image waves A _IM cosω _IM t is input, sin .omega _LO t to the mixer 3a as the local oscillation signal, cos .omega _LO t is supplied to the mixer 3b Is shown. Since the high-frequency component contained in the output signal of the mixer 3a is removed when passing through the LPF (low-pass filter) 50a, the output signal of the LPF 50a is expressed by Expression (1). The signal that has passed through the 90-degree phase shifter 51 is expressed by Equation (2).

(A _RF / 2) * sin (ω _LO −ω _RF ) t + (A _IM / 2) * sin (ω _LO −ω _IM ) t (1)
(A _RF / 2) * cos (ω _RF -ω _LO ) t-(A _IM / 2) * cos (ω _LO -ω _IM ) t (2)
On the other hand, the signal after the output signal of the mixer 3b passes through the LPF 50b is expressed by Expression (3).

(A _RF / 2) * cos (ω _RF −ω _LO ) t + (A _IM / 2) * cos (ω _LO −ω _IM ) t (3)
Therefore, the output of the adder 52 is A _RF cos (ω _RF −ω _LO ) t, and the image signal A _IM cos (ω _LO −ω _IM ) t is removed.

  As the 90-degree phase shifter 51, a CR-RC circuit that utilizes the fact that the voltage across the capacitor and the voltage across the resistor are 90 degrees out of phase is used. However, the bandwidth of the 90-degree phase shifter 51 is narrow, and there is a problem that image rejection characteristics deteriorate due to variations in the elements of capacitors and resistors and the amplitude and phase error of two signals having a 90-degree phase difference. there were. Therefore, it is attempted to use a polyphase filter instead of the 90-degree phase shifter 51 (see, for example, Patent Document 2 and Patent Document 3 described above).

  An example of the configuration of the passive polyphase filter is shown in FIG. The passive polyphase filter shown in FIG. 10 includes four-phase polyphase filters 53-1, 53-2,... 53-n connected in n stages. The polyphase filter 53-1 includes resistors R11 to R14 and capacitors C11 to C14, the polyphase filter 53-2 includes resistors R21 to R24 and capacitors C21 to C24, and the polyphase filter 53-n includes The resistors Rn1 to Rn4 and capacitors Cn1 to Cn4 are included.

  FIG. 11 shows the image rejection characteristics of the passive polyphase filter of FIG. When a desired signal is input, a characteristic indicated by a broken line 54 is shown. When an image signal is input, a characteristic indicated by a solid line 55 is indicated. The difference between the characteristic indicated by the broken line 54 and the characteristic indicated by the solid line 55 is image rejection. Since the polyphase filters are connected in multiple stages, the bandwidth is widened, and even if there are fluctuations in the elements, the image rejection characteristics are hardly deteriorated.

  FIG. 12 shows an example of an active polyphase filter for image rejection. In FIG. 12, the signals of inputs I, -I, Q, and -Q have the same amplitude and have phases of 0 degrees, -180 degrees, 90 degrees, and -90 degrees, respectively. 30-1, 30-2,..., 30-n constitute a BPF and are connected in n stages. The BPF 30-1 is composed of operational amplifiers 31-1, 32-1 and resistors R1a, R1b, R1c, R1a, and the BPF 30-2 is composed of operational amplifiers 31-2, 32-2, and resistors R2a, R2b, R2c, R2a. The BPF 30-n includes operational amplifiers 31-n, 32-n, Rna, Rnb, Rnc, and Cna.

  By using the polyphase filter, it is possible to reduce the degradation of the image rejection characteristics with respect to fluctuations in the characteristics of the respective elements. FIG. 13 shows an example of image removal characteristics of the active polyphase filter. In FIG. 13, a broken line 56 indicates the frequency characteristics when a desired signal is input, and a solid line 57 indicates the frequency characteristics when an image signal is input. The difference is image rejection. The active polyphase filter is a band filter, and has an action of removing out-of-band signals, so that it can be used as a part of a channel filter.

On the other hand, attempts have been made to replace passive components with active components in order to reduce costs (see, for example, Patent Document 1 described above). One example is shown in FIG. In the receiver of FIG. 14, the bandpass filter 6b in the receiver of FIG. 7 is replaced with a bandpass filter 6a composed of a switched capacitor filter (SCF) from the passive bandpass filter. The basic operation is the same as in FIG. In FIG. 14, reference numeral 58 denotes an anti-aliasing filter, which is used to prevent aliasing caused by using SCF. The unwanted signal after the mixer is removed from the signal that has passed through the anti-aliasing filter 58 by the band filter 6a, and only the desired intermediate frequency signal is extracted. The frequency divider 11 divides the output signal of the oscillator 4 to a desired frequency and supplies it as an SCF clock constituting the band filter 6a. The smoothing filter 42 removes the clock signal and its harmonics from the output of the bandpass filter 6a.
Special table 2001-513275 gazette JP 2003-298356 A Sharzad Tadjpour and three others “[A 900-MHz Dual- Conversion Low-IF GSM Receiver in 0.35-μm CMOS] ISSCC VOL.36, NO12, December, 2001

  By the way, in the radio receiver, the input reception signal band is wide, and signals of different modulation methods such as AM signals and FM signals are input. Therefore, a channel filter that amplifies only a desired signal for various frequency bands is necessary, and an image removal filter associated with the heterodyne system is necessary. For this reason, many reception channel filters must be used, and a large number of passive filters are required, which makes it difficult to reduce costs and reduce the mounting area. Also, as shown in Patent Document 1, it is possible to attempt to replace passive components with active components. However, depending on the input signal band and the type of signal, many active filters are required, which increases circuit current. The chip area is increased or the noise is increased.

  In the conventional examples shown in FIGS. 7 and 14, the RF filter 1 has functions of an image rejection filter and a channel filter, and the band-pass filters 6a and 6b remove unnecessary signals after mixing and desired IF signals. Only has a selection function. In the circuit of FIG. 7, these are mainly composed of a ceramic filter, a SAW filter or the like, and a filter having high selectivity and an image removal function is required. Therefore, when integrating with an active circuit, a highly accurate filter is required, and it is difficult to realize a filter that is stable with respect to element variations. Further, in the case of a configuration using a switched capacitor filter that is an active filter as the bandpass filter 6a as in the circuit of FIG. 14, the switched capacitor filter has a filter characteristic synchronized with the clock and does not require tuning. In addition, the anti-folding filter 58 is required. In order to realize a high-accuracy filter, a high-precision and large-scale anti-aliasing filter must also be used, and there are difficulties such as increased chip cost, increased power consumption, and increased noise. Integration was difficult.

  FIG. 15 shows an example of a receiving system in which RF signals of two different signal bands are input. In this system, the RF1 signal is input to the RF1 amplifier 2a through the RF1 filter 1a, and the RF2 signal is input to the RF2 variable gain amplifier 33 through the RF2 filter 1b.

  The RF1 signal is subjected to double down-conversion, and the output of the mixer 3 passes through the IF1 band filter 60 and is further mixed with the second local signal by the second mixer 61 to be converted to IF12. . The IF 12 is processed by the IF 12 band filter 62, the IF 12 amplifier 63, and the IF 12 detector 64, and the baseband signal 1 is output.

  The output of the RF2 variable gain amplifier 33 is processed in the same manner as the circuit of FIG. 7 by the mixer 3c, IF2 band filter 65, IF2 amplifier 66, IF2 detector 67, and AGC 68, and the baseband signal 2 is output.

  For RF1 and RF2 signals, RF filters 1a and 1b, IF1 band filter 60, and IF2 band filter 65 are required, respectively, depending on the functions of image rejection and channel selection filter and their frequency characteristics. Had to use the filter.

  An object of the present invention is to provide a reception IF circuit in which an image rejection filter and a channel selection filter are integrated at low cost and with high performance to reduce the cost of the receiver and the reception board area.

  In order to solve the above problems, a reception IF circuit of the present invention includes a variable gain amplifier that amplifies an RF input signal, and an intermediate for suppressing an image component by mixing the amplified RF input signal and a local oscillation signal. A frequency converter that generates a multi-phase signal, a polyphase filter that outputs the intermediate frequency signal to which the intermediate multi-phase signal is supplied and image components are suppressed, and a frequency response that is variable according to the supplied control signal A frequency variable bandpass filter for channel selection of the intermediate frequency signal, an IF detector for detecting the intermediate frequency signal, and a level of an output signal of the IF detector, and according to the detected level An automatic gain control unit for controlling the gain of the variable gain amplifier.

  According to the reception IF circuit of the present invention, the polyphase filter is folded back from the frequency variable bandpass filter by using the polyphase filter for image rejection and the frequency variable bandpass filter including the switched capacitor filter (SCF). The filter can also function as a prevention filter, and high-performance integration can be achieved at low cost.

  The reception IF circuit of the present invention preferably includes a reference signal generator that generates a reference signal, and the local oscillation signal and the control signal are generated based on the reference signal. As a result, the variable frequency band filter and the frequency converter operate synchronously, and a highly accurate operation is obtained.

  The reception IF circuit of the present invention switches a plurality of the frequency converters for obtaining the intermediate multiphase signal for suppressing an image component corresponding to a plurality of RF signals, and the plurality of the intermediate multiphase signals. And a switch for supplying the polyphase filter to receive a plurality of RF signals having different frequency bands.

  Preferably, in this configuration, a phase-locked loop that controls the frequency of the voltage-controlled oscillator, using the crystal oscillator signal as a reference signal, and the output signal of the voltage-controlled oscillator is divided to provide a plurality of signals corresponding to the plurality of RF signals. A plurality of frequency dividers each supplying a local oscillation signal to the frequency converter, and a variable frequency divider that divides the signal of the crystal oscillator and supplies it as the control signal of the frequency variable bandpass filter And As a result, a filter having a plurality of characteristics can be accurately realized by one basic filter using a clock with high accuracy, and a necessary chip area can be reduced.

  Further, the frequency converter for obtaining the intermediate multiphase signal for suppressing the image component corresponding to a part of the RF signals and the single-phase intermediate frequency signal corresponding to the other RF signals are obtained. And a switch for switching the intermediate multiphase signal and the single-phase intermediate frequency signal to supply to the polyphase filter, and configured to receive a plurality of RF signals having different frequency bands be able to.

  Preferably, in this configuration, a phase-locked loop that controls the frequency of the voltage-controlled oscillator, using the crystal oscillator signal as a reference signal, and the output signal of the voltage-controlled oscillator is divided to provide a plurality of signals corresponding to the plurality of RF signals. A plurality of frequency dividers each supplying a local oscillation signal to the frequency converter, and a variable frequency divider that divides the signal of the crystal oscillator and supplies it as the control signal of the frequency variable bandpass filter And

  The polyphase filter may be composed of a passive polyphase filter or a combination of active polyphase filters.

  The frequency variable band-pass filter is preferably composed of a switched capacitor filter. In that case, it is preferable that the polyphase filter is configured to function also as an anti-aliasing filter of the switched capacitor filter that constitutes the variable frequency band filter.

  Further, it is preferable that a clock frequency of the control signal supplied to the switched capacitor filter constituting the frequency variable band filter is higher than an input RF signal band.

  In addition, a plurality of variable gain amplifiers may be provided.

  Embodiments of the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 1 shows a reception IF circuit according to the first embodiment of the present invention. The input RF signal is frequency-selected by the RF filter 1 and amplified by the variable gain amplifier 2. The output signal of the variable gain amplifier 2 is supplied to the mixers 3a and 3b, mixed with the orthogonal local oscillation signal supplied from the oscillator 4, and an intermediate multiphase signal for suppressing the image component from the RF signal, that is, I , -I, Q, and -Q signals are converted into four-phase signals. The I, −I, Q, and −Q signals are supplied to the polyphase filter 5. An image rejection mixer 12 is configured by the range surrounded by the broken-line block, that is, the mixers 3a and 3b, the oscillator 4, and the polyphase filter 5.

  The output of the polyphase filter 5 is supplied to the variable frequency band filter 6 and only a desired IF signal is selected. The variable frequency band filter 6 is controlled based on a control signal obtained by dividing the output signal of the oscillator 4 by the frequency divider 11a, and the selected frequency is adjusted. The output of the variable frequency band filter 6 is amplified by an IF amplifier 7 and converted to a baseband signal by an IF detector 8. The output of the IF detector 8 is applied to the AGC 9, and the control voltage is supplied from the AGC 9 to the variable gain amplifier 2 and the IF amplifier 7, and the gain is controlled based on the control voltage so that the signal level becomes constant.

  In this circuit, the frequency selection characteristic can be changed by changing the frequency of the control signal supplied to the variable frequency band filter 6 by changing the frequency dividing ratio of the frequency divider 11a.

  As an example of the polyphase filter 5, the passive polyphase filter shown in FIG. 10 can be used. As inputs, four-phase signals I, -I, Q, and -Q, which are outputs of the mixers 3a and 3b, are input with the same amplitude. The center frequencies of the polyphase filters 53-1, 53-2,... 53-n are respectively f01 = 1 / (2πR11 × C11), f02 = 1 / (2πR21 × C21), f0n = 1 / (2πRn1 × Cn1). As a whole, the image signal has a notch characteristic indicated by a solid line 55 as shown in FIG. 11, and a desired signal has a frequency characteristic showing an almost all-pass characteristic as indicated by a broken line 54 as shown in FIG. Since polyphase filters are connected in multiple stages, even if there is variation in CR, it is possible to have a desired image rejection characteristic.

  As another example of the polyphase filter 5, the active polyphase filter shown in FIG. 12 can be used. Similarly, in this active polyphase filter, four-phase signals I, -I, Q, and -Q are input to the input with the same amplitude. The center frequencies f0 and −3 dB bandwidth BW of BPF 30-1, 30-2,... 30-n are f01 = 1 / (2πC1a × R1c), BW1 = 2 / (2πC1a × R1b), f02 = 1 / (2πC2a × R2c), BW2 = 2 / (2πC2a × R2b), f0n = 1 / (2πCna × Rnc), and BWn = 2 / (2πCna × Rnc). An example of the frequency characteristic is as shown in FIG. The desired signal has a BPF characteristic as indicated by a broken line 56 and the image signal has a characteristic as indicated by a solid line 57. Therefore, it is possible to have an image rejection characteristic.

  In the circuit of FIG. 1, since the signal after passing through the polyphase filter 5 is supplied to the frequency variable bandpass filter 6, the following effects can be obtained. That is, when the variable frequency band filter 6 is configured by a switched capacitor filter (SCF), the anti-aliasing filter 58 is usually required before the SCF as in the conventional example shown in FIG. On the other hand, when an active polyphase filter as shown in FIG. 12 is used, it has an image rejection function as shown in FIG. 13 and also operates as an active BPF. Is possible. Thus, the polyphase filter 5 can be used also as an anti-folding filter, and the chip area and power can be reduced.

  If the sampling frequency of the SCF is fs, the necessary attenuation can be obtained by connecting polyphase BPFs in multiple stages in order to achieve the necessary attenuation at fs / 2. Furthermore, another major advantage of SCF is that the frequency characteristics can be changed by changing the clock frequency.

  FIG. 2 shows an example of an SCF that can be used as the variable frequency band filter 6. The SCF is composed of capacitance selection networks 20 to 23 and an operational amplifier 24. Each of the capacitance selection networks 20 to 23 includes capacitors C1ap to Cnap, a capacitor C2a, capacitors C1an to Cnan, a capacitor C2a, and a switch SW. The capacitance value is selected according to the necessary frequency selection mode, and the clock selected at the necessary frequency is supplied to the switch SW. In the SCF of FIG. 2, an integrator or a primary basic filter can be configured, and a filter having a desired selection characteristic can be configured by selecting a capacitor network and selecting a clock frequency. A second-order or higher-order filter can be similarly configured. By using the operational amplifier 24 in common for all the filters, it is possible to reduce power and cost.

(Second Embodiment)
FIG. 3 shows a reception IF circuit according to the second embodiment of the present invention. In this circuit, RF input signals of two different frequency bands, that is, RF1 signal and RF2 signal are input. The RF1 signal and the RF2 signal are frequency-selected by the RF1 filter 1a and the RF2 filter 1b and input to the variable gain amplifier 2 and the amplifier 13, respectively. The signal amplified by the variable gain amplifier 2 is supplied to the mixers 3a and 3b, and the signal amplified by the amplifier 13 is supplied to the mixers 3c and 3d.

  To the mixers 3a, 3b, 3c, and 3d, a local oscillation signal obtained by dividing the output signal of the oscillator 4 by the frequency dividers 14a and 14b to obtain a predetermined IF frequency is supplied. The mixers 3a and 3b are supplied with local oscillation signals having phases different from each other by 90 degrees for image rejection. Similarly, local oscillation signals having phases different from each other by 90 degrees are also supplied to the mixers 3c and 3d. Thereby, four-phase IF signals I1, -I1, Q1, and -Q1 having the frequency of IF1 are output to the outputs of the mixers 3a and 3b. Similarly, four-phase IF signals I2, -I2, Q2, and -Q2 having the frequency of IF2 are output to the outputs of the mixers 3c and 3d.

  These two IF signals are selected by the switches 15 to 18 and supplied to the polyphase filter 5. An image rejection operation for two RF input signals is performed by the image rejection mixer 12 including the mixers 3a to 3d, the oscillator 4, the frequency dividers 14a and 14b, the switches 15 to 18, and the polyphase filter 5.

  The signal that has passed through the polyphase filter 5 is supplied to the variable frequency band filter 6. The variable frequency band filter 6 operates based on a control signal supplied by a frequency divider 11 having a variable frequency dividing ratio. Accordingly, it is possible to configure filters corresponding to different intermediate frequencies IF1 and IF2.

  In the configuration of FIG. 3, the operation after passing through the variable frequency band filter 6 in the circuit on the IF1 side is the same as the configuration of FIG. Although the circuit on the IF2 side does not have a feedback loop, the AGC operation can be performed in the same manner as the circuit on the IF1 side, and the amplifier 13 can be a variable gain amplifier.

  According to the reception IF circuit of the present embodiment, RF input signals of different frequencies can be processed by one polyphase filter 5 and one frequency variable bandpass filter 6, thereby suppressing an increase in chip cost and reducing power. Is possible. Although FIG. 3 shows the case of two different RF input signals, the same configuration can be applied to three or more RF input signals. The following features make this possible.

  (A) The control signal supplied to the variable frequency band filter 6 can be optimally selected by the value of the frequency divider 11.

  (B) A signal synchronized with the local oscillation frequency supplied to the mixers 3 a to 3 d is supplied to the frequency variable band-pass filter 6.

  (C) The polyphase filter 5 can perform image rejection and unnecessary signal removal corresponding to a plurality of RF signals.

(Third embodiment)
FIG. 4 shows a reception IF circuit according to the third embodiment of the present invention. In this circuit, RF input signals of two frequency bands, that is, an RF1 signal and an RF2 signal are input as in the circuit of FIG. The difference from the circuit of FIG. 3 is that the image rejection for the RF2 signal has a role in the RF2 filter 1c, which is an external filter.

  Also in this configuration, for the RF1 signal, similarly to the configuration of FIG. 3, the mixers 3a and 3b, the oscillator 4, the frequency divider 14a, the switches 25 to 28, and the polyphase filter 5 surrounded by the broken line blocks. The image rejection mixer 12 is configured. The polyphase filter 5 serves as a part of the selection filter of the frequency variable band-pass filter 6 for the RF2 signal, or when the frequency variable band-pass filter 6 is composed of SCF, serves as an anti-aliasing filter.

  Other operations are the same as those of the circuit of FIG. FIG. 5 shows an example of a shared circuit of the active polyphase BPF and the active biquad LPF. This circuit has a configuration in which switches SW1, SW2,... SWn are provided in the active polyphase filter shown in FIG. When the switches SW1, SW2,... SWn are on, the polyphase BPF is obtained, and when the switches SW1, SW2,... SWn are off, a normal bi-quad LPF is obtained. If the multiphase connection is made in the same way as the polyphase BPF, it is possible to realize the attenuation necessary for preventing the aliasing.

(Fourth embodiment)
FIG. 6 shows a reception IF circuit according to the fourth embodiment of the present invention. In this circuit, as in the circuit of FIG. 3, two frequency band RF input signals, that is, RF1 signal and RF2 signal are input to the variable gain amplifiers 2 and 33 via the RF1 filter 1a and RF2 filter 1b, respectively. . From the amplified signal, the first quadrature mixer composed of the mixers 3a and 3b and the second quadrature mixer composed of the mixers 3c and 3d, respectively, each of four-phase intermediate frequency signals IF1, IF2 is created.

  The local oscillation signal from the frequency divider 14a is supplied to the mixers 3a and 3b, and the local oscillation signal from the frequency divider 14b is supplied to the mixers 3c and 3d. The frequency dividers 14a and 14b are supplied with a phase-locked loop output signal based on the output signal of the crystal oscillator 37 and converted to a necessary local oscillation frequency. The phase-locked loop includes a voltage controlled oscillator 34, a phase comparator 35, an LPF 36, and a crystal oscillator 37.

  The four-phase intermediate frequency signals IF1 and IF2 are switched by the switches 38 to 41 and supplied to the polyphase filter 5. The output of the polyphase filter 5 is supplied to an SCF channel filter 6a composed of SCF. A control signal having a predetermined frequency obtained by frequency-dividing the reference signal from the crystal oscillator 37 by the frequency divider 11 is applied to the SCF channel filter 6a. The frequency of the control signal is switched by the frequency divider 11 according to the selection frequency switched by the switches 38-41.

  In this circuit, as the polyphase filter 5, a polyphase filter as shown in FIGS. 10 and 12 can be used, which can also serve as the anti-folding filter of the SCF channel filter 6 a. That is, the polyphase filter 5 is configured to sufficiently attenuate the input signal to the SCF channel filter 6a up to half the frequency of the clock of the SCF channel filter 6a. As a result, aliasing is prevented, and signals that have passed through the smoothing filters 42a and 42b become selected IF signals without distortion.

  The signals that have passed through the smoothing filters 42a and 42b are processed by the IF1 amplifier 7a, IF2 amplifier 7b, IF1 detector 8a, and IF2 detector 8b, respectively, and baseband signals 1 and 2 are output. The outputs of IF1 detector 8a and IF2 detector 8b are also supplied to AGCs 9a and 9b, respectively, and the gains of variable gain amplifiers 2 and 33, IF1 amplifier 7a and IF2 amplifier 7b are controlled.

  In the present embodiment, an image rejection function required for two different RF input signals is realized by a common image rejection mixer 12 (shown by a broken line block), and selection filters for two different frequencies are provided. By integrating the common SCF channel filter 6a in the same chip, the external filter can be reduced at low cost and with low power.

  Note that a switched capacitor filter is a time-discrete system and contains many harmonic components of the clock frequency. Therefore, when integrated on the same chip as the RF circuit, the harmonic component of the clock acts as noise for a small RF input circuit. Also, it becomes an unnecessary component for the mixer. If the clock frequency is higher than the frequency of the RF input signal, the harmonic components of the clock are attenuated when passing through the circuit, and at the same time, the influence of noise on the input signal band is reduced. Therefore, by making the SCF clock higher than the frequency of the RF input signal, the component of the signal processed by the SCF is prevented from becoming an interference wave of the RF circuit.

  According to the reception IF circuit of the present invention, image rejection is possible without using an external filter, and high density, low power consumption, low cost and various input signal bands and signal frequency characteristics are achieved. A highly accurate filter characteristic can be realized, and a low-cost and high-performance receiving system can be provided. It can also be applied to other receiving systems.

The block diagram which shows the reception IF circuit in the 1st Embodiment of this invention The figure which shows one example of SCF used as an active filter which comprises a frequency variable bandpass filter The block diagram which shows the reception IF circuit in the 2nd Embodiment of this invention The block diagram which shows the reception IF circuit in the 3rd Embodiment of this invention The figure which shows the specific example of a multistage connection biquad active LPF The block diagram which shows the reception IF circuit in the 4th Embodiment of this invention Block diagram showing a conventional reception IF circuit Diagram explaining the disturbance caused by the image signal Block diagram showing an overview of a conventional image rejection mixer Circuit diagram showing an example of a passive polyphase filter Image rejection characteristics of the same passive polyphase filter Circuit diagram showing an example of an active polyphase filter Diagram showing image rejection characteristics of the active polyphase filter The block diagram which shows the receiving IF circuit of the other conventional example The block diagram which shows the receiving IF circuit of other conventional examples

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 RF filter 1a RF1 filter 1b, 1c RF2 filter 2, 33 Variable gain amplifier 3, 3a, 3b, 3c, 3d Mixer 4 Oscillator 5 Polyphase filter 6 Frequency variable band filter 6a SCF channel filter 6b Band filter 7 IF amplifier 7a IF1 Amplifier 7b IF2 amplifier 8 IF detector 8a IF1 detector 8b IF2 detector 9, 9a, 9b AGC (automatic gain control circuit)
DESCRIPTION OF SYMBOLS 10 Integrated block 11a, 14a, 14b Frequency divider 12 Image rejection mixer 13 Amplifier 15-18, 25-28, 38-41 Switch 20-23 Capacitance selection network 24 Operational amplifier 30-1, 30-2, 30- n BPF
31-1, 32-1, 31-2, 32-2, 31-n, 32-n Operational amplifier 34 Voltage controlled oscillator 35 Phase comparator 36 LPF
37 Crystal oscillator 42a, 42b Smoothing filter 50a LPF (low-pass filter)
51 90-degree phase shifter 52 Mixer 53-1, 53-2, 53-n Polyphase filter 58 Antialiasing filter 60 IF1 band filter 61 Second mixer 62 IF12 band filter 63 IF12 amplifier 64 IF12 detector 65 IF2 band filter 66 IF2 amplifier 67 IF2 detector 68 AGC

Claims (11)

A variable gain amplifier for amplifying an RF input signal;
A frequency converter that mixes the amplified RF input signal and the local oscillation signal to generate an intermediate multiphase signal for suppressing image components;
A polyphase filter that is supplied with the intermediate polyphase signal and outputs an intermediate frequency signal in which an image component is suppressed;
A frequency response variable according to a supplied control signal, and a frequency variable bandpass filter for channel-selecting the intermediate frequency signal;
An IF detector for detecting the intermediate frequency signal;
A reception IF circuit comprising: an automatic gain control unit that detects a level of an output signal of the IF detector and controls a gain of the variable gain amplifier according to the detected level.   The reception IF circuit according to claim 1, further comprising a reference signal generator that generates a reference signal, wherein the local oscillation signal and the control signal are generated based on the reference signal. A plurality of frequency converters for obtaining the intermediate polyphase signal for suppressing image components corresponding to a plurality of RF signals;
A switch for switching the plurality of intermediate polyphase signals to supply to the polyphase filter;
The reception IF circuit according to claim 1, configured to receive a plurality of RF signals having different frequency bands. Using a crystal oscillator signal as a reference signal, a phase-locked loop for controlling the frequency of the voltage controlled oscillator,
A plurality of frequency dividers that divide the output signal of the voltage controlled oscillator and supply each of them as a local oscillation signal to a plurality of frequency converters corresponding to the plurality of RF signals;
The receiving IF circuit according to claim 3, further comprising: a variable frequency divider that divides a signal of the crystal oscillator and supplies the divided signal as the control signal of the frequency variable bandpass filter. The frequency converter for obtaining the intermediate polyphase signal for suppressing image components corresponding to some of the RF signals;
A frequency converter for obtaining a single-phase intermediate frequency signal corresponding to the other RF signal;
A switch for switching the intermediate multiphase signal and the single-phase intermediate frequency signal to supply to the polyphase filter,
The reception IF circuit according to claim 1, configured to receive a plurality of RF signals having different frequency bands. Using a crystal oscillator signal as a reference signal, a phase-locked loop for controlling the frequency of the voltage controlled oscillator,
A plurality of frequency dividers that divide the output signal of the voltage controlled oscillator and supply each of them as a local oscillation signal to a plurality of frequency converters corresponding to the plurality of RF signals;
The receiving IF circuit according to claim 5, further comprising: a variable frequency divider that divides a signal of the crystal oscillator and supplies the divided signal as the control signal of the frequency variable bandpass filter.   The reception IF circuit according to claim 1, wherein the polyphase filter is configured by a combination of a passive polyphase filter or an active polyphase filter.   The reception IF circuit according to claim 1, wherein the variable frequency band filter is configured by a switched capacitor filter.   The reception IF circuit according to claim 8, wherein the polyphase filter is configured to function also as an anti-aliasing filter of the switched capacitor filter constituting the frequency variable bandpass filter.   9. The reception IF circuit according to claim 8, wherein a clock frequency of the control signal supplied to the switched capacitor filter constituting the frequency variable band filter is higher than an input RF signal band. The reception IF circuit according to claim 1, comprising a plurality of the variable gain amplifiers.

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