Patent Publication Number: US-2007099590-A1

Title: Frequency converter

Description:
TECHNICAL FIELD  
      The present invention relates to a frequency converter, and more particularly relates to a mixer.  
     BACKGROUND ART  
      Conventionally, as a single balance type harmonic mixer has been known one disclosed in a patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. 2003-69345), and the principle of an even harmonic mixer using an antiparallel diode pair has been known as described in a non-patent document 1 (MARVIN COHN, JAMES E. DEGENFORD, BURTON A. NEWMAN, “Harmonic Mixing with an Antiparallel Diode Pair”, IEEE Transaction on Microwave Theory and Techniques, August 1975, vol. MTT-23, No. 8, p667-673). The single balance type harmonic mixer uses a balanced balun to branch a locally oscillated signal Lo into two signals which are different from each other in phase by 180 degrees, and have the same amplitude, and respectively supplies antiparallel diode pairs with the resulting signals. The antiparallel diode pairs are also supplied with a high frequency received signal RF. The locally oscillated signals Lo and the high frequency received signal RF are mixed by the antiparallel diode pairs, resulting in intermediate frequency signals IF.  
      The frequency fIF of the intermediate frequency signal IF is represented as: 
 
 fIF=fRF− 2 N·fLo  or 
 
 fIF=fLo− 2 N·fRF,  
 
 where fLo denotes the frequency of the locally oscillated signal Lo, and fRF denotes the frequency of the high frequency received signal RF. It should be noted that N denotes a positive integer (1, 2, 3, . . . ). 
 
      The single balance type harmonic mixer provides such an advantage that the locally oscillated signal Lo and harmonics thereof do not leak to the input side of the high frequency received signal RF.  
      However, in the above-mentioned single balance type harmonic mixer, the impedance of the output terminal of the balanced balun is the impedance of a terminal of the antiparallel diode pairs connected to the balanced balun. Moreover, the balanced balun is designed to adapt to the band of the fLo, and it is difficult to design it to adapt to the band of fRF. As a result, the impedance of the output terminal of the balanced balun largely changes. Thus, a frequency characteristic of a conversion loss on the conversion of the high frequency received signal RF into the intermediate frequency signal IF largely changes according to the frequency fRF of the high frequency received signal RF. The frequency characteristic of the conversion loss is preferably constant, and the large change of the frequency characteristic of the conversion loss thus poses a problem.  
      The object of the present invention is to maintain the frequency characteristic of the conversion loss to generally constant on the conversion of the high frequency received signal into the intermediate frequency signal.  
     DISCLOSURE OF INVENTION  
      According to an aspect of the present invention, a frequency converter includes: a signal branching unit that branches a locally oscillated signal into two signals; a constant impedance element that passes the two signals; and a mixing unit that respectively mixes an output from the constant impedance element with a high frequency received signal and generates an intermediate frequency signal, wherein the constant impedance element have a generally constant impedance in a frequency band of the high frequency received signal.  
      According to the thus constructed frequency converter, a signal branching unit branches a locally oscillated signal into two signals. A constant impedance element passes the two signals. A mixing unit respectively mixes an output from the constant impedance element with a high frequency received signal and generates an intermediate frequency signal. The constant impedance element have a generally constant impedance in a frequency band of the high frequency received signal.  
      According to the thus constructed frequency converter, the two signals may be two signals that are different from each other in phase by 180 degrees, and have the same amplitudes.  
      According to the thus constructed frequency converter, an impedance of the constant impedance element may be generally 0Ω across almost an entire frequency band of the high frequency received signal.  
      According to the thus constructed frequency converter, the constant impedance element may pass a signal with a frequency within the frequency band of the respective two signals more than a signal within the frequency band of the high frequency received signal.  
      According to the thus constructed frequency converter, the constant impedance element may be a low-pass filter whose cut-off frequency is an upper limit of the frequency band of the two signals.  
      According to the thus constructed frequency converter, the constant impedance element may be a band-pass filter whose passband is the frequency band of the two signals.  
      According to the thus constructed frequency converter, the constant impedance element may be a diplexer whose passband is the frequency band of the two signals, and which presents a termination characteristic in the frequency band of the high frequency received signal.  
      According to the thus constructed frequency converter, the signal branching unit may be a balanced balun corresponding to the frequency band of the locally oscillated signal.  
      According to the thus constructed frequency converter, the mixing unit may include: one diode; the other diode which is connected at the anode to the cathode of said one diode, and at the cathode to the anode of said one diode; a first terminal to which the cathode of said one diode and the anode of said the other diode are connected; and a second terminal to which the cathode of said the other diode and the anode of said one diode are connected; the first terminal receives an output from the constant impedance element; the second terminal receives the high frequency received signal; and the second terminal outputs the intermediate frequency signal.  
      The thus constructed frequency converter may further include: a high frequency input terminal which is connected to the second terminal, and receives an input of the high frequency received signal; an intermediate frequency band filter which is connected to the second terminal, and passes a signal within the frequency band of the intermediate frequency signal; and an intermediate frequency signal output terminal which is connected to the intermediate frequency band filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram showing a configuration of a frequency converter  1  according to a first embodiment of the present invention;  
       FIG. 2  is a chart showing an impedance characteristic of low-pass filters (constant impedance elements)  12   a  and  12   b;    
       FIG. 3  is a diagram showing an example of a circuit configuration of the low-pass filters  12   a  and  12   b;    
       FIG. 4  is an impedance chart showing an example of an impedance characteristic of the low-pass filters  12   a  and  12   b;    
       FIG. 5  is a circuit diagram showing a configuration of a frequency converter  1  according to a second embodiment of the present invention;  
       FIG. 6  is a chart showing an impedance characteristic of diplexers (constant impedance elements)  22   a  and  22   b ; and  
       FIG. 7  is a circuit diagram showing a circuit configuration of the diplexers  22   a  and  22   b , wherein  FIG. 7 ( a ) shows an example where the diplexers  22   a  and  22   b  are constituted by band-pass filters and  FIG. 7 ( b ) shows an example where the diplexers  22   a  and  22   b  are constituted by circuit elements L, C, and R. 
    
    
     BEST MODE FOR CARING OUT THE INVENTION  
      A description will now be given of embodiments of the present invention with reference to drawings.  
     First Embodiment  
       FIG. 1  is a circuit diagram showing a configuration of a frequency converter  1  according to a first embodiment of the present invention. The frequency converter  1  includes a locally oscillated signal input terminal  10   a , a balanced balun (signal branching means)  10 , low-pass filters (constant impedance elements)  12   a  and  12   b , DC return coils  14   a  and  14   b , antiparallel diode pairs (mixing means)  16   a  and  16   b , an antiparallel diode pair connection point  17 , and an RF/IF signal separating unit  18 . The frequency converter  1  mixes a locally oscillated signal Lo and a high frequency received signal RF to extract an intermediate frequency signal IF.  
      The locally oscillated signal input terminal  10   a  is a terminal which receives an input of a locally oscillated signal Lo (frequency fLo). The locally oscillated signal Lo input to the locally oscillated signal input terminal  10   a  is supplied to the balanced balun  10 . It should be noted that the frequency fLo is 4 to 8 GHz, for example.  
      The balanced balun (signal branching means)  10  branches the locally oscillated signal Lo into two signals which are different from each other in phase by 180 degrees, and have the same amplitude. The frequency of the two signals is the same as the frequency of the locally oscillated signal Lo. When the phase of one signal is 0°, then the phase of the other signal is 180°(refer to  FIG. 1 ). The balanced balun  10  is designed to adapt to the frequency band (4 to 8 GHz, for example) of the locally oscillated signal Lo. As a result, the impedance largely changes in a frequency band exceeding the frequency band of the locally oscillated signal Lo (the frequency band of the high frequency received signal RF for example).  
      The low-pass filter (constant impedance element)  12   a  receives the one signal output from the balanced balun  10 . The low-pass filter (constant impedance element)  12   b  receives the other signal output from the balanced balun  10 . The low-pass filters  12   a  and  12   b  are low-pass filters whose cut-off frequency is the upper limit of the frequency band of the signals output from the balanced balun  10 . It should be noted that the frequency band of the signals output from the balanced balun  10  is the same as the frequency band of the locally oscillated signal Lo. Thus, the upper limit of the frequency band of the signals output from the balanced balun  10  is 8 GHz, and the cut-off frequency is 8 GHz. As a characteristic of the low-pass filter a signal at a frequency equal to or lower than the cut-off frequency (the signal output from the balanced balun  10 ) is passed more than a signal at a frequency exceeding the cut-off frequency (a signal within the frequency band of the high frequency received signal RF, for example).  
      A description will now be given of an impedance characteristic of the low-pass filters (constant impedance elements)  12   a  and  12   b  with reference to a chart in  FIG. 2 . The impedances of the low-pass filters  12   a  and  12   b  are generally constant in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF. Specifically, while the impedance is 50Ω at 8 GHz, the impedance rapidly approaches 0Ω as the frequency increases (the impedance is considerably smaller than 50Ω at 9 GHz, for example), and finally reaches 0Ω. Namely, the impedance is approximately 0Ω across almost the entire frequency band of the high frequency received signal RF.  
       FIG. 3  shows an example of a circuit configuration of the low-pass filters  12   a  and  12   b . The low-pass filters  12   a  and  12   b  include a reactance element L which is connected to the balanced balun  10  on one end, and to the antiparallel diode pair  16   a  or  16   b  on the other end, a capacitance element C which is connected to the one end of the reactance element L and is grounded, and a capacitance element C which is connected to the other end of the reactance element L and is grounded.  
       FIG. 4  shows an impedance chart (Smith chart) of the low-pass filters  12   a  and  12   b  configured as shown in  FIG. 3 . With reference to  FIG. 4 , the impedance is 50Ω at the frequency of 8 GHz, rapidly decreases when the frequency becomes 9 to 10 GHz, and approaches generally 0Ω when the frequency becomes 20 GHz.  
      The DC return coil  14   a  is a coil which is connected on one end to an output side (opposite side of the balanced balun  10 ) of the low-pass filter  12   a , and is grounded on the other end. The DC return coil  14   b  is a coil which is connected on one end to an output side (opposite side of the balanced balun  10 ) of the low-pass filter  12   b , and is grounded on the other end. It should be noted that DC power supplies which supply the antiparallel diode pairs  16   a  and  16   b  with desired DC voltages may be connected in place of the DC return coils  14   a  and  14   b.    
      The antiparallel diode pair (mixing means)  16   a  includes diodes  162   a  and  164   a , a first terminal  166   a , and a second terminal  168   a . The diode  162   a  is connected to the RF/IF signal separating unit  18  at the anode, and is connected to the low-pass filter  12   a  at the cathode. The diode  164   a  is a diode which is connected at the anode to the cathode of the diode  162   a , and is connected at the cathode to the anode of the diode  162   a . The first terminal  166   a  is a terminal to which the cathode of the diode  162   a  and the anode of the diode  164   a  are connected. The second terminal  168   a  is a terminal to which the cathode of the diode  164   a  and the anode of the diode  162   a  are connected.  
      To the first terminal  166   a  is input the output from the low-pass filter  12   a . To the second terminal  168   a  is input the high frequency received signal RF. From the second terminal  168   a  is output the intermediate frequency signal IF.  
      The antiparallel diode pair (mixing means)  16   b  includes diodes  162   b  and  164   b , a first terminal  166   b , and a second terminal  168   b . The diode  162   b  is connected to the RF/IF signal separating unit  18  at the anode, and is connected to the low-pass filter  12   b  at the cathode. The diode  164   b  is a diode which is connected at the anode to the cathode of the diode  162   b , and is connected at the cathode to the anode of the diode  162   b . The first terminal  166   b  is a terminal to which the cathode of the diode  162   b  and the anode of the diode  164   b  are connected. The second terminal  168   b  is a terminal to which the cathode of the diode  164   b  and the anode of the diode  162   b  are connected.  
      To the first terminal  166   b  is input the output from the low-pass filter  12   b . To the second terminal  168   b  is input the high frequency received signal RF From the second terminal  168   b  is output the intermediate frequency signal IF.  
      The antiparallel diode pair connection point  17  is a connection point to which the second terminals  168   a  and  168   b  and the RF/IF signal separating unit  18  are connected.  
      The RF/IF signal separating unit  18  receives the high frequency received signal RF, and outputs the high frequency received signal RF to the second terminals  168   a  and  168   b . Then, the RF/IF signal separating unit  18  receives the intermediate frequency signals IF from the second terminals  168   a  and  168   b , and extracts the intermediate frequency signal IF.  
      The RF/IF signal separating unit  18  includes a high frequency band filter  182 , a high frequency input terminal  182   a , an intermediate frequency band filter  184 , and an intermediate frequency signal terminal  184   a.    
      The high frequency band filter  182  is connected to the second terminals  168   a  and  168   b . The high frequency band filter  182  is a filter which passes a signal in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF. It should be noted that the high frequency band filter  182  passes a signal at the frequency fIF (1 GHz, for example) of the intermediate frequency signal IF less than a signal in the frequency band of the high frequency received signal RF (preferably cuts off the signal at the frequency fIF).  
      The high frequency input terminal  182   a  is connected to the second terminals  168   a  and  168   b  via the high frequency band filter  182 . The high frequency input terminal  182   a  receives the input of the high frequency received signal RF.  
      The intermediate frequency band filter  184  is connected to the second terminals  168   a  and  168   b . The intermediate frequency band filter  184  is a filter which passes a signal at the frequency fIF (1 GHz, for example) of the intermediate frequency signal IF. It should be noted that the intermediate frequency band filter  184  passes a signal in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF less than a signal at the frequency fIF (1 GHz, for example) of the intermediate frequency signal IF (preferably cuts off the signal in the frequency band of the high frequency received signal RF).  
      The intermediate frequency signal output terminal  184   a  is connected to the second terminals  168   a  and  168   b  via the intermediate frequency band filter  184 . The intermediate frequency signal output terminal  184   a  is a terminal which outputs the intermediate frequency signal IF.  
      A description will now be given of an operation of the first embodiment.  
      To the locally oscillated signal input terminal  10   a  is input the locally oscillated signal Lo (frequency fLo). It should be noted that the frequency fLo is 4 to 8 GHz, for example. The locally oscillated signal Lo is branched by the balanced balun  10  into the two signals which are different from each other in phase by 180 degrees, and have the same amplitude. These two signals respectively pass the low-pass filters  12   a  and  12   b , and supplied to the first terminals  166   a  and  166   b  of the antiparallel diode pairs  16   a  and  16   b.    
      Moreover, to the high frequency input terminal  182   a  of the RF/IF signal separating unit  18  is input the high frequency received signal RF (frequency fRF). The high frequency received signal RF passes through the high frequency band filter  182 , and is supplied to the second terminals  168   a  and  168   b.    
      The antiparallel diode pairs  16   a  and  16   b  respectively mix even harmonics of the two signals (frequency fLo) which have passed the low-pass filters  12   a  and  12   b  and the high frequency received signal RF (frequency fRF) with each other. As a result, there are obtained the intermediate frequency signals IF (frequency fIF).  
      It should be noted that: 
 
 fIF=fRF− 2 N·fLo,  
 
or 
 
 fIF=fLo− 2 N·fRF,  
 
 where N denotes a positive integer (1, 2, 3, . . . ). 
 
      Moreover, when the frequency fLo=4 to 8 GHz, the frequency fRF=9 to 49 GHz, and there is obtained the signal fIF=fRF−2N·fLo, the frequency fIF=1 GHz.  
      Namely, 
 
 fIF=fRF− 2 ·fLo ( fRF= 9 to 17 GHz), 
 
 fIF=fRF− 4 ·fLo ( fRF= 17 to 33 GHz), and 
 
 fIF=fRF− 6· fLo ( fRF= 25 to 49 GHz). 
 
      On this occasion, since the balanced balun  10  respectively supplies the antiparallel diode pairs  16   a  and  16   b  with the two signal which are different from each other in the phase by 180 degrees, and have the same amplitude, odd harmonics (2N−1)·fLo (N is a positive integer) of harmonics generated by the antiparallel diode pairs  16   a  and  16   b  cancel each other at the connection point  17 .  
      Moreover, since the direction of the current of the diode  162   a  ( 162   b ) and the direction of the current of the diode  164   a  ( 164   b ) are opposite to each other in the antiparallel diode pair  16   a  ( 16   b ), even harmonics 2N·fLo (N is a positive integer) of the harmonics generated by the antiparallel diode pair  16   a  ( 16   b ) cancel each other at the second terminal  168   a  ( 168   b ).  
      Consequently, the harmonics of the locally oscillated signal Lo do not leak to the high frequency input terminal  182   a.    
      Moreover, in the antiparallel diode pair  16   a  ( 16   b ), regardless of the phase of the supplied locally oscillated signal Lo, it is considered that either one of the diodes  162   a  and  164   a  ( 162   b  and  164   b ) opposite to each other is turned on. As a result, the impedance of the antiparallel diode pair  16   a  ( 16   b ) observed from the antiparallel diode pair connection point  17  is approximately equal to the input/output impedance of the low-pass filter  12   a  ( 12   b ).  
      The input/output impedance of the low-pass filter  12   a  ( 12   b ) is generally constant in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF as described above. As a result, the frequency characteristic of the conversion loss upon the high frequency received signal RF being converted into the intermediate frequency signal IF is generally constant even if the frequency RF of the high frequency received signal RF changes.  
      If there is not the low-pass filter  12   a  ( 12   b ) as a prior art technology the impedance of the antiparallel diode pair  16   a  ( 16   b ) observed from the antiparallel diode pair connection point  17  is approximately equal to the impedance of the balanced balun  10 . The impedance of the balanced balun  10  largely changes in the frequency band of the high frequency received signal RF. Thus, the frequency characteristic of the conversion loss on the conversion of the high frequency received signal RF into the intermediate frequency signal IF largely changes as the frequency fRF of the high frequency received signal RF changes.  
      Moreover, in signal mixing by means of a non-linear element, the efficiency of the mixing generally increases if the impedance beyond the non-linear element observed from a signal input terminal is 0 (short circuit). As a result, since the impedances (impedances of the low-pass filters  12   a  and  12   b ) beyond the non-linear elements (antiparallel diode pairs  16   a  and  16   b ) observed from the input terminal (antiparallel diode pair connection point  17 ) of the high frequency received signal RF are generally 0Ω across approximately entire frequency band of the high frequency received signal RF, the efficiency to convert the high frequency received signal RF into the intermediate frequency signal IF increases, resulting in a low loss.  
      The intermediate frequency signals IF generated by the antiparallel diode pairs  16   a  and  16   b  are supplied to the RF/IF signal separating unit  18 . The intermediate frequency signals IF cannot pass the high frequency band filter  182 , and pass the intermediate frequency band filter  184 . The intermediate frequency signal IF is thus output from the intermediate frequency signal output terminal  184   a . It should be noted that the high frequency received signal RF which has passed the high frequency band filter  182  cannot pass the intermediate frequency band filter  184 , and the high frequency received signal RF will not be mixed with the signal obtained from the intermediate frequency signal output terminal  184   a.    
      According to the first embodiment, the input/output impedance of the low-pass filter  12   a  ( 12   b ) is generally constant in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF. Thus, the frequency characteristic of the conversion loss on the conversion of the high frequency received signal RF into the intermediate frequency signal IF is generally constant even if the frequency fRF of the high frequency received signal RF changes. Moreover, the efficiency to convert the high frequency received signal RF into the intermediate frequency signal IF increases, resulting in a low loss.  
      It should be noted that the same effects can be provided when band-pass filters whose passband is the frequency band (4 to 8 GHz, for example) of the signal output from the balanced balun  10  (the impedance characteristic thereof is the same as that of the low-pass filters  12   a  and  12   b  (refer to  FIG. 2 )) are used in place of the low-pass filters  12   a  and  12   b.    
     Second Embodiment  
      The second embodiment includes diplexers  22   a  and  22   b  (constant impedance elements) in place of the low-pass filters  12   a  and  12   b  according to the first embodiment.  
       FIG. 5  is a circuit diagram showing a configuration of the frequency converter  1  according the second embodiment of the present invention. The frequency converter  1  includes the locally oscillated signal input terminal  10   a , the balanced balun (signal branching means)  10 , the diplexers (constant impedance elements)  22   a  and  22   b , the DC return coils  14   a  and  14   b , the antiparallel diode pairs (mixing means)  16   a  and  16   b , the antiparallel diode pair connection point  17 , and the RF/IF signal separating unit  18 . In the following section, similar components are denoted by the same numerals as of the first embodiment, and will be explained in no more details.  
      The locally oscillated signal input terminal  10   a , the balanced balun (signal branching means)  10 , the DC return coils  14   a  and  14   b , the antiparallel diode pairs (mixing means)  16   a  and  16   b , the antiparallel diode pair connection point  17 , and the RF/IF signal separating unit  18  are the same as those of the fast embodiment, and a description thereof is thus omitted.  
      The diplexers (constant impedance elements)  22   a  and  22   b  have the frequency band (4 to 8 GHz, for example) of the signal output from the balanced balun  10  as the passband, and exhibit a termination characteristic (have a characteristic as a terminator) in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF.  
      A description will now be given of the impedance characteristic of the diplexers (constant impedance elements)  22   a  and  22   b  with reference to a chart in  FIG. 6 . The impedances of the diplexers (constant impedance elements)  22   a  and  22   b  are generally constant at 50Ω in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF.  
       FIG. 7  shows examples of a circuit configuration of the diplexers  22   a  and  22   b.    
       FIG. 7 ( a ) shows an example where the diplexers  22   a  and  22   b  are constituted by band-pass filters. The diplexers  22   a  and  22   b  include a band-pass filter  222  which is connected to the balanced balun  10  on one end, and to the antiparallel diode pair  16   a  or  16   b  on the other end, a band-pass filter  224  which is connected to the other end of the band-pass filter, and a resistor  226  which is connected to the band-pass filter  224  and is grounded. It should be noted that the band-pass filter  222  has the frequency band (4 to 8 GHz, for example) of the signal output from the balanced balun  10  as the passband. Moreover, the band-pass filter  222  has the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF as the passband.  
       FIG. 7 ( b ) shows an example where the diplexers  22   a  and  22   b  are constituted by circuit elements L, C, and R. The diplexers  22   a  and  22   b  include a reactance element L which is connected to the balanced balun  10  on one end, and to the antiparallel diode pair  16   a  or  16   b  on the other end, a capacitance element C 2  which is connected to the one end of the reactance element L and is grounded, and a capacitance element C 1  which is connected to the other end of the reactance element L, and a resistance element R 1  which is connected to the capacitance element C 1  and is grounded.  
      An operation of the second embodiment is generally the same as that of the first embodiment.  
      It should be noted that, in the antiparallel diode pair  16   a  ( 16   b ), regardless of the phase of the supplied locally oscillated signal Lo, it is considered that either one of the diodes  162   a  and  164   a  ( 162   b  and  164   b ) opposite to each other is turned on. As a result, the impedance of the antiparallel diode pair  16   a  ( 16   b ) observed from the antiparallel diode pair connection point  17  is approximately equal to the input/output impedance of the diplexer  22   a  ( 22   b ).  
      The input/output impedance of the diplexer  22   a  ( 22   b ) is generally constant in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF as described above. Thus, the frequency characteristic of the conversion loss on the conversion of the high frequency received signal RF into the intermediate frequency signal IF is generally constant even if the frequency fRF of the high frequency received signal RF changes.  
      According to the second embodiment, the input/output impedance of the diplexer  22   a  ( 22   b ) is generally constant in the frequency band (9 to 49 GHz, for example) of the high frequency received signal RF. Thus, the frequency characteristic of the conversion loss on the conversion of the high frequency received signal RF into the intermediate frequency signal IF is generally constant even if the frequency fRF of the high frequency received signal RF changes.