Abstract:
In one preferred embodiment, the data receiver includes an input matching circuit for receiving an input RF data and forming an impedance matched signal and an amplifier circuit responsive to the impedance matched signal for forming an impedance transformed signal. The data receiver further includes a dual band RF tank/IF tank circuit forming respective RF and IF components and a diode mixer for mixing the impedance transformed signal with the dual band RF/IF components to form a mixed signal so that the RF tank filters the RF component from the mixed signal to form an IF tone signal as an output. The amplifier circuit preferably includes a source impedance control circuit for controlling high RF and IF impedances and a low noise amplifier (LNA) for forming, in conjunction with the source impedance control circuit, the impedance transformed signal.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention (Navy Case No. 100,808) is assigned to the United States Government and is available for licensing for commercial purposes. Licensing and technical inquiries may be directed to the Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; voice (619) 553-2778; email T2@spawar.navy.mil. 
    
    
     BACKGROUND 
     Data receivers typically down-convert signals from a high frequency RF to a lower intermediate frequency IF. This is done to reduce the number of complex and power hungry RF circuits. This down-conversion is done using a mixer. Mixers may either be passive or active. Passive mixers are beneficial because they consume little or no power. However, they exhibit poor conversion gain, especially at small signal levels. Active mixers can achieve higher conversion gain, but use large amounts of power. For systems requiring very long battery life, the power consumption of an active mixer may be too high. For these applications, typically a passive diode mixer is used. Receivers using passive diode mixers typically have poor sensitivity to low conversion gain of the diode. Thus, the transmitted signal must have large power. This places restraints on the transmitter and message transmission distance. 
     SUMMARY 
     In one preferred embodiment, the data receiver includes an input matching circuit for receiving an input RF data and forming an impedance matched signal and an amplifier circuit responsive to the impedance matched signal for forming an impedance transformed signal. The data receiver further includes a dual band RF tank/IF tank circuit forming respective RF and IF components and a diode mixer for mixing the impedance transformed signal with the dual band RF/IF components to form a mixed signal so that the RF tank filters the RF component from the mixed signal to form an IF tone signal as an output. The amplifier circuit preferably includes a source impedance control circuit for controlling high RF and IF impedances and a low noise amplifier (LNA) for forming, in conjunction with the source impedance control circuit, the impedance transformed signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the drawings, where like components are represented by like reference numerals: 
         FIG. 1  shows a block diagram of a dual band diode mixer for an RF data receiver. 
         FIG. 2A  shows a circuit diagram of a data receiver including a diode mixer for two-tone or AM signals. 
         FIG. 2B  shows an equivalent RF circuit for the diagram shown in  FIG. 2A . 
         FIG. 2C  shows an equivalent IF circuit for the diagram shown in  FIG. 2A . 
         FIG. 3  shows a circuit diagram of a data receiver for mixing using a reference oscillator. 
         FIG. 4  shows a diagram of output voltage (log scale) of the improved passive mixer vs input power. 
         FIG. 5  shows a diagram of output voltage (log scale) of improved mixer circuit vs input power. 
         FIG. 6  shows an alternative configuration of AM or two-tone signals. 
         FIG. 7  shows an alternative configuration of mixing using a reference oscillator. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A data receiver utilizing a dual band diode mixer can be used to lower the power and improve the sensitivity of the data receivers. Any receiver that performs conversion from high frequency RF to low frequency IF could use this invention for mixing. Applications can include locating (lost hiker, animal search, or valuables), AM radios, and remote controlled systems (garage door openers, radio controlled robotics, etc.), medical implants, as well as wake-up signals for remotely located devices. 
     The dual band diode mixer provides advantages over prior art mixers, which have several disadvantages. First, the input match is to the antenna&#39;s impedance, typically 50 ohms, meaning the input voltage to the diode is very small for low input power levels. The voltage across the diode should be made as large possible for conversion gain. Providing a larger impedance seen by the diode will improve conversion gain. Also, this inductor allows no DC current to flow through the diode. A DC bias greatly increases the conversion gain of the diode. A preferred solution is to allow DC biasing, present large impedances at both RF and IF, and allow for the impedances to be set independently. 
     Other passive solutions include using multiple diodes and mixing with more harmonics. These improvements are minimal. One possible way to improve the sensitivity of a passive mixer is to provide an amplifier before the mixer. However, if the amplifier&#39;s output impedance is matched to the input impedance of the mixer (typically 50 ohms), the sensitivity of the mixer will only improve at most by the gain of the amplifier. It is worth noting the added amplifier will cause retuning the IF portion due to the impedance of the amplifier (the RF and IF impedances are not independent). 
       FIG. 1  shows a block diagram of a dual band diode mixer for an RF data receiver  10  which includes an input impedance matching circuit  10 , amplifier  14 , source impedance control  15 , diode mixer  20 , and a dual band tank configuration  30 . The dual band tank configuration  30  includes an RF tank  32 , IF tank  34  and RF/IF isolation  36 . The data receiver  10  receives an RF data signal  11  and forms an IF tone signal output  40 , as will now be described in more detail in conjunction with  FIGS. 2A-2C . 
       FIG. 2A  shows a data receiver  10  including a diode mixer for two-tone or AM signals. The data receiver includes an antenna  50  which receives an RF data signal  11  (modulated RF signal) for input to the input matching circuit  12  of  FIG. 1 . 
     The input circuit  12  includes DC blocking capacitor  52  which isolates the DC bias of the LNA (low noise amplifier)  60  from the LNA. Some LNAs require a DC bias, which would come from Node  56 , The capacitor  52  blocks this DC voltage from the antenna  50 , if required. Some antennas (typically active) cannot have a DC bias on their output. 
     The shunt capacitor  53  adjusts the input impedance of the LNA (if needed). The series inductor  54  adjusts the input impedance of the LNA (if needed). The node  56  for DC bias of LNA adjusts the gate-to-source voltage of the LNA (if DC bias is required). The node  57  for DC bias of LNA adjusts the gate-to-source voltage of the cascode transistor  65  for LNA. 
     The IF source impedance control circuit  15  is a resonant tank which is set to resonate at the IF frequency, providing several benefits. One, it increases the source impedance at IF. The results in lower noise gain from the input. Also, the output impedance of the LNA is increased at IF. These features provide higher conversion gain with lower noise. Since the resonate frequency of the tank is much lower than the RF, the tank has negligible effect on the RF performance. 
     The resonant capacitor  62  is one half of the IF resonate circuit for the source. This capacitor must be set with the resonant inductor  63  to resonate at IF. For the resonant inductor  63 : the larger this inductor, the larger the conversion gain. 
     LNA (low noise amplifier)  60 . This amplifier  60  increases the voltage across the mixing diode  85  to increase conversion gain. The benefit in using the LNA  60  in a high output impedance configuration (as opposed to usual 50 ohm) is that the diode  85  is provided a larger voltage for the same power. To illustrate this, consider a 0 dBm input with a 20 dB gain LNA. If the diode mixer is set with an input impedance of 50 ohms (prior art), the voltage across the diode is given by 
     
       
         
           
             
               P 
               IN 
             
             = 
             
               
                 0 
                 → 
                 
                   P 
                   diode 
                 
               
               = 
               
                 
                   0 
                   + 
                   20 
                 
                 = 
                 
                   
                     20 
                     → 
                     
                       
                         V 
                         2 
                       
                       R 
                     
                   
                   = 
                   
                     
                       .1 
                       → 
                       V 
                     
                     = 
                     
                       2.25 
                       ⁢ 
                       V 
                     
                   
                 
               
             
           
         
       
     
     Now consider the same input power and amplifier in the configuration of  FIG. 2 . If the output impedance is not 50 ohms, but large (10000 ohms) the voltage across the diode is much larger. 
     
       
         
           
             
               P 
               IN 
             
             = 
             
               
                 0 
                 → 
                 
                   P 
                   diode 
                 
               
               = 
               
                 
                   0 
                   + 
                   20 
                 
                 = 
                 
                   
                     20 
                     → 
                     
                       
                         V 
                         2 
                       
                       R 
                     
                   
                   = 
                   
                     
                       .1 
                       → 
                       V 
                     
                     = 
                     
                       31.6 
                       ⁢ 
                       V 
                     
                   
                 
               
             
           
         
       
     
     This much larger voltage across the diode greatly increases the conversion gain by more than the gain of the amplifier. 
     LNA input transistor  64  sets the gain, noise figure, and current consumption of the LNA  60 . The cascode transistor  65  improves the reverse isolation of the LNA  60 . This is important for mixing circuits to prevent unwanted signals to leak to the antenna  50 . 
     The source impedance control  67  for RF is an inductor which helps tune the input impedance of the LNA at RF (if needed). This inductor is also used to improve the stability of the circuit. 
     The RF resonant inductor  70  resonates at RF with the combined capacitance of the LNA  60  and mixing diode  85 . If the combined capacitance of the LNA  60  and diode  85  is called C Load , the value of the resonant inductor is given by 
     
       
         
           
             
               L 
               RF 
             
             = 
             
               1 
               
                 
                   w 
                   RF 
                   2 
                 
                 · 
                 
                   C 
                   Load 
                 
               
             
           
         
       
     
     The DC blocking capacitor  72  blocks the DC current for the RF resonant inductor  70 . This inductor  70  should be set to provide a small impedance at the RF frequency. Elements  72  and  70  are in series and may be interchanged. 
     The RF isolation circuit  36  filters the RF component of the mixed signal so only the IF tone is left. In a preferred embodiment, the circuit  36  is a LC resonant tank that is designed to resonate at a frequency greater than the IF and lower than the RF. 
     The resonant capacitor  83  is used with the resonant inductor  84  to resonate at a frequency greater than the IF and lower than the RF. The resonant inductor  84  is used with the resonant capacitor  83  described above. 
     The mixing diode  85  performs the mixing function as a result of the Taylor series expansion of the voltage dependence of the diode  85 . The diode  85  should be a high performance diode with low equivalent resistance and capacitance. 
     The mixer bias setting resistor  87  is used to set the DC bias of the mixing diode  85  to provide a DC bias to increase the conversion gain of the diode  85 . 
     The capacitor  89  is an RF short for the mixing diode  85 . The capacitor  89  should be set to be a small impedance at RF. This provides an AC short for the diode for proper mixing. 
     The IF resonant inductor  92  resonates with the DC blocking capacitor  72  to provide a high impedance at IF. By providing a high impedance at IF, the down-converted tone is not shorted by a small impedance, but supported by the impedance of the tank. Note the resonant inductor  83  of the RF isolation circuit is in series with the IF resonant inductor  92 . The RF isolation circuit  36  is preferably a small inductor compared to the IF resonant inductor and can be ignored. The value of the IF resonant inductor is set by 
     
       
         
           
             
               L 
               IF 
             
             = 
             
               1 
               
                 
                   w 
                   IF 
                   2 
                 
                 · 
                 
                   ( 
                   
                     
                       C 
                       Load 
                     
                     + 
                     
                       C 
                       
                         D 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Bloack 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     Note that both the IF and R tanks are set independently of each other, which is a significant improvements over previous efforts. 
     The voltage supply  94  and circuit output  40  shown in  FIG. 2A  are self-explanatory. 
       FIGS. 2B and 2C  show an equivalent RF circuit and an equivalent IF circuit, respectively, for the data receiver diagram shown in  FIGS. 1 and 2A . 
     In a preferred embodiment shown in  FIGS. 1 and 2A , circuit  36  resonates at a frequency greater than IF but less than RF. The idea is that at IF, circuit  36  effectively reduces to the inductor. At RF, circuit  36  reduces to the capacitor. If circuit  36  resonates at IF, the mixed IF tone will be “blocked” by the large impedance and will be attenuated at the output. In that preferred embodiment, the resonant frequency of circuit  36  should also be lower than the RF frequency. 
     At RF, the circuit has passed its resonant frequency, and effectively reduces to the capacitor. This isolates any load capacitance from the RF tank by putting the load capacitor in series with this capacitor in circuit  36 , since capacitors in series have a reciprocal relationship. Thus, if the capacitor in circuit  36  is small (for example, around 100 fF), then load capacitance can be fairly large (&gt;10 pF) without effecting the RF tank at all. 
     This is very desirable because the load capacitance tunes the IF tank. The IF tank can be tuned without adjusting the RF tank&#39;s tune. 
     At both frequencies (RF and IF), the circuit effectively reduces to an LC tank at the node of the diode. The inductor&#39;s impedance cancels the impedance of the capacitor, leaving the diode seeing a large impedance, which is the beauty of this circuit. At RF and IF, the circuit reduces to simple tanks that are tuned independently of each other. 
       FIG. 3  shows an embodiment of a mixer circuit configured to perform mixing using a reference oscillator. The analysis of  FIG. 3 . circuit is identical to that of  FIG. 2A  with the additions of the node for reference oscillator  97  and the power transformer  98 . 
     The dual band diode mixer provides many benefits: first, the invention allows DC biasing of the diode, increasing conversion gain. Second, the RF and IF tanks are set independently of each other. Thus the RF or IF tank can be tuned without disturbing the other match. Also, the output impedance of the amplifier is large at both RF and IF. This means both the RF and IF tone are supported by a large impedance. This maximizes conversion gain by presenting a larger voltage across the diode. Since the conversion gain of the diode is exponential with voltage, the conversion gain is increased by more than the gain of the amplifier. These advantages are shown in the following circuit simulation. 
       FIG. 4  shows a circuit simulation of output voltage (log scale) of improved passive mixer vs. input power. The circuit configuration of  FIG. 2  is a significant improvement over the previous effort. The conversion gain is considerably higher as evident by the increased slope of the curve. This is due to the dual resonant tanks supported the down-converted tone as well as the ability to DC bias the diode. Prior approaches exhibit very poor conversion gain in which slope of the curve is low. These prior approaches require large input power to achieve detectable voltage, and thus, sensitivity is very poor. 
       FIG. 4  shows the huge improvements of the circuit of  FIG. 2 . The circuit simulation includes a 33 dB amplifier, but the conversion gain is improved by 50 dB. The conversion gain can improve by more than the gain of the amplifier due to the exponential relationship of the diode. This large conversion gain—even at very small input power—allows the diode mixer to be used in applications that require high sensitivity. Also, the noise figure of this circuit is improved dramatically by amplifying the input signal while only adding small amounts of noise (typically 1-3 dB). This allows the mixing signal to be larger than the inherent noise of the diode, increasing sensitivity. 
     This mixer improves receiver performance by improving sensitivity, noise figure, and power consumption. 
       FIG. 5  shows a diagram of output voltage (log scale) of improved mixer circuit vs input power. 
     In various other embodiments, the mixer can down-convert AM signals. The mixer can also be used to down-convert two-tone signals. The mixer can also be used in down-convertors with a reference oscillator by using a transformer. The mixer can also be used without the amplifier by using an impedance transformation. This provides large impedances at RF and IF that can set independently. This configuration also allows DC biasing. However, the conversion is greatly reduced by eliminating the amplifier. 
       FIG. 6  shows an alternative configuration  100  for AM or Two-tone signals. 
     The data receiver  100  in  FIG. 6  includes antenna  101  with an AM or two-tone signal input to input matching circuit  102 . In the input matching circuit  102 , capacitors  104  and  105  match the circuit  102  to the impedance of antenna  101 . 
     The RF resonant inductor  110  resonates at RF with the combined capacitance of the mixing diode and the input match capacitors. The value of the resonant inductor is given by 
     
       
         
           
             
               L 
               RF 
             
             = 
             
               1 
               
                 
                   w 
                   RF 
                   2 
                 
                 · 
                 
                   ( 
                   
                     
                       
                         C 
                         Load 
                       
                       + 
                       
                         C 
                         104 
                       
                     
                     ∥ 
                     
                       C 
                       105 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     The DC blocking capacitor  112  blocks the DC current for the RF resonant inductor  134 . This inductor  134  should be set to provide a small impedance at the RF frequency. 
     Mixer bias setting resistor  114  is used to set the DC bias of the mixing diode  120 , providing a DC bias increases the conversion gain of the diode  120 . 
     Capacitor  115  is an RF short for the mixing diode  120 . The capacitor  115  should be set to be small impedance at RF, providing an AC short for the diode for proper mixing. 
     Mixing diode  120  performs the mixing function as a result of the Taylor series expansion of the voltage dependence of the diode. The diode  120  should be a high performance diode with low equivalent resistance and capacitance. 
     RF isolation circuit  130  filters the RF component of the mixed signal so only the IF tone is present. The circuit  130  is a LC resonant tank that is designed to resonate at a frequency greater than the IF and lower than the RF. 
     Resonant capacitor  132  is used with the resonant inductor  134  to resonate at a frequency greater than the IF and lower than the RF. 
     The output  136  of circuit  100  is self-explanatory. 
     {Changed output node  134  to  136 .  134  is used for the inductor.} 
     IF resonant inductor  140  resonates with the DC blocking capacitor  112  to provide a high impedance at IF. By providing a high impedance at IF, the down-converted tone is not shorted by a small impedance, but supported by the impedance of the tank. Note the resonant inductor  134  of the RF isolation circuit is in series with the IF resonant inductor  140 : The RF isolation circuit is usually a small inductor compared to the IF resonant inductor and can be ignored. The value of the IF resonant inductor is set by 
     
       
         
           
             
               L 
               IF 
             
             = 
             
               1 
               
                 
                   w 
                   IF 
                   2 
                 
                 · 
                 
                   ( 
                   
                     
                       
                         C 
                         Load 
                       
                       + 
                       
                         C 
                         
                           D 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Bloack 
                         
                       
                       + 
                       
                         C 
                         
                           2 
                           ⁢ 
                           a 
                         
                       
                     
                     ∥ 
                     
                       C 
                       
                         2 
                         ⁢ 
                         b 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
     Note that both the IF and RF tanks are set independently of each other. This is big improvements over previous efforts. 
     The voltage supply  142  is self-explanatory. 
     This configuration can be used to down-convert using a reference oscillator. This is shown in  FIG. 7 , which shows an alternative configuration for mixing using a reference oscillator. The operation of the circuit is identical as the  FIG. 6  with the addition of the power transformer  144  and the node for reference oscillator  146 . 
     From the above description, it is apparent that various techniques may be used for implementing the concepts of the present invention without departing from its scope. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that system is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.