Abstract:
A frequency mixer that generates out of phase sum signals and uses the out of phase relationship to absorb sum signal energy in an energy absorbing component. A frequency mixer according to the present teachings includes a set of mixing devices that generate a first sum signal and a second sum signal in response to an input signal and a drive signal such that the first sum signal is out of phase in relation to the second sum signal. A frequency mixer according to the present teachings includes an energy absorbing component that absorbs the out of phase sum terms from the mixing devices.

Description:
BACKGROUND  
       [0001]     Frequency mixers may be employed in a variety of electronic systems. For example, frequency mixers may be employed in radio systems to down convert a received radio frequency (RF) signal by combing the received radio frequency (RF) signal with a local oscillator (LO) signal. The combination of an input signal and an LO signal in a frequency mixer may yield an intermediate frequency (IF) signal having a frequency that corresponds to the difference between the frequencies of the input and LO signals.  
         [0002]     In addition, the combination of an input signal and an LO signal in a frequency mixer may yield a sum signal having a frequency that corresponds to the sum of the frequencies of the input and LO signals. The sum signal may be ignored as an unwanted byproduct of a frequency mixer that is designed to yield an IF signal. Unfortunately, the sum signal may reflect back and re-enter into a mixing device and cause a degradation the performance of a frequency mixer.  
         [0003]     Prior frequency mixers may include filter circuits that are designed to steer the sum signal to a load resistor that dissipates the energy of the sum signal. For example, a prior frequency mixer may include a diplex filter or a triplex filter coupled to the output port of the mixer. Unfortunately, a diplex filter may not have sufficient bandwidth to capture a sum signal, particularly in the case of a double balanced mixer. A triplex filter may be used in a single balanced mixer but the design of a triplex filter may be relatively complex, may require additional components, and compromise the performance of a frequency mixer.  
       SUMMARY OF THE INVENTION  
       [0004]     A frequency mixer is disclosed that generates out of phase sum signals and uses the out of phase relationship to absorb sum signal energy in an energy absorbing component. A frequency mixer according to the present teachings includes a set of mixing devices that generate a first sum signal and a second sum signal in response to an input signal and a drive signal such that the first sum signal is out of phase in relation to the second sum signal. A frequency mixer according to the present teachings includes an energy absorbing component that absorbs the out of phase sum terms from the mixing devices.  
         [0005]     Other features and advantages of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0007]      FIG. 1  shows a frequency mixer according to the present teachings;  
         [0008]      FIG. 2  shows another frequency mixer according to the present teachings.  
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  shows a frequency mixer  100  according to the present teachings. The frequency mixer  100  includes a pair of mixing devices  110 - 112  that are driven by a drive signal generator  104 . The drive signal generator  104  generates a drive signal  106  to the mixing device  110  that is out of phase from a drive signal  108  to the mixing device  112 .  
         [0010]     The mixing device  110  generates a sum signal and a difference signal on an output node  120  in response to the drive signal  106  and an input signal on an input node  124 . The mixing device  112  generates a sum signal and a difference signal on an output node  122  in response to the drive signal  108  and the input signal on the input node  124 .  
         [0011]     The frequency mixer  100  includes a pair of capacitors C 1  and C 2  having values that are selected to cause the sum signals on the output nodes  120 - 122  to be dissipated in an energy absorbing component, e.g. a resistor R 1 . The frequency mixer  100  includes a pair of inductors L 1  and L 2  that are selected to appear as open circuits at the frequency of the sum signals and prevent escape of sum signal energy to the input node  124 .  
         [0012]     Both ends of the resistor R 1  have substantially equal potentials at the frequency of the input signal. Thus, there is no substantial current through the resistor R 1  at the frequency of the input signal. On the other hand, the sum signals at opposite ends of the resistor R 1  are out of phase. Thus, sum signal energy flows through and is dissipated in the resistor R 1 .  
         [0013]     The product of the values of the capacitor C 1  and the inductor L 1  determines the frequency of energy flow from the input port  124  through the inductor L 1  and the capacitor C 1  to the output node  120 . The values of the capacitor C 1  and the inductor L 1  are selected so that their product passes the desired frequency of the input signal on the input node  124 .  
         [0014]     The frequency mixer  100  dissipates sum signal energy in the resistor R 1  and avoids reflections of sum signal energy using the resistor R 1  and proper selection and arrangement of the remaining components. The frequency mixer  100  also improves port match at the input node  124  by absorbing sum energy thereby preventing sum signal energy from re-entering the mixing devices and re-converting back to the input frequency energy and emerging from the input node  124 .  
         [0015]      FIG. 2  shows a frequency mixer  10  according to the present teachings. The frequency mixer  10  is a single balanced mixer that includes a pair of transistors Q 1  and Q 2  that function as mixing devices. The transistors Q 1  and Q 2  in one embodiment are field effect transistors (FETs). In other embodiments, the transistors Q 1  and Q 2  may be bipolar junction transistors (BJTs) or diodes.  
         [0016]     The frequency mixer  10  includes an intermediate frequency port  30  (IF+), an intermediate frequency port  32  (IF−), and a radio frequency (RF) port  40 . The gates of the transistors Q 1  and Q 2  are driven by a local oscillator signal LO+ and a local oscillator signal LO−, respectively.  
         [0017]     The transistor Q 1  combines the local oscillator signal LO+with an RF signal received at the RF port  40 . The IF+port  30  carries the difference term generated by the transistor Q 1 . Similarly, the transistor Q 2  combines the local oscillator signal LO− with the RF signal received at the RF port  40 . The IF− port  32  carries the difference term generated by the transistor Q 2 .  
         [0018]     An inductor L 13  and a capacitor C 13  provide a low pass filter between an output node  50  of the mixing transistor Q 1  and the IF+ port  30 . A capacitor C 11  and an inductor L 11  provide a band pass filter from the output node  50  to the RF port  40 . Similarly, an inductor L 14  and a capacitor C 14  provide a low pass filter between an output node  52  of the mixing transistor Q 2  and the IF− port  32 , and a capacitor C 12  and an inductor L 12  provide a band pass filter from the output node  52  to the RF port  40 .  
         [0019]     The local oscillator signals LO+ and LO− are complementary phase signals that have a 180 degree phase relationship with respect to one another. As a consequence, the difference term generated by the transistor Q 1  is 180 degree out of phase with respect to the difference term generated by the transistor Q 2 . Thus, the signal at the IF+ port  30  is 180 degree out of phase with respect to the signal at the IF-port  32 .  
         [0020]     The values for the capacitors C 11  and C 12  and the inductors L 11  and L 12  are selected so that the sum signals at the output nodes  50  and  52  are absorbed by a resistor R 11 . The inductors L 11  and L 12  are selected to be large enough to appear as open circuits at the frequency of the sum signals. This prevents escape of sum term energy through the inductors L 11  and L 12 . The capacitors C 11  and C 12  are selected to be large enough to couple the sum signals at the output nodes  50  and  52  to the resistor R 11  efficiently. The values of capacitors C 11  and C 12  and the inductors L 11  and L 12  and the resistor R 11  are chosen to provide an impedance matched termination of sum signal energy.  
         [0021]     The product of the values of the capacitor C 11  and the inductor L 11  determines the frequency of energy flow from the RF port  40 , through the inductor L 11  and the capacitor C 11 , to the drain of the transistor Q 1  at the output node  50 . The values of the capacitor C 11  and the inductor L 11  are selected so that their product passes RF frequencies. The values of the capacitor C 12  and the inductor L 12  are selected so that their product passes RF frequencies.  
         [0022]     Both ends of the resistor R 11  have substantially equal RF potentials. Thus, there is no substantial RF current through the resistor R 11 . On the other hand, the sum signals at opposite ends of the resistor R 1  are 180 degrees out of phase. Thus, sum signal energy flows through and is dissipated in the resistor R 11 .  
         [0023]     The value of the inductor L 11  may be equal to the value of the inductor L 12 , and the value of the capacitor C 11  may be equal to the value of the capacitor C 12 .  
         [0024]     It will be appreciated that the RF signal appears at each of the mixing devices Q 1  and Q 2  at the same phase angle, i.e. in phase. The IF signal emerging from the transistor Q 1  is filtered by the low pass filter composed of the inductor L 13  and the capacitor C 13 , and appears at the IF+ port  30 . In a similar manner, the IF signal from the transistor Q 12  is filtered by the inductor L 14  and the capacitor C 14 , and appears at the IF− port  32 .  
         [0025]     The sum signal also emerges from the mixing transistors Q 1  and Q 2  in an opposite phase condition, but cannot pass through the IF low pass filters. Instead, the sum signal passes though the capacitors C 11  and C 12  and is terminated by sum the absorbing resistor R 11 . The sum signal is transformed up in impedance by the capacitor C 11  and the inductor L 11 , as well as by the capacitor C 12  and the inductor L 12 . The degree of impedance transformation is determined by the values of C 11 , C 12 , L 11 , and L 12 . The value of the resistor R 11  is chosen to match the transformed impedance of the sum signal so that it is efficiently absorbed.  
         [0026]     The resistor R 11  does not degrade the RF signal path because it is connected to two nodes that are at equal RF potential. The effective sum termination without RF signal degradation is possible because the RF energy enters the structure in common mode, while the sum signal exits the mixing transistors Q 1  and Q 2  in differential mode. This may be referred to as mode conversion, and it affords complete isolation of the RF signal from the sum termination resistor R 11 .  
         [0027]     The filtering of the RF signal from the IF signal using the inductors L 13 , L 14  and the capacitors C 13 , C 14  may in other embodiments be performed by other arrangements of components.  
         [0028]     The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.