COMPENSATED RING MIXERS

Abstract: A compensated mixer including a radio frequency (RF) balun (18) through which RF signals are transferred, and a local oscillator (LO) balun (14) for receiving an LO signal. In an exemplary implementation the mixer further includes a diode ring quad circuit (12) for mixing RF signals received by the RF balun (18) with the LO signal received by the LO balun (14). During operation the diode ring quad circuit develops, between the first and second nodes therein (50, 52), a capacitive reactance arising from switching at the LO frequency. The mixer further includes a compensation inductor (56), connected between the first and second nodes (50, 52), for cancelling the effect of the capacitive reactance at the LO signal frequency.

Description: COMPENSATED RING MIXERS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to radio frequency communication devices and more specifically to mixers having high third order intercepts. 2. Description of the Prior Art The dynamic range of many prior art microwave front-ends is controlled by the single and two-tone intermodulation levels of a Schottky diode mixer. (S. Weiner, D. Neuf, S. Spohrer, "2 to 8 GHZ Double Balanced MESFET Mixer With +30 dBm Input 3rd Order Intercept," 1988 IEEE MTT-S Digest, pp.1097-1099.) A typical Schottky mixer obtains third order intercept points approximately equal to the local oscillator (LO) power minus the conversion loss plus 10-dB, wherein third-order two-tone intercept points are commonly used as a measure of a figure of merit for a mixer. The popular diode-ring double-balanced mixer often requires the LO power to exceed the signal compression level by 6-dB. Trade-offs between LO power levels and third order intercept and one dB compression points are generally necessary, even in multiple diode schemes attempting to improve isolation, bandwidth, and single-tone intermodulation levels. Low distortion mixing is known to be possible with small amounts of LO power when an unbiased channel of a GaAs MESFET is used as the mixing element. (See, Stephen A. Maas, "A GaAs MESFET Balanced Mixer With Very Low Intermodulation," 1987 IEEE MTT-S Digest, pp.895-896.) . In quad-ring demodulator/mixer devices, the switching action of the four ring-connected diode or FET switching elements effects the desired mixing function. The switching elements act, essentially, as a pair of switches reversing the phase of a signal at a rate determined by the LO frequency. The switching elements exhibit a finite on-resistance that is expressed as a conversion efficiency loss. The loss results from the channel resistance of the switching elements relative to both the signal and IF impedances, and from signal conversion to unwanted frequencies. The switching elements may also manifest a parasitic capacitance tending to introduce signal distortion and otherwise adversely affect mixer performance. OBJECTS OF THE INVENTION It is therefore an object of this invention to produce a radio frequency mixer characterized by low overall signal distortion. It is a further object of this invention to providing a technique for improving the third order intercept point of radio frequency mixers in a manner which does not adversely affect other aspects of mixer operation. SUMMARY OF THE INVENTION In summary, the present invention is directed to a novel compensated mixer including an RF balun coupled to an RF port, and an LO balun for receiving an LO signal. In an exemplary embodiment the mixer further includes a diode ring quad circuit for mixing an RF signal applied to the RF port with the LO signal. During the mixing process, the diode ring quad circuit develops a capacitive reactance between first and second nodes as a result of switching at the LO frequency. A compensation inductor is connected between the first and second nodes, and becomes resonant at the LO signal frequency with the capacitive reactance. In this way the capacitive reactance exhibited by the diode ring quad is cancelled by the compensation inductor, thereby preventing from adversely affecting mixer performance. A specific advantage of the present invention is that the compensation inductor enables an improved third order two tone intercept point to be achieved without degrading RF conversion efficiency. These and other objects and advantages of this invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. IN THE DRAWINGS FIG. 1 schematically illustrates an exemplary embodiment of a ring quad mixer of the present invention. FIG. 2A shows an equivalent circuit representation of one of the diodes included in the ring quad mixer of FIG. 1. FIG. 2B depicts variation in the junction capacitance (C j ) associated with a ring quad diode as a function of the LO signal voltage (V) applied thereto. FIG. 3 schematically illustrates an exemplary embodiment of a dual ring quad mixer of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically illustrates an exemplary embodiment of a ring quad mixer 10 of the present invention. The mixer 10 is comprised a diode ring quad 12, to which are coupled a local oscillator (LO) balun 14, and a radio frequency (RF) balun 18. During operation of the mixer 10 as a frequency downconverter, an RF signal is applied across capacitor 24 between signal ground and an RF input 28, an LO signal is applied across capacitor 30 between LO port 32 and signal ground, and an intermediate frequency (IF) output is coupled from an IF port 34 provided by the RF balun 18. When the mixer 10 is operating as a frequency upconverter, the roles of the RF and IF ports 28 and 34 are interchanged. In the exemplary embodiment of FIG. 1 the RF balun 18 is realized using an RF transformer 36, which is center-tapped to provide the IF port 34. The LO balun 14 includes first and second transformers 37 and 38 for providing, to the diode ring quad 12, the LO signal applied to the LO port 32. It is noted that other types of RF and LO baluns may substituted for those depicted in FIG. 1 without departing from the scope of the present invention. The diode ring quad 12 includes a set of four switching elements, which in FIG. 1 are realized as first, second, third and fourth diodes 41-44. The diodes 41-44 are serially connected in a ring configuration; that is, the output of each of the diodes 41-44 is coupled to the input of another of the diodes 41-44. During those half-cycles of the applied LO signal for which the voltage at mixer node 50 exceeds the voltage at mixer node 52, diodes 41 and 42 are turned on and diodes 43 and 44 are turned off. Similarly, during the remaining half-cycles of the LO signal the diodes 43 and 44 are turned on and the diodes 41 and 42 are turned off. The relative phase angle of the signals at the IF port 34 will depend on whether the first and second diodes 41 and 42, or the third and fourth diodes 43 and 44, are forward-biased. That is, if the diodes 41 and 42 are forward-biased, then diodes 43 and 44 must be reverse-biased, and vice versa. The LO signal applied at the LO port 32 is therefore caused to beat against the RF signal applied at the RF port 28, with the products (RF, LO, RF-LO, and RF+LO) appearing at the IF port 34. (The RF and LO product terms are considerably suppressed by virtue of the balanced topology and are therefore insignificant.) As is discussed further below, during mixer operation a parasitic capacitive reactance develops between the mixer nodes due to the junction capacitance inherent within the diodes 41-44. This capacitive reactance, which varies as a function of the frequency of the applied LO signal, has been found to have deleterious effects on the third-order intercept point of the mixer 10. In accordance with the invention, a compensation element comprising a compensation inductor 56 is connected between the mixer nodes 50 and 52 for the purpose of effectively eliminating this parasitic capacitive reactance. The inductive reactance of the compensation inductor 56 is selected such that it resonates with the parasitic capacitive reactance at the frequency of the applied LO signal. This improves the mixer third order intercept point by preventing the junction capacitance of the diodes 41-44 from introducing nonlinearity during mixing at the frequency of the LO signal. Turning now to FIG. 2A, there is shown an equivalent circuit representative of each of the diodes 41-44. Each diode 41-44 may be characterized as a series resistance (R s ) , connected in series with the parallel connection of a junction resistance (R:) and junction capacitance (C:) . It is the aggregate parasitic capacitive reactance of the diode ring quad 12, to which the junction capacitance (C:) of diode 41-44 contributes, which the compensation inductor 56 is designed to neutralize. FIG. 2B depicts an exemplary variation in the junction capacitance (C:) as a function of the LO signal voltage (V) across a given diode 41-44. In FIG. 2, it is assumed that the LO signal voltage varies from a maximum forward-bias voltage of Vf to a maximum reverse-bias voltage of V r . At a forward-bias voltage of Vf the junction capacitance is equivalent to C j , and at a reverse-bias voltage of V_ the junction capacitance is equivalent to C^. When the applied LO signal voltage causes either the first and second diodes 41 and 42, or the third and fourth diodes 43 and 44, to become forward-biased (i.e., > V f ) , the impedance between terminals 50 and 52 approximates a short circuit. Conversely, when the applied LO signal voltage is such that the voltage across each of the diodes 41-44 is between V r and Vf, then the aggregate parasitic capacitive reactance (C p ) between terminals 50 and 52 may be determined by reference to FIG. 2B. In accordance with the invention, the inductive reactance (L c ) of the compensation inductor 56 is selected to cancel the effect of C_, and thereby improve mixer linearity. The inductive reactance L c necessary to resonate at the LO frequency with the estimated value of C_ may be obtained from the following expression: 2π v / ^ It should be noted that when commercially available inductors are used to realize the compensation inductor 56, any stray capacitance inherent therein may be accounted for through appropriate adjustment of the value of C p . One feature of the present invention that the compensation inductor 56 improves the third order intercept point of the mixer 10 without adversely affecting other RF and IF operating characteristics, such as RF conversion efficiency. This is achieved by connecting the inductor 56 between the internal mixer nodes 50 and 52, which are at signal ground with respect to the RF signal port 28. Accordingly, the compensation inductor 56 is precluded from introducing distortion into RF signals coupled from the mixer 10. FIG. 3 schematically illustrates an exemplary embodiment of a dual ring quad mixer 100 of the present invention. The mixer 100 is comprised of first and second diode ring quads 104 and 108, to which are coupled a local oscillator (LO) balun 114, and a radio frequency (RF) balun 118. During operation of the mixer 100 as a frequency downconverter, an RF signal is applied at an RF input 128, and an LO signal is applied at LO port 132. The resultant intermediate frequency (IF) output is coupled from an IF port 134, which is connected to first and second internal nodes 136 and 138 of the first and second diode ring quads 104 and 108, respectively. When the mixer 100 is operating as a frequency upconverter, the roles of the RF and IF ports 128 and 134 are interchanged. That is, an IF input signal is applied via IF port 134 to the first and second diode ring quads 104 and 108, and the resultant RF output signal is coupled from the RF port 128. In FIG. 3, the RF balun 118 is comprised of first and second RF transformers 136 and 138, which are respectively connected to the ring quad devices 104 and 108. Similarly, the LO balun 114 includes first and second LO transformers 142 and 144 cross-coupled to the ring quad devices 104 and 108. In the dual-ring implementation of FIG. 3, first and second compensation inductors 150 and 152 serve to compensate for the parasitic capacitive reactance developed by the diode switching elements within the first and sr.cond diode ring quads 104 and 108. It is observed that for one-half of each period of the LO signal applied at LO port 132, a first diode 161 within the first diode ring quad 104 and a first diode 171 within the second diode ring quad 108 will be turned on. In addition, a fourth diode 164 within the first diode ring quad 104 and a fourth diode 174 within the second diode ring quad 108 will be turned off. During the other one-half of each LO signal period, the first diodes 161, 171 are turned off and the fourth diodes 164, 174 are turned on. In accordance with the invention, the first compensation inductor 150 is selected to provide an inductive reactance resonant with the average parasitic capacitive reactance developed by the first diodes 161, 171 in conjunction with the fourth diodes 164, 174 at the LO signal frequency. In like manner, for one-half of each period of the LO signal applied at LO port 132, a second diode 162 within the first diode ring quad 104 and a second diode 172 within the second diode ring quad 108 will be turned on. During this time a third diode 163 within the first diode ring quad 104 and a third diode 173 within the second diode ring quad 108 will be turned off. During the other one-half of each LO signal period, the second diodes 162, 172 are turned off and the third diodes 163, 173 are turned on. Over each LO signal period, an average parasitic capacitive reactance arises from the set of diodes comprised of the second diodes 162, 172 and the third diodes 163, 173. By selecting the value of the second compensation inductor 152 such that it provides an inductive reactance at the LO signal frequency resonant with this average capacitive reactance, the nonlinearity which would otherwise be engendered is substantially eliminated. Although this invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. For example, in alternate embodiments appropriately connected transistor devices may be used as switching elements in lieu of the diode ring quads described herein. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.

Claims: WHAT IS CLAIMED IS: 1. A mixer, comprising: an input port for receiving an input signal; a switching port for receiving a switching signal of a predetermined switching frequency; ring mixer means for mixing said input signal with said switching signal, said ring mixer means exhibiting a capacitive reactance at said switching frequency; and a compensation element coupled to said ring mixer means, said compensation element becoming resonant at said switching frequency with said capacitive reactance. 2. The mixer of claim 1 wherein said compensation element comprises an inductor having an inductive reactance selected in accordance with said capacitive reactance. 3. The mixer of claim 1 wherein said ring mixer means includes: a first switching element having a first input and a first output, a second switching element having a second input and a second output, said second input being connected to said first output, a third switching element having a third input and a third output, said third input being connected to said second output, and a fourth switching element having a fourth input and a fourth output, said fourth input being connected to said third output and said fourth output being connected to said first input. 4. The mixer of claim 3 wherein said input port includes an RF balun for impressing said input signal between said first input and said third input, said input signal comprising an RF signal. 5. The mixer of claim 4 wherein said switching port includes an LO balun for impressing said switching signal between said second input and said fourth input, said switching signal comprising an LO signal. 6. The mixer of claim 1 wherein said ring mixer means comprises a ring quad arrangement of having first, second, third and fourth diodes, each of said diodes being characterized by a junction capacitance contributing to said capacitive reactance. 7. The mixer of claim 6 wherein said compensation element comprises an inductor having an inductive reactance selected in accordance with a total capacitance formed from said junction capacitances of said first, second, third and fourth diodes. 8. The mixer of claim 4 wherein said RF balun includes means for providing an IF signal port. 9. A mixer, comprising: an RF balun coupled to an RF port; an LO balun for receiving an LO signal of an LO frequency; ring mixer means for mixing said RF signal with said LO signal, said ring mixer means exhibiting a capacitive reactance between first and second nodes therein at said LO frequency; and a compensation inductor connected between said first and second nodes, said compensation inductor becoming resonant at frequency of said LO signal with said capacitive reactance. 10. The mixer of claim 9 wherein said compensation element comprises an inductor having an inductive reactance selected to compensate for said capacitive reactance. 11. The mixer of claim 9 wherein said ring mixer means comprises a ring quad arrangement of first, second, third and fourth diodes, each of said diodes being characterized by a junction capacitance contributing to said capacitive reactance. 12. A dual ring mixer, comprising: an RF balun coupled to an RF port; an LO balun for receiving an LO signal of an LO frequency; a first ring quad device coupled to said RF balun and to said LO balun; a second ring quad device coupled to said RF balun, to said LO balun, and to said first ring quad device; and inductive compensation means for compensating for capacitive reactance developed within said first and second ring quad devices during switching operation at said LO frequency. 13. The dual ring mixer of claim 12 wherein said first and second ring quad devices each include a set of four switching elements connected between a corresponding set of four of four internal nodes. 14. The dual ring mixer of claim 13 wherein said inductive compensation means includes: a first inductor connected between a first of said four internal nodes of said first ring quad device and a first of said four internal nodes of said second ring quad device, and a second inductor connected between a second of said four internal nodes of said first ring quad device and a second of said four internal nodes of said second ring quad device. 15. The dual ring mixer of claim 14 wherein said LO balun includes an LO input port and first and second LO output ports, said first LO output port being connected in parallel to said first inductor and said second LO output port being connected in parallel with said second inductor. 16. In a mixer including an input port for receiving an input signal, and a switching port for receiving a switching signal of a predetermined switching frequency, a method of impedance compensation comprising the steps of: providing a ring quad device for mixing said input signal with said switching signal, said ring quad device exhibiting a capacitive reactance at said switching frequency; and compensating for said capacitive reactance by coupling a compensation element to said ring quad device, said compensation element becoming resonant with said capacitive reactance at said switching frequency. 17. In a mixer including an RF balun coupled to an RF port and an LO balun for receiving an LO signal, a method of impedance compensation comprising the steps of: providing a ring quad device for mixing said RF signal with said LO signal, said ring quad device exhibiting a capacitive reactance between first and second nodes therein at said LO frequency; and connecting a compensation inductor between said first and second nodes, said compensation inductor becoming resonant with said capacitive reactance at frequency of said LO signal. 18. In a mixer including an input port for receiving an input signal, a switching port for receiving a switching signal of a predetermined switching frequency, and a ring mixer circuit for mixing said input signal with said switching signal wherein said ring mixer circuit exhibits a capacitive reactance at said switching frequency, the improvement comprising: a compensation element coupled to said ring mixer means, said compensation element becoming resonant at said switching frequency with said capacitive reactance. 19. The improvement to the mixer according to claim 18, wherein said compensation element comprises an inductor having an inductive reactance selected in accordance with said capacitive reactance.

Inventors: LI XIAOHUI, LI, XIAOHUI
Applicants: WATKINS JOHNSON CO, WATKINS-JOHNSON COMPANY
Application year: 1996
Main classification: H04B1
IPC codes: H03D7, H04B1