Abstract:
A stripline tuning fork microwave balanced mixer circuit which includes two diodes (21, 22) and a differential transformer (61) whereby matching is accomplished in the differential transformer (61) and in the circuit arrangement between the differential amplifier and the diodes (21, 22) for matching the resistance to the input terminals (1, 2) and a transforming circuit (24, 25, 26) which suppresses the frequency range of the input signals following the diodes and has simultaneous high frequency blocking for matching to the intermediate frequency signal output at terminal (3). This latter circuit can be utilized so that it functions as a blocker for the sum frequencies of the two input signals and the balanced push-pull mixer circuit of the invention can be used in sensitive receivers used in the microwave range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to a stripline microwave balanced mixer circuit for push-pull mixer circuits manufactured with stripline technology and comprises a differential transformer for applying two input voltages which are to be mixed and has two input terminals to which one of the input terminals is connected through two quarterwave length striplines to two transformer output terminals and the other input terminal is connected to the one transformer output terminal through the inner conductor of a coaxial line which has an arc-like shape and from the transformer extends to the output terminal. An arcuately shaped stripline with through contact to the ground plate is symmetrically disposed at the end and the outer conductor end is connected to the ground plate through a through contact and the other outer conductor end is connected to the other transformer output terminal. Two semiconductor diodes with one connected between one transformer output terminal and an intermediate frequency signal output and the other diode is connected between the other transformer output terminal and the intermediate frequency signal output. The diodes have opposite poling to the intermediate frequency signal output and components for blocking the frequency range of the two input voltages is also provided. Sometimes a problem exists for connecting a mixer which is to be operated with a noisy oscillator so as to prevent the mixer noise factor from becoming poor and which is to have low mixing losses then a balanced or push-pull mixer is utilized which functions with broadband high isolation between the two input terminals, i.e., between the oscillator branch and the input signal branch and also functions with matching for the two input signal terminals as well as for the intermediate frequency signal output to convert it for example to an intrinsic inpedance Z 0  of 50Ω. 
     SUMMARY OF THE INVENTION 
     The invention utilizes a not so well known tuning fork circuit for producing a balanced or push-pull mixer which contains a differential transformer for microwaves and which has the shape that appears like a tuning fork and allows a frequency independent high isolation between the two input signal terminals. In band width, it is superior to known circuits such as the 90° hybrid or the 180° hybrid. For example, the oscillator noise is greatly suppressed by the circuit of the invention even when the signal and the oscillator frequencies are greater spaced from each other. 
     According to the invention, a microwave balanced or push-pull mixer circuit is formed in stripline technology to match the intrinsic impedance Z 0  between the two input terminals and at the intermediate frequency signal terminal. 
     Two symmetrical striplines are mounted between the output terminals of the differential transformer and the diode terminals on the side away from the intermediate frequency signal output. 
     By enlarging the width of the stripline parallel capacitances are formed in the areas of the diode terminals. 
     The arc-shaped coaxial line and the stripline extending symmetrically relative thereto which extends from the two differential transformer output terminals are designed to be shorter than a quarter wave length so that parallel inductance is obtained. 
     The measures listed in the above three paragraphs are accomplished such that the diode impedances are respectively transformed at the differential transformer output terminals to the intrinsic impedance value Z 0 . The width of two quarter wave lengths striplines is dimensioned such that a respective intrinsic impedance value of Z 0 .√2 is obtained. 
     A stripline is inserted between the inner conductor end of the coaxial line on the side away from the differential transformer output terminals and the associated input terminal so that the overall length of the coaxial line and the stripline is equal to a quarter wave length and the intrinsic impedance of these two lines is equal to Z 0 .√2. For blocking the frequency range of the input signals, a blocking circuit consisting of stripline arrangements and/or discrete components is inserted between the common diode terminal on the side facing the intermediate frequency signal output and the blocking circuit is dimensioned such that the impedance value Z 0  occurs at the intermediate frequency signal output at the desired intermediate frequency. 
     Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure and in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of the invention; 
     FIG. 1B is a side view of the invention; 
     FIG. 2 illustrates a plan view of a modification of the invention; and 
     FIG. 3 illustrates a further modification of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A comprises a plan view of a first sample embodiment of a balance push-pull mixer according to the invention in stripline technology and FIG. 1B comprises a side view of the mixer. The stripline is formed on a substrate plate 69 and a grounded layer 70 of conductive material is formed on the second side of the substrate as illustrated in FIG. 1B and the device operates at a frequency of approximately 1GHz and at an intermediate frequency of approximately 60MHz. A differential transformer 61 is mounted on the plate 69 in an area above the broken line 71 in FIGS. 1A and 1B. The portion 68 below the broken line 71 converts and augments the differential transformer and produces a balanced push-pull mixer. 
     The differential transformer 61 has the job of conducting respective halves of an oscillator signal supplied at terminal 1 in opposite phase and an input signal supplied at terminal 2 in equi-phase to two terminals 4 and 5. With symmetrical wiring of the two terminals 4 and 5, the signals from terminals 1 and 2 are frequency independent and are decoupled. The equi-phase distribution occurs by means of the two striplines 11 and 12 which are approximately a quarter wave length long. The out of phase distribution is achieved with the assistance of a thin coaxial line 10 which is applied to a stripline 6 which is approximately a quarter wave length long and is short circuited to the ground plate 70 at one end by means of a through contact 8 of the outer conductor. The outer conductor of the coaxial line 10 terminates at the terminal 4 and the inner conductor of the coaxial line 10 terminates at the terminal 5 which for balancing circuits connects to stripline 7 with a through contact 9 to the ground plate 70. At every external location, the coaxial line 10 is equal potential with the stripline 6 but is only electrically and mechanically connected, for example, by means of soldering to the stripline 6 at its ends in an advantageous manner. Differences of thermal expansion between the coaxial line 10 and the stripline 6 do not result in mechanical stresses which are too large due to the curve form of the lines. The diameter of the coaxial line 10 is at the most equal to the width of the strip line 6 when a decoupling of approximately 30 decibels is to be achieved without correcting measures. Broader coaxial lines 10 reduce the intrinsic impedance of the line with the stripline 6 so that in order to obtain symmetry the line with stripline 7 must receive the same intrinsic impedance, in other words, must be broadened. It suffices for circuit balance however, to provide a parallel capacitance at terminal 5 and balance capacitor can be realized by means of a stripline length 16 at the terminal 5 which is made larger in area than a balance capacitor which is formed by stripline length 15 which extends from terminal 4. 
     A matching circuit is inserted between two semiconductor diodes 21 and 22 and the differential transformer 61 with the matching circuit assuring that the two input signal levels, in other words, both the oscillator level supplied to terminal 1 as well as the signal level supplied to terminal 2 are efficiently utilized. 
     The impedance of the two diodes 21 and 22 is transformed to the resistance Z 0  by means of parallel capacitors 19 and 20 as well as striplines 17 and 18 and parallel inductances at the terminals 4 and 5. The resistance Z 0  is the resistance which is to exist at the input terminals 1 and 2 as well as an intermediate frequency signal output terminal 3 and which equals for example 50Ω. The two parallel capacitors 19 and 20 are formed by enlarging the stripline widths at the terminal locations of the two diodes 21 and 22 as shown in the drawing. The parallel inductance is formed in that the two striplines 6 and 7 of the differential transformer 61 are shorter than a quarter wave length. This type of matching produces a low space requirement which is very advantageous particularly at the relatively low microwave frequencies such as, for instance, 1GHz. 
     The differential transformer 61 thus becomes a part of the matching circuit. Its lines formed from the two striplines 6 and 7 also represent the DC return path required for the two mixer diodes 21 and 22. 
     The further matching within the differential tramsformer 61 at the external signal terminal to the intrinsic impedance Z 0  of, for example, 50Ω results in a manner such that the two quarter wave lengths lines with striplines 11 or respectively 12 are executed with an intrinsic impedance of Z 0 .√2. 
     The connected load for the coaxial line 10 between the two terminals 4 and 5 is 2.Z 0  which would be in the example 100Ω. This load is transformed to the resistance Z 0  after the impedance terminal 1 being transformed through a line transformer which is a quarter wave length long and also exhibits an intrinsic impedance of Z 0 .√2. The quarter wave length long transformer consists of two line pieces since the length of the coaxial line 10 is expediently only as long as the stripline 6. A stripline 14 which is connected at a junction 13 to the inner conductor of the coaxial line 10 completes the overall line length. In other words, the sum of the coaxial line 10 and the stripline 14 equals one quarter wave length. The stripline 14 also has an intrinsic impedance of Z 0 .√2. Semi-rigid cable can be utilized as the coaxial line 10. 
     The arrrangement illustrated in FIGS. 1A and 1B has a particularly high frequency blocking between the mixer diodes 21 and 22 and the intermediate frequency signal output 3. Matching to the resistance Z 0  should also exist at intermediate frequency signal output 3 in the same manner as at the signal output terminals 1 and 2. The capacity value of a blocking capacitor 24 is selected such that together with the inductance of an integrated stripline coil 26 matching to the resistance Z 0  is produced at the intermediate frequency signal output 3 utilizing a desired intermediate frequency of, for example, 60 MHz. This simple type of matching although not being very broad band is sufficient for many applications. 
     It is possible to produce broad band circuits using four reactance elements. The high frequency blocking is made highly effective in that the series resonance which is already present which is obtained from the blocking capacitor 24 and its feed inductance including the inductance of a through contact 25 to ground is placed in the area of the signal or respectively oscillator frequency. This can be accomplished for example, in that the distance between the common diode terminal point 23 and the through contact location 25 is suitably selected. The stripline coil 26 is connected on one side to the common diode point 23 over a narrow stripline 62 and is connected at the other side to the intermediate frequency signal output 3 over a wire bridge 27. 
     An advantageous measure which leads to low mixing losses can be produced by creating a high frequency short circuit at the diode interconnection point 23 for the sum frequency of the two input signals. This is achieved by using a quarter wave length long stripline 28 for the sum frequency which is under no load operation at this point. At the intermediate frequency the capacitance of the line with the stripline 28 switches parallel to the blocking capacitance 24. 
     FIG. 2 is an enlarged plan view illustrating a second embodiment of a stripline balance push-pull mixer according to the invention. Somewhat different execution of the diode portion allows the oscillator voltage rectified by the mixer diodes 21 and 22 to be conducted out for check purposes or allows a bias voltage to be supplied. The high frequency blocking occurs over the two blocking capacitors 30 and 31 which relative to the input signal frequencies, for example, are mounted in series resonance with the inductances of the two through contacts 32 and 33. The short circuit for the sum frequency of the two input voltages supplied to the input terminals 1 and 2 results due to the two no load striplines 28 and 29. The DC component after the diodes 21 and 22 arrives through two chokes 34 and 35 at the two terminal points 36 and 37. These two terminal points 36 and 37 are respectively connected to ground through the two resistors 38 and 39 at through contacts 32 and 33 so as to terminate the DC path for the diodes 21 and 22 when no low resistance external wiring occurs. The intermediate frequency signal is supplied to the intermediate frequency signal output 3 over two separating capacitors 40 or 41 and an integrated stripline coil 26 as well as over a wire bridge 27. The transformation of the intermediate frequency internal resistance to the value Z 0  (for example 50Ω) results in a manner similar to that in the sample embodiment according to FIG. 1 with two blocking capacitors 30 and 31 as parallel capacitors and the coil 26 as a series inductance. The chokes 34 and 35 are high impedance for the intermediate frequency and practically have no significance. The inner connection location of the two separating capacitors 40 and 41 which forms one terminal of the stripline 26 is designated by reference 67. The separating capacitors 40 and 41 are respectively connected to the chokes 34 and 35 with the two striplines 65, 65a and respectively, 66 and 66a whereby terminal location 63 and 64 is provided for the diodes 21 and 22 in the path of the striplines 65, 65a or, respectively, 66 and 66a. The no-load striplines 28 and 29 and the blocking capacitors 30 and 31 extend from terminals 63 and 64, respectively. 
     The part of the differential transformer of the sample embodiment according to FIG. 2 lying above the broken line is provided with two semi-circular coaxial lines 10 and 50 which are mounted over striplines 6 and 7. The outer conductor ends of the coaxial lines 10 and 50 are electrically and mechanically connected for example by soldering to the ends of the two striplines 6 and 7, respectively. By means of special shaping and mounting of the coaxial lines 10 and 50 mechanical stresses are practically entirely suppressed. The coaxial line 50 on the stripline 7 was not utilized in the FIG. 1 embodiment but it produces the advantage of an even greater band width for the matching of the input signal terminal 1. The coaxial line 50 is produced to be approximately one quarter wave length using the stripline attached to its inner conductor at the junction 51. Therefore, the no-load at the end of the line with the stripline 52 is transformed at the center frequency into a short circuit at the input of the coaxial line 50 at the terminal 5. In other words, the inner conductor of the coaxial line 10 is connected to terminal 5. The inner conductors of the two coaxial lines 10 and 50 are electrically connected together. At frequencies which vary from the center frequency and at which the lines with the striplines 6 and 7 vary from the desired for example inductive reactive impedance, a reactive impedance which has a value can be set with the intrinsic impedance of the coaxial line 50 which resists the deviation that appears at the input of the coaxial line 50 at the terminal location 5. 
     The respective one quarter wave length long striplines 11 and 12 between the input signal 2 and the terminals 4 or 5 are partially formed so that they meander as illustrated in FIG. 2. The remaining portions of the example of FIG. 2 coincide with that illustrated in FIG. 1. 
     The precondition for the diode circuits illustrated in FIGS. 1 and 2 is that for high frequency blocking after the mixer diodes 21 and 22 is possible due to the concentrated capacitors 24, or respectively 30 and 31, utilizing series resonance. At operating frequencies above 3 GHz this will not occur without further measures. Therefore, capacitive stripline patches can be employed, for example. Quarter wave length line pieces which for instance produce a short circuit at the mixer diode 21 and 22 at the operating frequency can also be utilized. 
     FIG. 3 illustrates an example of such a circuit. Here a bent down quarter wave length low resistance stripline 42 accomplishes the blocking. A no-load stripline 28 additionally short circuits the sum frequency of the input signals. Since, however, the capacitance of the two lines 28 and 42 would usually probably not suffice to enable the required transformation of the intermediate frequency internal resistance to the value Z 0  for example 50Ω together with the series inductance of the stripline coil 26 an auxiliary capacitor in the form of a discrete capacitor which is through connected to ground at location 45 is provided in the embodiment illustrated in FIG. 3. For the purpose of decoupling from the blocking a high resistance approximately one quarter wavelength stripline 43 is additionally inserted. The remaining portion of the push-pull amplifier illustrated in FIG. 3 corresponds to the example illustrated in FIG. 1. 
     It is seen that the various components which have been described herein are clearly illustrated in FIGS. 1A 1B, 2 and 3 and the shape and arrangement of the various striplines capacitors, coils and other components are generally as illustrated. It is to be realized that minor modifications of the arrangements can be made and still would be within the scope of the present invention which is defined by the appended claims.