Abstract:
A transformer circuit disclosed performs wideband impedance transformation (matching) and can be used for voltage step-up and step-down applications. The circuit disclosed is comprised of two transmission line transformers and two reactive impedances that compensate the circuit for impedance versus frequency. The circuit disclosed performs a very wideband impedance ratio of 1:9. The circuit may have different turns ratios for T1 and T2 and different values for Z1 and Z2 to accommodate rational, fractional and integer values of transformation up to an impedance ratio of 16:1. The addition of a balun to the output provides a balanced output.

Description:
TECHNICAL FIELD 
     This invention relates to impedance transformers and more particularly to high impedance ratio transformer circuits operable at a higher wide range of frequencies on the order of 5 MHz to 1.2 GHz. 
     BACKGROUND ART 
     Conventional single core autotransformers (also known as transmission line transformers) are well known in the electronics industry for impedance or voltage step-up and step-down applications. Autotransformers are known to work fairly well up to 1 GHz with an impedance ratio of 4:1 or less. At higher impedance ratios of greater than 4:1, however, the high frequency response is limited by the length of the windings as they approach a quarter wavelength and thereby induce a resonance. More specifically, the higher ratio impedance transformer&#39;s performance is limited by the electrical length of the secondary windings. 
     Broadhead, Jr. U.S. Pat. No. 3,244,998 discloses an impedance transformer circuit using a single toroid core that is suitable for operating up to about 80 MHz. 
     Wandel U.S. Pat. No. 5,216,393 discloses an impedance transformer circuit in which a double aperture ferrite core is used as well as an autotransformer having the first and second windings twisted together and wrapped through the apertures of the double ferrite aperture core. This patent discloses performance within a band width from 47 MHz up through 860 MHz. 
     DISCLOSURE OF THE INVENTION 
     In accordance with the present invention there is provided a transformer circuit having two transmission line transformers with the first winding of the first transformer connected to a low impedance port and a second winding of the first transformer connected to the first winding of a second transformer via a first reactive impedance with the second winding of the second transformer connected to a high impedance port. The transformers shown are wound on a double aperture ferrite core and utilize a twisted pair configuration between the primary and secondary windings. A second reactive impedance is connected between the low impedance port and ground. The magnitude of the first and second impedances are balanced to give a response in which impedance ratios of about 9:1 to 16:1 and frequency responses from about 5 MHz to 1.2 GHz may be achieved. The addition of an unbalanced to a balanced transmission line transformer known as a balun and specifically a 1:1 balun connected to the output provides an unbalanced input to a balanced output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings which like parts bear similar reference numerals in which: 
     FIG. 1 is a basic electrical schematic circuit diagram of a high impedance ratio wideband transformer circuit embodying features of the present invention. 
     FIG. 2 is a more detailed electrical circuit diagram of the basic circuit shown in FIG. 1. 
     FIG. 3 is a sectional view of the first transformer shown in FIGS. 1 and 2. 
     FIG. 4 is a perspective view of the core shown in FIG. 3 showing the locations of the core dimensions for reference purposes. 
     FIG. 5 is a sectional view of the second transformer shown in FIGS. 1 and 2. 
     FIG. 6 is a Smith chart showing the composite contribution of the impedances in the circuit for the transformer circuit shown in FIGS. 1-5. 
     FIG. 7A is a representation of reactive impedance on the Smith chart showing the response of the circuit if the second reactive impedance is excessive. 
     FIG. 7B is a representation of reactive impedance on the Smith chart showing the response of the circuit if the second reactive impedance is insufficient. 
     FIG. 7C is a representation of reactive impedance on the Smith chart showing the response of the circuit if the first reactive impedance is excessive. 
     FIG. 7D is a representation of reactive impedance on the Smith chart showing the response of the circuit if the first reactive impedance is insufficient. 
     FIG. 7E is a representation of reactive impedance on the Smith chart showing the correct balance of the first and second reactive impedances which is the desired response for the circuit. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1 there is shown a basic high impedance ratio wideband transformer circuit embodying features of the present invention including a first transformer T1 having a first winding W1 connected to a low impedance port I and having a second winding W2. The first and second windings W1 and W2 are wound on a core B1 with the windings inductively coupled to one another. There is also provided a second transformer T2 having a first winding W3 and a second winding W4. The second winding W2 of the first transformer T1 is connected to the first winding W3 of the second transformer T2 via a first reactive impedance Z1. The first and second windings W3 and W4 are wound on a core B2 with the windings inductively coupled to one another. The second winding W4 of the second transformer T2 is connected to a high impedance port O. A second reactive impedance Z2 is connected between the low input port I and ground. The output O is an unbalanced output. A 1:1 balun may be connected to the output O of the transformer circuit to provide a balanced output at terminals OA and OB. 
     Referring now to FIG. 2 a more detailed diagram of the transformer circuit of the present invention is shown and is designated by letter D. Transformer circuit D is shown as having a pair of input terminals I and a pair of output terminals O. The first reactive impedance Z1 is connected in series between nodes 2 and 3, respectively, and the second reactive impedance Z2 is connected between node 1 and ground. A reactive impedance is an impedance whose reactance changes with frequency. Reactive impedances Z1 and Z2 provide compensation for impedance versus frequency in the circuit. First winding W1 has one side connected to ground and the other side connected to node 5 which is also connected to node 1. Second winding W2 has one side connected to node 5 and the other side connected to node 2. The second reactive impedance Z2 is connected between node 1 and ground. The second transformer T2 has a first winding W3 connected between ground and node 6. The second winding W4 of transformer T2 is connected between node 6 and node 4 which is connected to an output terminal O. 
     More specifically, the transformer T1 is connected as an autotransformer with first winding W1 being a primary winding and second winding W2 being a secondary winding. Similarly, transformer T2 is connected as an autotransformer with first winding W3 being a primary winding and winding W4 being a secondary winding. 
     Generally stated, transformers T1 and T2 are transmission line transformers. Referring now to FIG. 3 there is shown a transformer T1 found suitable to practice the present invention and is later referred to herein in more detail in Example 1. Transformer T1 has a miniature double aperture ferrite core having a rectangular core body B1 with rounded ends and having a pair of spaced apertures H1 and H2 formed in the core body. There is shown in FIG. 3 two and one half (2.5) turns of twisted wire designated wires W1 and W2 which are wound on core body B1 through apertures H1 and H2 and the wire is then separated. At the point of separation wire *W2 is designated wire W2. Wire *W1 is wound two and a half turns more and is designated wire W1. Wires *W1 and W2 are then connected to form a center tap which is the input to the transformer T1 and specifically is connected to the ungrounded side of winding W1 and is designated I1. Wires *W2 and W1 are designated O1 and G1, respectively, for the transformer. The transformer T1 is installed in the circuit such that I1 is node 5 and O1 is node 2 and G1 is grounded as shown in FIG. 2. The primary winding W1 of T1 then has 5 turns and the secondary winding W2 of T1 has seven and one half (7.5) turns. 
     A reactive inductance Z1 found suitable for practicing the present invention is a single strand of enamel coated magnet wire of which the gauge is not critical provided the wire is wound on a phenolic toroid core. With this structure the inductance of the inductor can be adjusted to close the loop as shown in FIG. 7E. 
     The locations of the dimensions on the core B1 are shown in FIG. 4 for reference purposes and are designated as width &#34;b&#34;, height &#34;h&#34;, diameter of apertures &#34;d&#34;, distance between centers of the apertures l&#39; and length of the core &#34;l&#34;. 
     Referring now to FIG. 5 transformer T2 is similar in construction to transformer T1 previously described. There is shown in FIG. 5 a twisted pair of separate wires designated *W3 and *W4. Two and a half turns of the twisted wire are wound on the double aperture core B2 through apertures H3 and H4. The wires are then separated. At the point of separation wire *W4 is designated wire W4. Wire *W3 is wound one and a half turns more and designated W3. Wires *W3 and W4 are then connected to form a center tap which is the input to the transformer designated I2. Wires *W4 and W3 are designated O2 and G2, respectively. The circuit is arranged so that with reference to FIG. 2 I2 is node 6 and O2 is node 4 and G2 is grounded. The primary winding W3 of T2 has four turns and the secondary winding W4 of T2 has six and one half (6.5) turns. 
     Referring now to FIG. 6 the Smith chart shown is a graph having real impedance plotted along a horizontal axis with increasing impedance designated +X being to the right and at one end being infinite impedance (open circuit) and decreasing impedance designated -X to the left and at the other end being minimum (short circuit). Inductive reactance is above the axis and capacitive reactance is below the axis. Constant resistance is indicated on the diagram as a dashed line intersecting the horizontal axis. The effect of changing reactive impedance Z1 is indicated in the direction of the arrow and less reactive impedance Z1 in the direction opposite the arrow. The effect of changing reactive impedance Z2 is indicated with more reactive impedance Z2 and less reactive impedance Z2. The desired response is a perfect circle with a minimum diameter central about the locus indicating the desired match between input and output impedances. 
     Referring now to FIGS. 7A through 7E, 7A shows a circuit response if reactive impedance Z2 is excessive, 7B shows the circuit response if reactive impedance Z2 is insufficient, 7C shows the circuit response if reactive impedance Z1 is excessive, 7D shows the circuit response if reactive impedance Z1 is insufficient and 7E shows the correct balance of reactive impedance Z1 and Z2 which is the desired response of the circuit. 
     In general, implementation of the circuit dictates that transformer T1 and transformer T2 (and balun for the balanced output) must be designed such that they perform satisfactorily independently. It is well known that the A L  (L P ) parameter for the core in conjunction with the numbers of turns on the low impedance side of a transformer dictates the low frequency performance. 
     Inherent in all transformers wound on a core is a component resistance Rp (parallel resistance associated with core loss and is material dependent) that must be considered in all transformer designs. Ideally the value of resistance Rp would be infinite for minimum loss, but in reality has some finite value and must be considered. Resistance Rp which is a function of the number of turns squared is critical for transformer T2 if the circuit is used in an impedance step up configuration but is also an important consideration with respect to transformer T1. For example, if the circuit is configured as a 9:1 step up transformer and the input and output impedances are 50 and 450 ohms, respectively, then the resistance Rp of the transformer T2 secondary must be at least 3 to 4 times the value of the output impedance at point 0 to minimize insertion loss. 
     Referring to FIG. 2, the resistance Rp of transformer T1 would be a resistor connected from node 2 to ground (in parallel with the secondary of transformer T1). Since resistance Rp is a function of the core material and the number of turns of wire on the secondary of the transformer, it is therefore not shown in the figure as an additional component. The resistance Rp of transformer T2 would be a resistor connected from node 4 to ground (in parallel with the secondary of transformer T2). As with transformer T1, resistance Rp is a function of the core material and the number of turns of wire on the secondary of the transformer and therefore is not shown in the figure as an additional component. 
     Resistance Rp is an important consideration in overall design of the circuit since the resistance Rp of transformer T2 is in parallel with the load that would be connected to point 0. This effectively lowers the impedance at node 4. In addition, the same phenomenon is present at node 2 of transformer T1 and has the same effect on transformer T1. With the ratio multiplying nature of the embodiment, the overall effect is that the resultant input impedance at I is lower than the expected 9:1. Referring to FIG. 6, the lower impedance shifts the response in the -X direction on the Smith chart. 
     In the circuit shown and described transformers T1 and T2 in general compensate for the lower equivalent impedance seen at I by decreasing the impedance step-up ratio of transformers T1 and T2. Decreasing the impedance step-up ratio of transformers T1 and T2 shifts the circuit response in the +X direction of FIG. 6. In summary, the change of the turns ratio is used to shift the circuit response to the right and correct for the lower equivalent impedance seen at I. 
     While two transformers connected in series and with the reactive impedances Z1 and Z2 as shown has been found to provide higher impedance ratios over a wider range of frequencies, it is understood that the same principle may be used employing only a single transformer. For example, only reactive impedances Z1 and Z2 with transformer T1 may be used with the output at node 3. This circuit would have lower impedance ratios but the Smith chart, balancing of the reactive impedances and adjustment of turns on the transformers would be the same. 
     EXAMPLE 1 
     The impedance and the characteristics of the transformers that provide a 1:9 impedance ratio and bandwidth of about 5 MHz to 1.2 GHz are summarized as follows: 
     
         ______________________________________Z1 = Inductor = 2.7 nanohenriesZ2 = Capacitor = 1.0 picofarad*W1 and *W2 = 2.5 turnsW1 = 5 turnsW2 = 7.5 turnsW3 = 4 turnsW4 = 6.5 turnsCore Rp for T1 and for T2 = 32 ohms/turns squaredCore dimensions      l = 2.70 mm      l&#39; = 1.35 mm      h = 1.30 mm      d = 0.50 mmCore material    82 material of KrystinelRp for T1 =    (W2).sup.2 × Core Rp    (7.5).sup.2 × (32) = 1800 ohmsRp for T2 =    (W4).sup.2 × (32)    (6.5).sup.2 × (32) = 1352 ohmsWhere I = 50 ohms O = 450 ohmsWhere I = 50 ohmsZ at node 2 = 150 ohmsZ at node 4 = 450 ohmsImpedance ratio 1:9______________________________________ 
    
     EXAMPLE 2 
     The impedances and the characteristics of the transformers that provide a 1:12 impedance ratio and a bandwidth of about 10 MHz to 1000 MHz are summarized as follows: 
     
         ______________________________________Z1 = Inductor = 2.7 nanohenriesZ2 = Capacitor = 1.8 picofarad*W1 and *W2 for T1 = 2.0 turns*W1 and *W2 for T2 = 2.5 turnsW1 = 4.5 turnsW2 = 6.5 turnsW3 = 3.0 turnsW4 = 5.5 turnsCore Rp for T1 = 32 ohms/turns squaredCore Rp for T2 = 80 ohms/turns squaredCore dimensions for T1         l = 2.70 mm         l&#39; = 1.35 mm         h = 1.30 mm         d = 0.50 mmCore dimensions for T2         l = 3.5 mm         l&#39; = 1.40 mm         h = 2.06 mm         d = .76 mmCore material for T1        82 material of KrystinelCore material for T2        81 material of KrystinelRp for T1 =    (W2).sup.2 × Core Rp    (6.5).sup.2 × (32) = 1352 ohmsRp for T2 =    (W4).sup.2 × Core Rp    (5.5) × (80) = 2420 ohms______________________________________ 
    
     EXAMPLE 3 
     The impedances and the characteristics of the transformers that provide a 1:16 impedance ratio and a bandwidth of about 10 MHz to 600 MHz are summarized as follows: 
     
         ______________________________________Z1 = Inductor = 4.8 nanohenriesZ2 = Capacitor = 2.2 picofarads*W1 and *W2 for T1 = 1.5 turns*W1 and *W2 for T2 = 2.5 turnsW1 = 4.5 turnsW2 = 6.0 turnsW3 = 2.5 turnsW4 = 5.0 turnsCore Rp for T1 = 32 ohms/turns squaredCore Rp for T2 = 80 ohms/turns squaredCore dimensions for T1         l = 2.70 mm         l&#39; = 1.35 mm         h = 1.30 mm         d = 0.50 mmCore dimensions for T2         l = 3.5 mm         l&#39; = 1.40 mm         h = 2.06 mm         d = 0.76 mmCore material for T1        82 material of KrystinelCore material for T2        81 material of KrystinelRp for T1 =    (W2).sup.2 × Core Rp    (6.0).sup.2 × (32) = 1080 ohmsRp for T2 =    (W4).sup.4 × Core Rp    (5.0).sup.2 × (80) = 2000 ohms______________________________________ 
    
     It is understood the above described circuits can be used for step-down applications by using the output 0 as an input. 
     Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.