Abstract:
A mixer with improved isolation between the RF and local oscillator frequency signals. The mixer has a substrate. A pair of cores are mounted to the substrate. Windings are wound on the cores to from a pair of baluns. A diode ring is connected between the baluns. The windings are formed from a first and second twisted pair of wires on one core and a third and fourth twisted pair of wires on another core. Some of the wires are wound on the cores but not twisted together. This winding configuration achieves superior balance between the baluns and good isolation.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to mixers in general and more particularly to a mixer that has high local oscillator to RF (L-R) isolation, low conversion loss and good VSWR at a low cost.  
           [0003]    2. Description of Related Art  
           [0004]    A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates inter-modulation products due to the non-linearity response.  
           [0005]    [0005]FIG. 1 shows a schematic drawing of a prior art mixer. Mixer  20  has a local oscillator input terminal LO for receiving a local oscillator signal, an RF input terminal RF for receiving an RF signal and an intermediate frequency output terminal IF for providing an intermediate frequency output signal.  
           [0006]    A diode ring QD 1  has four diodes D 1 , D 2 , D 3 , and D 4 . The diodes are connected in a ring configuration. The cathode of each diode is connected to the anode of the adjacent diode. Node  22  is connected between diodes D 1  and D 2 . Node  23  is connected between diodes D 2  and D 3 . Node  24  is connected between diodes D 3  and D 4 . Node  25  is connected between diodes D 1  and D 4 .  
           [0007]    Mixer  20  has a local oscillator port LO that is connected to local oscillator balun transformer T 1 . Transformer T 1  has windings T 1   a  and T 1   b  that are wound on a core C 1 . Windings T 1   a  and T 1   b  are magnetically coupled. Winding T 1   a  has one end connected to port LO and the other end connected to ground G. Winding T 1   b  has one end connected to node  22  and the other end connected to node  24 . The midpoint of winding T 1   b  is connected to ground. Transformer T 2  has windings T 2   a  and T 2   b  that are wound on a core C 2 . Windings T 2   a  and T 2   b  are magnetically coupled. Winding T 2   a  has one end connected to port RF and the other end connected to ground G. Winding T 2   b  has one end connected to node  23  and the other end connected to node  25 . The midpoint of winding T 2   b  is connected to port IF.  
           [0008]    The turns ratio of balun transformers T 1  and T 2  determine the VSWR at the local oscillator (LO) and RF terminals. The amplitude unbalance and phase unbalance of balun transformers T 1  and T 2  determine the L-R isolation. The insertion loss and matching of diode ring QD 1  to balun transformers T 1  and T 2  determines the conversion loss of the mixer. Due to parasitic capacitance between the primary and secondary windings, the amplitude unbalance of balun transformers T 1  and T 2  becomes worse at high frequency. The VSWR and insertion loss of balun transformers T 1  and T 2  becomes worse at high frequency.  
           [0009]    While various mixers have been used, they have suffered from not having high local oscillator to RF (L-R) isolation, low conversion loss and good VSWR at a low cost.  
           [0010]    A current unmet need exists for a mixer that has high L-R isolation, low conversion loss, good VSWR and that can be assembled at low cost.  
         SUMMARY  
         [0011]    It is a feature of the invention to provide a mixer for mixing an RF input signal with a local oscillator signal to provide at an output an intermediate frequency signal that is easily assembled at low cost.  
           [0012]    Another feature of the invention to provide a mixer that has improved isolation between the RF signal and the local oscillator signal.  
           [0013]    Another feature of the invention to provide a mixer that has improved VSWR and conversion loss.  
           [0014]    Another feature of the invention is to provide a mixer that mixes an RF input signal with a local oscillator signal to provide an intermediate frequency signal. The mixer includes a first balun transformer that has a first winding, a second winding and a third winding. The first winding is coupled to the second and third windings. The first winding is connected between a local oscillator terminal and ground. The second winding is connected between ground and a first node. The third winding is connected between ground and a second node. A second balun transformer has a fourth winding, a fifth winding and a sixth winding. The fourth winding is coupled to the fifth and sixth windings. The fourth winding is connected between an RF terminal and ground. The fifth winding is connected between an intermediate frequency terminal and a third node. The sixth winding is connected between the intermediate frequency terminal and a fourth node. The mixer has four diodes. One diode is connected between each of the first, second, third and fourth nodes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic drawing of a prior art double balanced mixer.  
         [0016]    [0016]FIG. 2 is a schematic drawing of a double balanced mixer in accordance with the present invention.  
         [0017]    [0017]FIG. 3 is a diagrammatic view of the layout of the windings of FIG. 2.  
         [0018]    [0018]FIG. 4 is a top view of a physical package layout of the mixer of FIG. 2.  
         [0019]    [0019]FIG. 5 is a left side cross-sectional view of FIG. 4.  
         [0020]    [0020]FIG. 5 is a right side cross-sectional view of FIG. 4.  
         [0021]    [0021]FIG. 7 is a graph of conversion loss versus RF frequency for the mixer of FIG. 2.  
         [0022]    [0022]FIG. 7A is a graph of conversion loss versus RF frequency for a prior art mixer.  
         [0023]    [0023]FIG. 8 is a graph of L-R isolation versus frequency for the mixer of FIG. 2.  
         [0024]    [0024]FIG. 8A is a graph of L-R isolation versus frequency for a prior art mixer.  
         [0025]    [0025]FIG. 9 is a graph of L- 1  isolation versus frequency for the mixer of FIG. 2.  
         [0026]    [0026]FIG. 9A is a graph of L- 1  isolation versus frequency for a prior art mixer.  
         [0027]    [0027]FIG. 10 is a graph of VSWR at the RF terminal for the mixer of FIG. 2.  
         [0028]    [0028]FIG. 10A is a graph of VSWR at the RF terminal for a prior art mixer.  
         [0029]    [0029]FIG. 11 is a graph of VSWR at the LO terminal for the mixer of FIG. 2.  
         [0030]    [0030]FIG. 11A is a graph of VSWR at the LO terminal for a prior art mixer.  
         [0031]    [0031]FIG. 12 is a graph of VSWR at the IF terminal for the mixer of FIG. 2.  
         [0032]    [0032]FIG. 12A is a graph of VSWR at the IF terminal for a prior art mixer. 
     
    
       [0033]    It is noted that the drawings of the invention are not to scale. In the drawings, like numbering represents like elements between the drawings.  
       DETAILED DESCRIPTION  
       [0034]    Referring to FIG. 2, a schematic drawing of double balanced mixer  30  in accordance with the present invention is shown. Mixer  30  has a local oscillator port LO that is connected to local oscillator balun transformer T 4 . Transformer T 4  has windings T 4   a,  T 4   b  and T 4   c  that are wound on a core C 1 . Winding T 4   a  is a primary winding Windings T 4   b  and T 4   c  are secondary windings. Windings T 4   a  and T 4   b  are magnetically coupled as are windings T 4   a  and T 4   c.  Winding T 4   a  has one end connected to port LO and the other end connected to ground G. Winding T 4   b  has one end connected to node  22  and the other end connected to ground G. Winding T 4   c  has one end connected to node  24  and the other end connected to ground G. A set of four diodes or diode quad QD 1  is arranged in a ring configuration. Diode quad QD 1  has four diodes D 1 , D 2 , D 3  and D 4 . The diodes are arranged such that the cathode of one diode is connected to the anode of another diode. Diode quad QD 1  has nodes  23 ,  23 ,  24  and  25 . Transformer T 5  has windings T 5   a,  T 5   b  and T 5   c  that are wound on a core C 2 . Winding T 5   a  is a primary winding. Windings T 5   b  and T 5   c  are secondary windings T 5   a  and T 5   b  are magnetically coupled as are windings T 5   a  and T 5   c.  Winding T 5   a  has one end connected to port RF and the other end connected to ground G. Winding T 5   b  has one end connected to node  23  and the other end connected to port IF. Winding T 5   c  has one end connected to node  25  and the other end connected to port IF.  
         [0035]    Turning to FIG. 3, a layout diagram of the windings of balun transformers T 4  and T 5  are shown. Wires  32 ,  34  and  36  are wound on core C 1  to form LO balun transformer T 4 . Wire  32  has 3.5 turns, wire  34  has 2.5 turns and wire  36  has 2.5 turns. Wire  32  has ends  32 A and  32 B. Wire  34  has ends  34 A and  34 B. Wire  36  has ends  36 A and  36 B. Wires  32  and  34  are twisted together to form a pair of twisted wires TW 1  for 2 turns. Wires  32 ,  34  and  36  are twisted together to form twisted wires TW 2  for 1 turn. Winding T 4   a  corresponds to wire  32 . Winding T 4   b  corresponds to wire  34 . Winding T 4   c  corresponds to wire  36 . This design of transformer T 4  provides a differential LO signal to diode ring QD 1  that has better amplitude unbalance.  
         [0036]    Wires  38 ,  40  and  42  are wound on core C 2  to form balun transformer T 5 . Wire  38  has 3.5 turns, wire  40  has 2.5 turns and wire  42  has 2.5 turns. Wire  38  has ends  38 A and  38 B. Wire  40  has ends  40 A and  40 B. Wire  42  has ends  42 A and  42 B. Wires  38  and  40  are twisted together to form twisted wires TW 5  for  1  turn. Wires  38 ,  40  and  42  are twisted together to form twisted wires TW 4  for  1  turn. Wires  38  and  42  are twisted together to form twisted wires TW 3  for  1  turn. Winding T 5   a  corresponds to wire  38 . Winding T 5   b  corresponds to wire  40 . Winding T 5   c  corresponds to wire  42 .  
         [0037]    Referring now to FIGS.  4 - 6 , mixer  30  is realized in a physical package. Mixer  30  has a carrier  50 . Carrier  50  has a cavity  52 , rim  54 , bottom surface  56  and support  58 . Metal leads  62  are attached to rim  54 . Leads  62  would be soldered to an external printed circuit board (not shown). Diode ring QD 1  is assembled as a chip on board on a ceramic substrate and is attached to support  58  by an epoxy (not shown). Wires  34 B,  36 A,  40 B and  42 A are connected to pads  66  on diode QD 1 .  
         [0038]    Ferrite cores C 1  and C 2  are attached to surface  56  with an epoxy  60 . Cores C 1  and C 2  each have apertures  70  through which the windings pass. Core C 1  has legs  72  and  74 . Core C 2  has legs  76  and  78 .  
         [0039]    Balun transformer T 4  has core C 1  with legs  72  and  74 . Wires  32 ,  34  and  36  are wound on leg  74 . Twisted wires TW 1  and TW 2  are wound on leg  74 . The wire ends  32 A,  34 A,  32 B and  36 B are welded or soldered to leads  62 . Balun transformer T 5  has core C 2  with legs  76  and  78 . Twisted wires TW 3 , TW 4  and TW 5  are wound on leg  74 . The wire ends  38 A,  38 B,  40 A and  42 B are welded or soldered to leads  62 .  
         [0040]    The present invention has several advantages. The mixer has very good L-R isolation over a broad frequency range. The improved isolation is due to configuring the windings of the transformers to achieve superior balance. The turns ratio of transformers T 4  and T 5  were selected to match the impedance presented by diode ring QD 1 . This results in a mixer with low conversion loss, low conversion loss flatness, excellent LO, RF and IF port matching and superior L-R isolation.  
         [0041]    A mixer  30  was fabricated and tested for electrical performance as was the prior art mixer  20 . The results are shown graphically in the following figures.  
         [0042]    [0042]FIG. 7 is a graph of conversion loss versus RF frequency for mixer  30 . The conversion loss is very flat. It is within +/−0.2 dB over the frequency range of 10 to 1000 MHz. This is a vast improvement over prior art mixers.  
         [0043]    [0043]FIG. 7A is a graph of conversion loss versus RF frequency for prior art mixer  20 . The conversion loss is +/−1.6 dB over the frequency range. Therefore, the conversion loss of the present mixer  30  is much better than that of prior art mixer  20 .  
         [0044]    [0044]FIG. 8 is a graph of L-R isolation versus frequency for the mixer of FIG. 2. The isolation starts at 80 dB at 10 MHz, drops to 75 dB at 60 MHz, stays at 75 dB until 300 MHz, drops to 60 dB at 410 MHz and then levels off at 48 dB. This is a 15 to 30 dB improvement over the L-R isolation of the prior art mixer seen in FIG. 8A.  
         [0045]    [0045]FIG. 8A is a graph of L-R isolation versus frequency for a prior art mixer.  
         [0046]    [0046]FIG. 9 is a graph of L-I isolation versus frequency for the mixer of FIG. 2.  
         [0047]    [0047]FIG. 9A is a graph of L-I isolation versus frequency for a prior art mixer. The L-I isolation is similar for the new mixer  30  and the prior art mixer.  
         [0048]    [0048]FIG. 10 is a graph of VSWR at the RF terminal for the mixer of FIG. 2.  
         [0049]    [0049]FIG. 10A is a graph of VSWR at the RF terminal for a prior art mixer. The RF port match of mixer  30  (1.1:1 to 1.5:1) is improved over the prior art mixer (1.4:1 to 2.5:1).  
         [0050]    [0050]FIG. 11 is a graph of VSWR at the LO terminal for the mixer of FIG. 2.  
         [0051]    [0051]FIG. 11A is a graph of VSWR at the LO terminal for a prior art mixer. The LO port match of mixer  30  (1.5:1 to 2.0:1) is improved over the prior art mixer (2.5:1 to 3.7:1) for 7 dBm LO power level. The performance is substantially improved at 4 and 10 dBm LO power levels.  
         [0052]    [0052]FIG. 12 is a graph of VSWR at the IF terminal for the mixer of FIG. 2.  
         [0053]    [0053]FIG. 12A is a graph of VSWR at the IF terminal for a prior art mixer. The IF port match of mixer  30  is about the same as the prior art mixer.  
         [0054]    High isolation mixer  30  has high L-R isolation, low conversion loss, good VSWR and can be assembled at low cost providing an improvement over previous mixers.  
         [0055]    While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.