AU2009100734A4 - HF mixer circuit arrangement - Google Patents

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR AN INNOVATION PATENT ORIGINAL Name of Applicant: Codan Limited Actual Inventor: Bruce Henry Johnson Address for Service: C/- MADDERNS, First Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia Invention title: HF MIXER CIRCUIT ARRANGEMENT The following statement is a full description of this invention, including the best method of performing it known to us.

FIELD OF THE INVENTION The present invention relates to radio frequency (RF) mixer circuit arrangements. In a particular form the present invention relates to doubly balanced mixer circuit arrangements. 5 BACKGROUND OF THE INVENTION A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF signal and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The 10 difference or IF frequency is a result of the non-linearity of the mixer. Along with the IF frequency, the mixer typically generates intermodulation products due to the non-linearity response. Third-order intermodulation products can be close in frequency to the fundamental IF frequencies and therefore are difficult to remove by filtering. Third-order intermodulation distortion is a measure of the third-order products generated by a second input signal arriving 15 at the input of a mixer along with the desired signal. Whilst mixer circuits are typically used in RF front ends to down convert a received RF signal, the symmetry of the system allows the same mixer circuit to be used for mixing an IF signal with a LO to provide at an output an RF signal with low intermodulation products. 20 One technique to measure the suppression capability of a mixer is the "third-order intercept" approach. The third-order intercept point (IP3) is a theoretical point on the RF input versus IF output curve where the desired output signal and third-order products become equal in amplitude as RF input is raised. A mixer is usually specified in terms of input IP3. A mixer with a higher input IP3 value for a given conversion loss will usually have better 25 performance. Output IP3 is the difference between input IP3 and conversion loss, and is not generally used for mixer specification. Conversion loss is a measure of the efficiency of the mixer in providing frequency translation between the input RF signal and the output IF signal. Conversion loss of a mixer is equal to 30 the ratio of the IF output to the RF input level. Mixers are typically designed with one of three topologies: single ended, balanced, and double balanced. The double balanced mixers are capable of isolating both the RF signal and the local oscillator LO voltages from the output and thus allow overlap of the RF and IF 35 frequency bandwidths. Several prior art mixer circuits are known. One mixer design uses a Schottky diode quad or ring circuit that uses four diodes with all of the diodes pointed in the same direction. Another mixer circuit is called a star circuit, which uses two diodes pointing 2 toward the central node and two diodes pointing away from the central node. Schottky diode mixers approaching +30 dBm IP3 are difficult to tune and are expensive. Diode mixers also require large LO signal levels to obtain a high IP3 which is not practical in many systems. 5 Another type of mixer uses field effect transistors (FET) as the mixing element instead of a Schottky diode in the circuits described above. Mixers fabricated using FETs in this way can achieve a higher value of 1P3. Unfortunately, mixers using FETs in this way can have several other disadvantages such as higher conversion losses and a higher noise figure. 10 Another type of mixer, known as the H-mode mixer, uses FETs as the mixing element in a bridge arrangement with the FETs alternately switching to a ground node. The FETs used in this arrangement have isolated substrate allowing the drain terminal voltage to vary both positive and negative relative to the source terminal. Mixers fabricated using this arrangement can achieve a higher value of 1P3 with lower conversion losses compared to that described 15 above. Unfortunately, mixers using this arrangement are more complex, requiring an additional transformer, or windings on a transformer, and on each LO cycle two of the transformer windings are unterminated resulting in reduced mixer efficiency, particularly at higher frequencies. 20 Today's communication receivers operate under multiple frequency carrier environments. Some of these multiple frequency carriers are wanted and some are unwanted. When these frequency carriers are present in a receiver or transmitter simultaneously, they generate inter modulation products that are unwanted. The level of the intermodulation products is dictated by the linearity of the mixer in the receiver. The degree of non-linearity of the mixer is 25 measured by IP3. High IP3 mixers are desired for receivers and transmitters operating in multiple carrier environments to minimize third-order inter-modulation products. There is thus a need to provide a mixer circuit arrangement that has a high third-order intercept point and minimizes one or more of the problems discussed or at least provide an 30 alternative possibly including a comparatively simpler configuration than prior art mixer circuits. SUMMARY OF THE INVENTION 35 According to a first aspect of the present invention, there is provided a doubly balanced mixer circuit arrangement including: 3 a) a Local Oscillator (LO) terminal, a Radio Frequency (RF) terminal and an Intermediate Frequency (IF) terminal; b) an RF transformer including at least three windings, each winding having a first and a second terminal, wherein the first winding is for connection across the RF 5 terminal, wherein the first and second terminals of the first winding are connected to the RF terminal; c) an IF transformer including at least three windings, each winding having a first and a second terminal, wherein the first and second terminals of the first winding are connected to the IF terminal; and 10 d) at least two switches which are switched under the control of a LO signal at the rate of the LO signal, wherein the second and third windings of the RF transformer and the second and third windings of the IF transformer are connected at four junctions to form a continuous loop including a non electrically connected cross over portion, such that 15 the first terminal of the second winding of the RF transformer is connected to the first terminal of the second winding of the IF transformer at the first junction; the second terminal of the second winding of the RF transformer is connected to the first terminal of the third winding of the IF transformer at the second junction; the first terminal of the third winding of the RF transformer is connected to the 20 second terminal of the second winding of the IF transformer at the third junction; and the second terminal of the third winding of the RF transformer is connected to the second terminal of the third winding of the IF transformer at the fourth junction; and the at least two switches are connected to the loop such that switching of the at least two switches alternatively connects the first and fourth junctions, and the second and 25 third junctions. The RF transformer and the IF transformer may be trifilar wound on balun transformer cores. The at least two switches may comprise four switches, with one side of each of the four 30 switches connected to a common ground, and pairs of switches alternatively connect the first and fourth junctions, and the second and third junctions, to the common ground. Each switch may be a FET, with one terminal of each FET being connected to a common RF ground node. A detailed description of one or more embodiments of the invention is provided below along 35 with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any embodiment. On the contrary, the scope of 4 the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. 5 The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured. 10 BRIEF DESCRIPTION OF THE DRAWINGS Illustrative embodiments of the present invention will be discussed with reference to the accompanying drawings wherein: FIGURE 1 is a circuit diagram of a doubly balanced mixer arrangement according to an 15 embodiment of the present invention; FIGURE 2 is an alternative circuit diagram of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 3 is an alternative circuit diagram of a doubly balanced mixer arrangement according to an embodiment of the present invention; 20 FIGURE 4 is an alternative circuit diagram of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 5 is a circuit layout diagram of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 6 is an isolation measurement of a doubly balanced mixer arrangement according to 25 an embodiment of the present invention; FIGURE 7 is an IP3 measurement of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 8 is a graph of IP3 measurements as a function of frequency of a doubly balanced mixer arrangement according to an embodiment of the present invention; 30 FIGURE 9 is a graph of conversion loss measurements as a function of frequency of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 10 is a graph of LO feedthrough measurements as a function of frequency of a doubly balanced mixer arrangement according to an embodiment of the present invention; FIGURE 1 1 is a graph of RF isolation measurements as a function of frequency of a doubly 35 balanced mixer arrangement according to an embodiment of the present invention; and FIGURE 12 is a graph of 45MHz isolation measurements as a function of frequency of a doubly balanced mixer arrangement according to an embodiment of the present invention. 5 In the following description, like reference characters designate like or corresponding parts. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 5 Referring now to Figure 1, there is shown a circuit diagram 100 of a doubly balanced mixer arrangement according to an embodiment of the present invention. The circuit includes a Local Oscillator (LO) terminal, a Radio Frequency (RF) terminal and an Intermediate Frequency (IF) terminal and two transformers TI and T2, each having three windings although other embodiments may have more, wherein in this embodiment, each winding 10 having a first and a second terminal. The first winding (primary) of the TI transformer has a first terminal 110 and second terminal 112 for connection across the RF terminal, and thus this transformer will also be referred to as the RF transformer. The second winding of the TI or RF transformer has a first terminal 120 and a second terminal 122 and the third winding of the TI or RF transformer has a first terminal 130 and a second terminal 132. Similarly the 15 first (primary) winding of the T2 transformer has a first terminal 150 and second terminal 152, for connection across the IF terminal, and thus this transformer will also be referred to as the IF transformer. The second winding of the T2 or IF transformer has a first terminal 160 and a second terminal 162, and the third winding of the T2 or IF transformer has a first terminal 170 and a second terminal 172. 20 The second and third windings (secondaries) of the RF transformer and the second and third windings (secondaries) of the IF transformer are connected at four junctions, 142, 144, 146, 148 to form a continuous loop including a non electrically connected cross over portion 180, which in Figure 1 appears as a somewhat distorted figure "8". The first terminal 120 of the 25 second winding of the RF transformer is connected to the first terminal 160 of the second winding of the IF transformer at the first junction 142. The second terminal 122 of the second winding of the RF transformer is connected to the first terminal 170 of the third winding of the IF transformer at the second junction 144. The first terminal of the third winding 130 of the RF transformer is connected to the second terminal 162 of the second winding of the IF 30 transformer at the third junction 146. The second terminal 132 of the third winding of the RF transformer is connected to the second terminal 172 of the third winding of the IF transformer at the fourth junction 148. It is to be understood that the positions of the junctions are indicative, and the terminals may be joined by intervening electrical connectors. Preferably the number of turns on the secondary windings of the transformers TI and T2 can be equal so 35 that the transformer can be easily wound using trifilar winding wire, whereas the number of turns on the RF and IF signal port windings (the "primaries") can be changed to suit different impedances if necessary. 6 The mixer circuit also includes two switches SI and S2 which are switched under the control of a LO signal connected across the LO terminals. Switching of the switches occurs at the rate of the LO signal, and the switches are connected to the loop such that switching of the two 5 switches alternatively connects the first and fourth junctions (142, 148), and the second and third junctions (144 and 146). That is one pair is connected together whilst the other pair is unconnected. In this embodiment the switches are two single pole single throw switches. The switches S1 and S2 are alternately switched ON and OFF on each half-cycle of the LO signal, which in this embodiment is a square wave signal. The LO frequency is preferably selected 10 as the sum of the nominal RF and desired IF frequencies. It is preferable that the switches are capable of switching quickly, at least the rate of the LO signal, and of passing current in a linear manner in both directions when in the ON condition, and of not passing current in either direction for the voltages of interest when in the OFF condition. It is preferable that the ON, OFF and switching characteristics of each switch is identical for best balance of the 15 doubly balanced mixer. The balance of the doubly balanced mixer could be further enhanced using balun transformers on the RF and IF signal ports. In other embodiments the junctions may be connected together by the switch which is also connected to a ground node. Operation of the circuit is described with current flowing from the dot end (the "top") of the 20 secondary windings of TI. If switch SI is closed and switch S2 is open, current flowing from the top of the windings of TI flows into the top of the windings of T2. If switch SI is open and switch S2 is closed (the arrangement shown in Figure 1), current flowing from the top of the windings of TI flows into the bottom of the windings of T2, resulting in phase inversion. This operation and the transformer structure results in the required double balanced mixer 25 operation. Figure 2 illustrates an alternative circuit diagram 200 of a doubly balanced mixer circuit arrangement according to an embodiment of the present invention in which the two switches have been replaced by 4 switches SI, S2, S3, and S4 which are connected at junctions 144, 30 142, 146 and 148. The switches are operated in pairs, with SI and S3 operated together, and S2 and S4 operated together under the control of the LO signal. This allows one side of the switches to be connected to a common RF ground node, which in some circumstances makes the implementation of the switching easier. Alternatively, the four single pole single throw switches could be replaced by two single pole double throw switches. In both cases it is 35 preferable that the ON, OFF and switching characteristics of each switch is identical for best balance of the double balanced mixer. 7 Figure 3 illustrates an alternative circuit diagram 300 of a doubly balanced mixer circuit arrangement according to an embodiment of the present invention in which the four switches in Figure 2 have been replaced by 4 FETs, Q1, Q2, Q3 and Q4. The FETs are operated in pairs QI / Q3, and Q2 / Q4. The FETs are in the arrangement where one terminal of each FET 5 is connected to the common RF ground node. This assists with ensuring the switching behaviour of the FETs is as close as possible to identical. Preferably the FETs are of a type that does not pass current in either direction for drain-source voltages of either polarity when in the OFF state. Again, the ON, OFF and switching characteristics of each FET should be identical for best balance of the double balanced mixer. Suitable FET switches are available 10 in integrated circuit form, with four matched FETs in one device. These switches require the common RF ground node to be biased to a DC level to provide correct operation with both positive and negative drain-source voltages. Figure 4 is an alternative circuit diagram 400 of a doubly balanced mixer circuit arrangement 15 according to an embodiment of the present invention. The schematic uses the same arrangement as Figure 3, but with the RF signal port replaced by a transformer coupled, grounded base transistor amplifier. DC bias supplies are not shown for clarity. The turns ratios on transformer TI are selected to provide the required gain and impedance to the RF signal port given a known termination impedance of the IF signal port. 20 The RF mixer circuit arrangement is suited to use in the HF band, with an input RF range from 1MHz to 30MHz. However it is to be understood that the mixer circuit arrangement is not limited to this frequency range and the circuit would be suitable for use at both lower (eg MF band) and higher frequencies (VHF, UHF and microwave bands). Further it is to be 25 understood that choice of specific components to use would be dictated by the operating frequency range of the circuit. Figure 5 shows a representative circuit layout 500 of the circuit diagram shown in Figure 3 utilising a FST3125 Quad Bus Switch which provides 4 matched FETs on a single IC chip. 30 This IC provides high speed switching (- 2ns) and low on resistance of approximately 70 at 2V DC bias. This chip requires a 2V DC bias for voltage across each switch so as to permit a swing of approximately ±2V peak to peak. This IC is manufactured by Fairchild Semiconductor as FST3125. Other suitable IC chips in this family include the Philips CBT3125, TI SN74CBT3125, ON Semiconductor 74FST3125, and Pericom P15C3125. The 35 layout shown in Figure 5 is clean and simple without crossovers of signals or LO, and in which LO signals are perpendicular to signal paths. 8 When operating in the HF band, the RF transformer should have adequate impedance over the range of RF operation (eg I to 30MHz). Similarly the IF transformer should have adequate impedance over a range from (LOmin - RFmax) to (LOmax + RFmax). This corresponds to approximately 15MHz to 105 MHz for 45MHz IF, and should ideally include harmonics. 5 Further the load resistance should be nominal over the same range as the IF transformer. In some embodiments this may require a diplexer to be used. In this example, if the RF transformer is wound on p = 3000 core, then the IF transformer needs only be on P= 300 core. This arrangement gives low loss and improved frequency response, although some care must be taken to reduce any transformer generated intermodulation distortion (IMD). 10 The minimum total conversion loss of the I-IF mixer circuit arrangement can be estimated for the circuit in Figure 5. An ideal mixer with square wave drive has a loss of 20log(0.5 * 4/n)= -3.92 dB. The two 7 i switch resistance at 50 0 have a loss of -l.14 dB. The loss due to transformers is unknown and will contribute to the conversion loss. The minimum total 15 conversion loss is thus -5.06 dB plus the loss due to the transformers, which is the same as the H mode mixer. A doubly balanced mixer arrangement using the layout in Figure 5 was constructed and tested in the HF frequency range over a RF input from 1 to 30MHz and using an IF of 45MHz. 20 Figure 6 is an isolation measurement 600 using a 13MHz OdBm single tone input signal. 610. The desired IF signal 620 occurs at approximately 45MHz. LO feed through signals 630 are also visible. The horizontal scale ranges spans 150MHz centred on 75MHz (15MHz divisions) and the vertical scale ranges from 0 down to -1OOdBm (IOdBm divisions). 25 Figure 7 is a typical IP3 measurement 700 using the layout in Figure 5. The third-order intercept point is found by calculating the required level of a 2-tone RF input signal 710 and 720 which would be necessary to produce equal in amplitude third-order products 730. For this measurement, the IP3 point was obtained by increasing the 2-tone 13MHz RF input signal to +4dBm, and the IP3 value is calculated by taking the amplitude from the peak (-1.77 30 dBm) to the third order product (-88.7dBm) indicated by arrow 740, and dividing this value by 2 and adding 4, giving an IP3 point of +47.5 dBm, although in this case a realistic limit of measurement is assumed to be +45dBm based on the accuracy of measurement of the intermodulation product. 35 Representative measurements were taken using the circuit built according to Figure 5 over the 1MHz to 30MHz range. Figure 8 is a graph 800 of IP3 measurements 810 as a function of 9 frequency. The IP3 point is greater than +40dBm for RF frequencies from I MHz to 30MHz. 820, and substantially at the limit of measurements (+45dBm) from 2 to 13MHz. Figure 9 is a graph 900 of conversion loss measurements 910 as a function of frequency. The 5 conversion loss is typically around -5.6 dB and always better than -6dB 920. Figure 10 is a graph 1000 of LO feedthrough measurements as a function of frequency. The LO to RF feedthrough 1010 and the LO to IF feedthrough 1020 are both better than -40dBm 1030. Figure 1I is a graph 1100 of RF isolation measurements 1110 as a function of frequency. The RF isolation is always better than -35dB 1120. Figure 12 is a graph 1200 of 45MHz isolation 10 measurements 1210 as a function of frequency illustrated that the 45MHz isolation is always better than -35 dB 1220. Both the RF and 45MHz isolation were obtained by providing a OdBm signal at the RF port. The mixer circuit arrangement described thus has the advantages that the circuit minimises 15 intermodulation products and has a high third-order intercept point (IP3). Further when used in the HF band the circuit has an intermediate frequency (IF) that does not include frequencies below a few hundred kilohertz (kHz). The mixer circuit arrangement described thus has all of the advantages of the prior art H 20 mode arrangement with the added advantages that it is simpler and cheaper to implement. In principle the circuit arrangement also provides better frequency response and lower loss, although the ultimate performance depends on the switches used (this applies to the H-mode arrangement as well). A further advantage is that unlike the H-mode, there are no redundant windings with all windings being active during each half cycle of the LO thereby providing a 25 more efficient circuit arrangement with reduced losses compared to the H mode mixer. Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the 30 exclusion of any other integer or group of integers. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge. 35 It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred 10 embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the 5 following claims. 11

Claims (5)

1. A doubly balanced mixer circuit arrangement including: e) a Local Oscillator (LO) terminal, a Radio Frequency (RF) terminal and an Intermediate Frequency (IF) terminal; 5 f) an RF transformer including at least three windings, each winding having a first and a second terminal, wherein the first winding is for connection across the RF terminal, wherein the first and second terminals of the first winding are connected to the RF terminal; g) an IF transformer including at least three windings, each winding having a first 10 and a second terminal, wherein the first and second terminals of the first winding are connected to the IF terminal; and h) at least two switches which are switched under the control of a LO signal s at the rate of the LO signal, wherein the second and third windings of the RF transformer and the second and third 15 windings of the IF transformer are connected at four junctions to form a continuous loop including a non electrically connected cross over portion, such that the first terminal of the second winding of the RF transformer is connected to the first terminal of the second winding of the IF transformer at the first junction; the second terminal of the second winding of the RF transformer is connected to the 20 first terminal of the third winding of the IF transformer at the second junction; the first terminal of the third winding of the RF transformer is connected to the second terminal of the second winding of the IF transformer at the third junction; and the second terminal of the third winding of the RF transformer is connected to the second terminal of the third winding of the IF transformer at the fourth junction; 25 and the at least two switches are connected to the loop such that switching of the at least two switches alternatively connects the first and fourth junctions, and the second and third junctions.

2. A doubly balanced mixer circuit arrangement according to claim 1, wherein RF transformer and the IF transformer are trifilar wound on balun transformer cores. 30

3. A doubly balanced mixer circuit arrangement according to claim I or 2, wherein the at least two switches comprises four switches, with one side of each of the four switches connected to a common ground, and pairs of switches alternatively connect the first and fourth junctions, and the second and third junctions, to the common ground. 12

4. A doubly balanced mixer circuit arrangement according to claim 3 wherein each switch is a FET, and one terminal of each FET is connected to a common RF ground node.

5. A doubly balanced mixer circuit arrangement substantially as herein described with reference to any one of the embodiments of the invention described with reference to the 5 accompanying drawings. 13