Cross Reference to Related Applications

[0001] This application claims the priority of US Patent Application Serial No. 15/637,286 filed 29 Jun 2017 and entitled "Mixer with Series Connected Active

Devices", the disclosure of which is hereby incorporated herein by reference.

Statement Regarding Federally Sponsored Research or Development

[ 0002 ] This invention was made with Government support under Contract No.

N00014- 11-C-0127 awarded by ONR (Office of Navy Research) STAR. The Government has certain rights in the invention.

Background

[0003] In communication systems there is often a need to shift a signal from one frequency band to a different frequency band, with very little or preferably no distortion occurring. This frequency shifting is done by a device or circuit called a mixer. The mixer typically generates the signal in a new frequency band (often called the Intermediate Frequency signal or IF signal when the mixer is used in a radio receiver) from the original signal (often called the Radio Frequency signal or RF signal when the mixer is used in a radio receiver) using an unmodulated signal (often called the Local Oscillator frequency or LO when the mixer is used in a radio receiver).

[0004] Mixers come in several forms , but they typically rely on well known non- linearities to achieve the desired mixing. Unfortunately, those non-linearities can also give rise to undesirable intermodulation distortion. A lot of the technical literature with respect to mixers discuss about how to take advantage of the non-linearities to achieve the desired mixing while reducing the unwanted intermodulation distortion. [0005] In a typical mixer, a down-convened Intermediate Frequency ("IF") signal is derived which is a nonlinear harmonic product of Radio Frequency ("RF") and Local Oscillator ("LO") signals. A typical mixer uses a device's non-linearity to achieve the mixing. Because of this, achieving highly linear mixer is challenging, especially for a large third order input intercept point ("IIΡ3").

[0006] IIΡ3 is a figure of merit used in describing the linearity of many circuits, including mixers. IIΡ3 is determined by finding the intersection of a plot of the output power versus the input power for a first order term of the Taylor series expansion of the circuit transfer function and a plot of the third order term of the series expansion for the circuit transfer function. The third order distortion producrs are undesirable, so the bigger the ratio, the better (less distorted) the circuit being analyzed. So n higher IIΡ3 is better than a lower IIΡ3.

[0007] Mixers can also have a conversion loss which is defined as the ratio of the power of the signal in the new frequency band (typically the signal at the IF frequency) to the power of the original signal in the original frequency band (typically the signal at the RF frequency). If the mixer hae a built in amplifier, then it can exhibit a conversion gain.

[0008] Mixers are often called resistive mixers when transistors are operated under turned-off DC bias condition to achieve the desired frequency mixing. When one or more Field Effect Transistors (FETs) are utilized in a resistive mixer, it is the non-linearities in the channel conductance of the one or more FETs which are utilized to achieve frequency mixing.

[0009] To reduce and/or minimize conversion loss and maximize 1IP3 (to lower third order distortion products), the mixer must be very linear during the intermodulation of RF frequencies and sufficiently non-linear during the intermodulation of RF and LO frequencies to achieve the desired mixing of the RF and LO frequencies. [0010] A typical balanced (or double balanced) resistive microwave mixer uses one device at one stage and usually contains matching components (such as micro-strip line or capacitors) to try to obtain the best conversion loss with reasonable linearity.

[0011] Typically, when a linear resistive mixer is designed, each unit cell has one transistor and its size and gate bias are adjusted to try to obtain the best linearity performance along with matching condition. However, since there is trade-off relationship between IIΡ3 and conversion loss in this approach, better IIΡ3 cannot typically be achieved without sacrificing conversion loss.

[0012] In some cases, selective drain bias can be applied to utilize device output conductance as described in JA. Garcia, J.C. Pedro, M.L. De la Fuente, N.B. Carvalho. A. Mediavilla, and Tazon, A.. "Resistive FET mixer conversion loss and IMD optimization by selective drain bias." Microwave Symposium Digest, IEEE MTT-S International Volume: 2 (1999) and in J. Kim and Y. Kwon, "Intermodulation analysis of dual-gate FET mixers," Microwave Theory and Techniques, IEEE Transactions on Volume: 50, Issue: 6 (2007). However, their resulting IIΡ3 may still be limited and a more complex layout may be required.

Summary

[0013] An aspect of the technology relates to a unit cell, wherein a mixer may comprises a plurality of such unit sells, each unit cells including a plurality of active devices arranged in series, wherein each of said plurality of active devices have a different size (gate width, for example). Another aspect of the technology relates to a resistive mixer, comprising a plurality of active devices connected in series with one another to form a unit cell, the plurality of active devices connected in series with one another to form a unit cell having different conductances and capacitances.

[0014] The unit cell as described above, wherein the plurality of active devices have different size gate widths. [0015] The unit cell as described above, wherein the size ratio of the active devices is 1:2 (IF side: RF side).

[0016] The unit cell as described above, wherein the size ratio of the active devices is between 1:1 and 1:4 (IF side: RF side).

[ 0017 ] The unit cell as described above, wherein the plurality of active devices are arranged in series to suppress a non-linear third harmonic of an RF signal. The unit cell as just described, wherein the series arrangement comprises a first active device drain connected to a second active device source.

[0018] The unit cell as described above, wherein a first active device source is connected to an first frequency source and a second active device drain is connected to a second frequency output and gates of the first and second active device are connected in common to a third frequency input. The unit cell as just described, wherein the gates of the active devices are connected to a local oscillator and have a common DC gate bias voltage.

[0019] The unit cell as just described, wherein half of the gates of the plurality of active devices receive a 180 degrees out of phase local oscillator signal.

[0020] The unit cell as described above, wherein one or more of the plurality of active devices comprises one or more from the group consisting of HEMTs, CMOSs, FETs and BJTs.

[0021] Tn another aspect the present invention involves a mixer, comprising: a plurality of unit cells each comprising a plurality of active devices of different sizes arranged in series to reduce a generation of a non-linear third harmonic by a mixer in which the plurality of unit cells are arranged. The unit cell as just described, wherein a size ratio of the active devices is 1 :2 (IF side: RF side). The unit cell as just described, wherein a size ratio of the active devices is 1:4 (IF aide: RF side). The unit cell as just described, wherein the plurality of active devices are arranged in series in the mixer. The unit cell as just described, wherein the series arrangement comprises a first active device drain connected to a second active device source. The unit cell as just described, wherein a first active device source is connected 10 an intermediate frequency and a second device drain is connected to a radio frequency. The unit eel as just described, wherein gates of the plurality of active devices are connected to a local oscillator with the same DC gate bias voltage. The unit cell as just described, wherein half of the gates of the plurality of active devices receive a 180 degrees out of phase local oscillator signal. The unit cell as just described, wherein a single balanced resistive mixer includes two unit cells, a double balanced resistive mixer includes four unit cells and a double-doubly balanced mixer includes eight unit cells.

[0022] In another aspect the present invention relates to a method of reducing a third harmonic generated by a mixer, the mixer comprising a plurality of unit cells, the unit cells being connected in a ring with an RF input and LO input and having a IF output, the method comprising: providing at least a pair of active devices in each of said unit cells, each of said active devices having a pair of current carrying electrodes and at least one control electrode; connecting the at least a pair of active device in each of said unit cells so their the current carrying electrodes are coupled in series with each other and also connected to current carrying terminals of an associated united cell; connecting the at least a pair of active device in each of said unit cells so their the control electrodes are coupled in parallel with each other and also connected to a control terminal of the associated united cell; and adjusting sines of at least one semiconductor element in each of the pair of active devices in each of said unit cells relative to other other in each of said unit cells to reduce said third harmonic.

[0023] In another aspect the present invention relates to a method tor producing a unit cell in a mixer, the method comprising: providing a plurality of active devices of different sizes connected in series. Brief description of the drawings

[ 0024] FIG. 1 is a circuit diagram of a typical prior art mixer. [ 0025] FLO. la is a simplified representation of the mixer in FIG 1 [ 0026] FIG lb is an equivalent circuit for FIG la.

[ 0027 ] FIG.2a is a circuit diagram of active devices connected in series in a unit cell while FIG.2b is schematic of an equivalent circuit for that of FIG 2a.

[ 0028] FIG.3 is a circuit diagram of a mixer with series transistors.

[0029] FIG.4 is a diagram showing a relationship between IIΡ3 and frequency.

[ 0030] FIG.5 is a diagram showing the relationship between conversion loss and frequency.

[ 0031 ] FIG.6 is an illustration of a FET device (such as a HEMT) showing how gate width (W) and length (L) are defined.

[0032 ] FIG.7 depicts a mixer made using the circuit of FIG. 1.

[0033] FIG.8 depicts a mixer made using the circuit of FIG.3.

[ 0034 ] FIG.9u is similar to FIG.2a but shows a circuit diagram of two bipolar transistors (BJTs) connected in series in a unit cell.

[0035] FIG.9b presents an equivalent circuit to that of FIG 9a. Detailed Description

[0036] The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0037] In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art thai the present invention may be practiced without necessarily being limited to these specific details. In other instances, well- known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.

[0038] The reader's attention is directed to all papers and documents which arc filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0039] Furthermore, any element in a claim that does not explicitly state "means for" performing a specified function, or "step for" performing a specific function, is not to be interpreted as a "means" or "step" clause as specified in 35 U.S.C. Section 112(f). In particular, the use of "step of' or "act of in the claims herein is not intended to invoke the provisions of 35 U.S.C. 112(f).

[0040] The present technology relates to an apparatus, system, and method for making the same, related to a mixer with series-connected active devices. Each apparatus, system and method may include one or more embodiments in any combination. The features of any embodiment may be used in combination with any other embodiment.

[004] ] Mixer linearization involves minimizing certain unwanted harmonics while maximizing other wanted harmonics. Under 2-tone RF signal (Frfi and Fin) injection, a mixer's IF signal frequency is defined by FLO-FRP, (where FRP is Frf ι or FIR) and an IF third harmonic frequency (IF3) defined by (2*F,n-F,n)-Fijoor (2*F,n-F,n)-FLo,

Accordingly, from this relationship, it is understood that the IF third harmonic is the down-converted RF third harmonic by FLO. The conversion loss is defined by PRF- PJF, where PIP denotes power at IF frequency and PRF denotes power at RF frequency. IIΡ3 is defined as PRF + 0_5*(PIP-PIF3), wherein PIP. denotes power of IF3 signal frequency defined by (2*F,f i -Ρ,η)-Ρυο or (2*F,f_-Frfi)-FLo.

[ 0042 ] Some typical microwave resistive mixers (including single-balanced and double balanced types) may utilize single transistors, while other typical resistive mixers may use dual-gate transistors as their basic units. In either of these designs, conversion loss or IIP3 may be optimized, typically by adjusting transistor sizes, bias conditions along with microwave matching using, for example, micro-strip lines and lumped components such as capacitors and inductors. In most such microwave resistive mixer designs, there can be a tradeoff between conversion loss and IIΡ3. Accordingly, it can be challenging to achieve very good conversion loss and IIΡ3 at the same time using these conventional mixer design techniques. For example, although increasing device sizes or using HEMTs with higher gate bias may lower conversion loss, this can be accompanied by a lower, less desirable IIP3. [0043] FIG. 1 shows a typical, prior art, standard double balanced-resistive mixer 100. In the typical standard double balanced-resistive mixer design, each unit cell 112 usually has a single transistor 102, 114, Ί16 or 118 whose size and gate bias are adjusted to achieve the best linearity performance along with matching condition. However, since there is trade-off between 1IP3 and conversion loss in this approach, it may be difficult to achieve a better (\A. higher) IIP3 without sacrificing conversion loss.

[0044] In some cases, selective drain bias can be applied to utilize device output conductance; but the resulting IIΡ3 may still be limited and the layout may be more complex. See, as examples, devices described by J A. Garcia, J.C. Pedro, ML. De la Fuente, K.B. Carvalho, A. Mediavilla, and Tazon, A., in "Resistive FET mixer conversion loss and IMD optimization by selective drain bias," Microwave Symposium Digest, IEEE MTT-S International Volume: 2 (1999) and by J. Kim and Y. Kwon, in "Intermodulation analysis of dual-gate FET mixers," Microwave Theory and Techniques, IEEE

Transactions on Volume: SO, Issue: 6 (2007). Both of these documents are incorporated by reference herein.

[0045] As shown in FIG. 1 , a conventional standard double balanced resistive mixer 100 typically can include HEMTs 102, 114, 116 and 118, each of which forms or is in a typical unit cell or transistor block 112. So the mixer 100 of FIG. 1 has four unit cells or transistor blocks 112. The typical prior art unit cell or transistor block 112 has only a single active device therein. Connected to the gate of each of the four HEMTs 102, 114, 116 and 118 is a Local Oscillator (LO) 106, which provides the same unmodulated signal to the gate of each of the HEMTs, except that the signal to HEMTs 114 and 118 is 1 B0 degrees out-of -phase with respect to the LO signal provided to HEMTs 102 and 116. The source of each of the four HEMTs, 102, 114, 116 and 118, provide an IF output 108, whereas the drain of each of the four HEMTs, 102, 114, 116 and 118 are connected to a RF input 110.

[0046] In a typical arrangement, the typical double balanced resistive mixer 100 usually relies on linearity during intermodulation of two RF frequencies and sufficient non-linearity during the intermodulaiion of RF and LO frequencies to reduce and/or minimize conversion loss and maximize 1IP3. In such a typical microwave mixer, a down converted IF signal is the nonlinear harmonic product of RF and LO signals. This typically means that the mixer relies on device non-linearity to generate the IF signal. Accordingly, achieving a highly linear mixer is challenging, especially for IIP3.

[0047] The unlabeled rectangles connote the use of micro-strips which are advantageous at microwave frequencies.

[0048] FTO. la depicts a conventional single unit cell or transistor block 11?.. In this case, a Field Effect Transistor (FET) is labeled with the numeral 102, but it could have been labeled instead with the numerals 114, 116 or 118. FIG. lb depicts an equivalent diagram. The resistive FET mixer of Fig. 1 a can be described equivalently with the circuit schematic of FIG. lb, which circuit consists of output resistance (Rds), output capacitance (Cds), gate source capacitance (Cgs) and gate drain capacitance (Cgd), which are all non-linear. Output conductance is thus equal to 1/Rds. An ideal resistive FET mixer assumes only non-linear output conductance for the IF to RF up-conversion or KF to IF down conversion by LO, excluding other non-linear capacitances. Under the down- conversion assumption with 2-tone RF signal (Frfl and FrfZ) injection, a mixer's IF signal frequency is defined by FLO-FRP. (where FRP is Frfl or Frf2) and an IF third harmonic frequency (IF3) defined by (2*Frfi-Fin)-FLo or (2*Fin-Fif I)-FLO. Accordingly, from this relationship, it is understood that the IF third harmonic is the down-converted RF third harmonic by FLO. The conversion loss is defined by PRP- PIF, where PIF denotes power at IF frequency and PHF denotes power at RF frequency. IIP3 is defined as PRF + 0.5*(ΡΐΡ--Ριη), wherein Pin denotes power of IF3 signal frequency defined by (2*F _I H- FIGO-FLO or (2*Frf2"Frfi) FLO. AS explained, non-linear output conductance itself, makes the signal down-conversion as well as the IF third harmonic. Once non-linear capacitances are all considered together, all non-linear capacitances generate third harmonics along with non-linear output conductance and resulting I1P3 becomes worse than in the ideal resistive mixer case. [0049] FIG.2a shows an embodiment of a unit cell in accordance with present disclosure where a plurality of active devices (preferably two active devices such aa HEMTs 202 and 204) is arranged in series in a new design unit cell 212. The new design unit cell 212 can replace the prior unit cell 112 and an improved mixer can result. A mixer usually has a number of unit cells, such as the four unit cells 112 of the prior art mixer of FIG. 1. It is to be noted that the unit cell 212 of FIG.2a is different from the prior art unit cell 112 in that it has a plurality of (at least two) separate active devices, namely, FETs 202 and 204. The plurality of active devices (here 202 and 204) within the new design unit cell 212 are preferably connected in series with each other. FETS 202 and 204 may be n-channel devices (arrow away from the gate) or p-channel devices (arrow towards the gate). In addition, FETS 202 and 204 may both be enhancement mode devices or both depletion mode devices. Without implying a limitation, FETS 202 and 204 may be HEMTs, JFETs, MESFETs, and/or CMOS devices including hybrid devices.

[0050] FIG.2a discloses first and second active devices 202 and 204, respectively, connected in series with each other to form a new design unit cell 212. The gate of the first and second active devices 202 and 204 are preferably electrically connected in phase to a local oscillator (IX)) source 206 (preferably via a micro-strip 218 when used at microwave frequencies) so that each gate in a new design unit cell 212 receives the same signal with same phase thereof from the local oscillator 206. The drain of the first active device 202 can, in a preferred embodiment, be connected to the source of the second active device 204, to form a series connection between them . The drain of any active device directly connected to the source of another active device may be referred to as an inner drain, and the source of the another active device may be referred to as an inner source. The source of the first active device 202 may be connected to IF (preferably via a micro-strip 214 when used at microwave frequencies), and the drain of the second active device 204 can be connected to (he RF (preferably via a micro-etrip 216 when used at microwave frequencies). The source of any active device connected to the IF connection and may be referred to as an outer source, and the drain of any active device connected to the RF connection may be referred to as an outer drain. Alternatively, in some embodiments (not shown), the source of the first active device 202 may be connected to RF, and the drain of the second active device 204 can be connected to IF.

[0051] FIG.2b is an equivalent circuit diagram to that of FIG.2a and the nomenclature generally follows that of FIG. lb except mat here two series-connected transistors in the unit cell 212 and therefore the numerals 1 and 2 have been added to the various capacitances and resistances compared to FIG. lb.

[0052] With two FETs connected in series, the equivalent circuit model appears in FIG.2b. Because of the two FETs 202, 204 being in series connection, Cdsl and Cds2 are also in series connection, so their effective capacitance (Cds_effective) is thereby reduced and the third harmonic generation from Cds_effective is reduced compared to a single FET unit cell 112 resistive mixer as used in the prior art of Fig. 1. If the two FETs 202, 204 are identically sized (so that they both have the same gate width W - see Fig.6), the non-linear behavior of Cgdl and Cgs2 would be almost or basically the same (see loop 220).

[0053] On the other hand, if the FET structures of FETs 202, 204 take on a non- symmetric shape so that the gate is closer to the source than it is to the drain (i.e. a relatively long drain to gate distance (Lao - see FIG.6) and relatively short source to gate distance (Los - see Fig.6) like a Field-plate GaN HEMT or LDMOS), then, the Cgdl and Cgs2 non-linear harmonic generation mechanisms are significantly different and their non-linear third harmonics can be cancelled out by adjusting the device width (W - see Fig.6) ratio, resulting in better linearity (better HP3). Similarly, with a series connection of two FETs (regardless symmetric or non-symmetric gate to drain Lgd or gate to source Lgs), more chances for nonlinear harmonic cancellation can occur between Cgdl and Rds2 (see loop 222 on Fig.2b), Cgs2 and Rdsl (see loop 224 on Eg.2b), Cgdl and Cgs2 (see loop 220 on Fig.2b). Since equation based non-linear components modeling and analysis is very complex and varies depending on what kind of device, device structure and/or device manufacturing process is used, it is easier to determine a best (or suitable) optimized size width W ratio of the two FETs through circuit simulation or trial and error. [0054] Since the series connection of the FETs does not increase the output capacitance at the IF connection or the input capacitance at the RF connection, this improved circuit can maintain the same bandwidth as the single FET prior art embodiment of FIG. 1b, but exhibiting a belter IIΡ3. In other words, the is no need to compromise improvement in IIΡ3 by a reduction in operating bandwidth when two or more series connected active devices replace the single active device cell 112 found in the prior art of FIG. 1a.

[0055] A series connection of Bipolar Junction Transistor (BJT) devices (see FIGs.9a and 9b) can also provide the similar linearity improvement with different device sizes. For BJT's the device size ratio is based on the areas of the emitters whereas for FETs the device size ratio is based on gate widths. So in the case of a BJT embodiment, the BJT connected to the RF side of the mixer is the larger device. Preferred ratios of the BJT emitter area on the RF side to the emitter area of the BJT on the BP side range from 1 : 1 to 4:1 with 2:1 being preferred.

[0056] In some embodiments, the active devices 202 and 204 may comprise one or more high-electron mobility transistors, or HEMTs. In other embodiments, other transistor types, such as CMOS FETs or Bipolar Junction Transistors (BJTs), may be used as the active devices 202 and 204 with similar termination. See FIGs.9a and 9b for a BJT embodiment. Tn some embodiments, active device 202 can be a different type of transistor than active device 204, for example, or can be the same type of transistor.

[0057] With active devices 202 and 204 having different sizes, the output conductances and capacitances of each active device can differ from the other one or more active devices within the same unit cell such that each generated RF third order harmonic from each output conductance and capacitance can effectively be cancelled out, reduced or minimized within the unit cell or between 180 degree out-phased two unit cells in a balanced structure. In a resistive mixer, signal mixing and harmonic generation can be achieved through non-linear output conductance and capacitances. The two series active devices 202 and 204 may be selected/configured to have size ratio (IF side: RF aide) between 1 :4 and 1 : 1. A preferred size ratio is 1:2. The size ratio for FETs (including transistors which are FBT based such as HEMTe, etc.) is based on the ratio of the gate width (W) of device 202 divided by the gate width of device 204 in FIG.2a using the definition for gate width (W) of Fig. 6. The active devices 202 and 204 within a unit cell 212 are preferably connected to the same DC gate bias voltage ted through a RF choke resistor or inductor (not shown), utilizing biasing techniques known to those skilled in the an.

[0058] In some embodiments, one or more of the new unit cells 212 can be arranged in a Resistive Microwave Mixer ("RMM") design. Preferably, four unit cells 212 may be arranged as a double balanced resistive mixer 300 as shown in FIG.3. FIG.3 illustrates a double balanced resistive mixer 300 that includes four unit cells 212 (individually numbered 308, 314, 316 and 318 in this figure), each having two active devices

(preferably series connected FETs or HEMT type FETs). Accordingly, a plurality of unit cells 212 may be arranged in the double balance resistive mixer 300. More or fewer unit cells can be included in a resistive mixer design. Alternative embodiments of double balance resistive mixers are not illustrated, but the use of the new design unit cells 212 disclosed herein is certainly contemplated.

[0059] Turning now to the resistive mixer 300 shown in a preferred embodiment at FIG.3, each of the enumerated unit cells 308, 314, 316 and 318 includes a plurality of active devices arranged in series with each other and, except for the numbering utilized, they are otherwise preferably the same as unit cell 212 of FIG.2a. For example, unit cell 308 includes active devices 310 and 312 connected in series with each other. Although two active devices are shown, the unit cells 308, 314, 316 and 318 or any other unit cell in other embodiments can include additional active devices, which may be added in pairs. The other unit cells 314, 316, and 318 can be arranged or configured preferably to similarly include a plurality of active devices arranged in series or other arrangements. In some embodiments, the active devices of a unit cell are different sizes so as to have a size ratio. [0060] As shown in FIG.3, the gates of all active devices in the unit cells 308, 314, 316 and 318 can be connected preferably to a LO frequency source 304 and thereby receive the same signal, though the active devices of unit cells 314 and 318 receive the signal 180 degree out-of-phase with respect to the active devices of unit cells308 and 316. Each of the plurality of active devices can take the same gate bias voltage or different bias voltages.

[0061] The outer source of each of the plurality of unit cells (308, 314, 316, 318) can be connected preferably to IF 306 and the outer drain of each of the plurality of unit cells (308, 314, 316, 318) can be connected preferably to a RF input 302. Within the unit cells, 308,314, 316 and 318, the inner source of an active device Is connected to the inner drain of another active device. Resistive elements may be arranged between each of the active devices and the corresponding IF, RF and LO ports, of a preferred embodiment.

[0062] The active devices 310 and 312 in series of a preferred embodiment, as shown for example in unit cell 308, may be arranged or configured within the unit cell such that the unit cell has an IF-side active device and an RF-side active device. In such an embodiment, in unit cell 308, active device 310 can be arranged in series with active device 312 such that the active device 310 may be considered an RF-side active device and active device 312 may be considered an IF-side active device due to their connections to RF 302 and IF 306, respectively.

[0063] The series connection of two active devices in all of the series connection embodiments can improve mixer linearity (I1P3) through adjusting the size ratio between the two active devices. Selecting proper device size ratio can improve the output conductance and capacitances between two devices and cancel out, or at least reduce, non-linear third harmonics, as the result of that selection as is explained herein. Thus, output conductance and capacitances are dependent on device size, with a bigger device having a bigger output conductance and capacitance. As noted earlier, determination of the size ratio may depend on the type of active devices employed. To achieve the best IIΡ3, non-linear harmonic cancellation is realized from two different output conductances and capacitances. An optimum size ratio of the IF-side active device to the RF-side active device, in some embodiments, is between 1:1 and 1:4. For FETs, the size ratio is based on the gate widths of the FETs. For bipolar transistors, the size ratio is based on the emitter areas.

[0064] As shown in Fig.3, in some embodiments, each unit cell 308, 314, 316 and 318 includes two series active transistors. In some embodiments each of the unit cells have the same conductance. Tn other embodiments, the transistors can be of any type, including, but not limited to HEMTs, each of whose outer ends (drain and source) may be connected to RF 302 and IF 306, respectively. A LO 304 produced signal can be provided through the two gates at the same phase. The two series active devices 310 and 312, shown for example in unit cell 308, may be selected/configured preferably to have size ratio (IF side: RF side) in a range between 1:2 and 1:4.

[0065] Although four unit cells are shown in FIG.3 , additional unit cells may be included in a similar configuration. In some embodiments, additional pairs of unit cells may be included. This can provide a balanced mixer having two unit cells, a double balanced mixer having four unit cells, a double-doubly balanced mixer having eight unit cells, or any other mixer with additional pairs of unit cells. In any of these configurations the larger size device is on the RF side and the smaller size device is on the IF side

[0066] Using FIG.2 as an example, one embodiment is a method for providing a unit cell having a plurality of active devices arranged in series, with each of said plurality of active devices having a different output conductance. In another embodiment, the method can optionally include providing a plurality of active devices of different sizes. The size ratios can optionally be, in some embodiments, between 1 : 1 (IF side: RF side) to 1 :4 (IF side: RF side) with 1 :2 (IF side: RF side)preferred. The method can optionally include arranging the active devices in series to suppress a non- linear third harmonic of an RF signal. The method can optionally include, as shown in FIG. 2, connecting a first active device 202 drain to a second active device 204 source. The method can optionally include connecting a first active device 202 source to an IF and a second active device 204 drain to a RF. The method can optionally include connecting gates of the plurality of active devices, 202 and 204, to a LO to have the same DC gate bias voltage. Half of the gates of the plurality of active devices, 202 and 204, receive a 180 degrees out of phase local oscillator signal. As further shown in FIG.3, 308 and 316 will be 180 degrees out of phase with 314 and 318, as further exemplified by RF, LO and IF ports each having "+" and "-" signs connoting in typical circuitry convention the 180 degree phase difference. Any devices connecting to a "+" sign have 180 degree phase difference with "-" connecting devices.

[0067] In other embodiments, the method employs a HEMT, CMOS and/or BJT as one or more of the plurality of active devices.

[0068] In still other embodiments, the shunt resistors for gate bias (not shown) can be replaced with choke inductors. Inductors are preferred when the devices 202 and 204 are BJT instead of FBTs to minimize DC power loss.

[0069] Embodiments according to the principles of the preset invention were simulated with QaN HHMTs. For comparison, the standard prior art mixer design of FIG. 1 was also optimized for the best conversion loss and IIΡ3.

[0070] A double balanced resistive mixer architecture was chosen since it can typically deliver wide broadband signal within the microwave frequency range and is a widely used architecture for communications. In this simulation, the design used all MMTC devices.

[0071] In one example, the simulated resistive double balanced mixer design according to the present invention using GaN HEMTs shows 5 to 15 dB IIΡ3 improvement compared with a standard prior art resistive double balanced mixer, without sacrificing conversion loss for example. PIG. 1 and FIG. 3 show the respective schematics of the standard prior art resistive double balance mixer and the double- balanced resistive mixer according to the present invention. [0072] The simulation employed 102 device sizes of 720 urn gate length for standard prior art mixer and 420um and 840um (gate lengths of devices 312 and 310, respectively in each unit cell 308, 314, 316, and 318. Since a double-balanced architecture was employed, the RF, LO and IF ports were balanced, which had + and - signal and gate biases (less than threshold voltage: -4.5V) provided through 1000 Ohm resistors. Boththe standard prior art mixer and a mixer according to the present invention had shunt resistor (1000 Ohm) termination at the RF and IF that acted like a DC current path to the ground. In a large signal swing condition, an undesired DC current can be generated through nonlinear harmonic distortion and such a DC current can shift the device's bias point and may cause undesired performance. Since each design sought the best 1IP3 and conversion loss in a given architecture, each device's size, transmission lines and matching condition differed between the two designs, resulting in different chip sizes.

[0073] FIGs.4 and S show the simulation comparisons for IIΡ3 and conversion loss of some examples between standard prior ait mixer and the mixer according to the present invention. For the simulation purposes, RF frequency was swept from 1 GHz to 9 GHz and IF frequency was selected at 0.S GHz.

[0074] As shown in FIG, 4 'a I1P3 results, the mixer according to the present invention can outperform the standard prior art mixer by 5 to 15 dB of IIP3. The mixer according to the present invention effectively suppressed the third harmonic by about 10 dB more than standard mixer. FIG. S shows the conversion loss of the mixer according to the present invention and the standard mixer.

[0075] FIGs.7 and 8 show layout examples of conventional and invented double- balanced resistive mixer using GaN HEMTs. The gate with sizes are 720um for the standard prior art mixer and gate widths of 420um and840um for 312 and 310 respectively. The total chip sizes are 3.6mmx2.9mm for the standard prior art mixer and 2.6mmx2.7mm for the 312 and 310 devices . Since they both use a double-balanced architecture, the RF, LO and IF ports arc the balanced type, which have plus and minus signal pads. Gate biases are provided through TaN resistors. Both the standard prior art mixer and the invented mixer have shunt resistor termination at the RF and IF probe pads that act like a DC current path to the ground. In a large signal swing condition, undesircd DC current is generated through non-linear harmonic distortion and such DC current shifts device bias point and brings undesired performance. Since each design attempts to achieve the best HPS and conversion loss in a given architecture, device sizes, transmission lines, and matching conditions are different between the two designs, resulting in different chip sizes.

[0076] It should be noted that the series connected active devices in a unit cell can be implemented fairly easily with little or not expansion of surface area.

[0077] FIG .9a is similar to FIG.2a but shows a circuit diagram of two bipolar transistors (BJTs) connected in series in a unit cell 212 while FIG.9b presents an equivalent circuit for the circuit schematic of FIG 9a.

[0078] The size ratio described herein is in term of the relative sizes of a semiconductor element of a pair of active devices. The semiconductor element In the case of FETs is the gate and the size ratio is based on gate widths as defined in Fig.6. The semiconductor element in the case of bipolar transistors is the emitter and the size ratio is based on emitter areas.

[0079] While the ratio described herein is in terms of the relative sizes of semiconductor element of a pair of active devices, it may well be possible to characterize this ratio in terms of the relative transconductances of the two devices as opposed to just the sizes of a semiconductive element thereof.

[0080] While certain features have been described herein, many other features are contemplated and fall within the scope of the disclosure. [0081] All elements, parts and steps described herein are preferably included. It is to be understood that any of these elements, parts and steps may be replaced by other elements, parts and steps or deleted altogether.

[0082] Concepts of this invention include:

Concept 1 : A mixer, comprising :

four cells wherein each cell has a drain terminal, a source terminal and a gate terminal'.

wherein the source terminals of the first and second cells are connected at a first RF input terminal;

wherein the source terminals of the third and fourth cells are connected at a second RF input terminal;

wherein die drain terminals of the first and third cells are connected at a first IF terminal;

wherein the drain terminals of the second and fourth cells are connected at a second IF terminal;

wherein the gate terminals of the first and fourth cells are connected at a first LO terminal;

wherein the gate terminals of the second and third cells are connected at a second LO terminal;

wherein each cell comprises first and second series connected devices;

wherein a ratio of a gate width of the first series connected device to a gate width of the second series connected device in each cell is between one to one and one to four.

Concept 2: The mixer of concept 1 , wherein the first device and the second device Is a field effect transistor

Concept 3: The mixer of concepts 1 or 2 , wherein the first device is connected to the source terminal of the cell and the second device is connected to the drain terminal of the cell. Concept 4: The mixer of any one of concepts 1 - 3, wherein the first device and the second device is a high electron mobility transistor (HEMT).

Concept 5: The mixer of any one of concepts 1 - 4, wherein a first device source is connected to a first frequency source and a second device drain is connected to a second frequency source.

Concept 6: The mixer of any one of concepts 1 - 5, wherein gates of the devices are connected to a local oscillator to have common DC gate bias voltage.

Concept 7: The mixer of concept 6, wherein half of the gates of the devices receive a 180 degrees out of phase local oscillator signal.

Concept 8: The mixer of any one of concepts 1 - 7, wherein one or more of the plurality of active devices comprises one or more from (he group consisting of HEMT, CMOS and BJT.

Concept 9: A method of reducing a third harmonic of an RF signal in a mixer comprising a plurality of unit cells, the unit cells being connected in a ring with an RF input and LO input and having a IF output, the method comprising:

providing at least a pair of active devices in each of said unit cells, each of said active devices having a pair of current carrying electrodes and at least one conirol electrode;

connecting the at least a pair of active device in each of said unit cells so said current carrying electrodes are coupled in series with each other and also connected to current carrying terminals of an associated united cell;

connecting the at least a pair of active device in each of said unit cells so said control electrodes are coupled in parallel with each other and also connected to a control terminal of the associated united cell; and adjusting sizes of at least one semiconductor element in each of the pair of active devices in each of said unit cells relative to other in each of said unit cells to reduce said third harmonic.

Concept 10: The method of concept 9 wherein the size of the semiconductor element in each of the pair of active devices in each of said unit cells which are adjusted is a width of a gate of each of the pair of active devices in each of said unit cells .

Concept 11 : The method of concepts 9 or 10 wherein the size of the semiconductor element in each of the pair of active devices in each of said unit cells which are adjusted is an area of an emitter of each of the pair of active devices in each of said unit cells.

Concept 12: A mixer, having an intermediate frequency (IF) side and a radio frequency (RF) side, the mixer comprising:

a plurality of unit cells each comprising a plurality of active devices of different sizes arranged in series to reduce or inhibit generation of a non-linear third harmonic by the mixer.

Concept 13: The mixer of concept 12, wherein a size ratio of the active devices in each unit cell is at least 1 :2 (IF side: RF side).

Concept 14: The mixer of concept 12 , wherein a size ratio of the active devices is between 1:2 and 1 :4 (IF side: RF side).

Concept 15 : The mixer of any one of concepts 12 - 14, wherein the plurality of active devices in each unit cell are connected in series between current carrying electrodes of said active devices while control electrodes of said active devices in each unit cell are connected in parallel. Concept 16: The mixer of any one of concepts 12 -15 _r wherein the series connection of the active devices in each unit cell comprises a connection of a drain of a first active device connected to a source of a second active device.

Concept 17: The mixer of any one of concepts 12 - 16, wherein a source of first active device is connected to IF side and a drain of a second device drain is connected to the RF side.

Concept 18: The mixer of any one of concepts 12 - 17, wherein gates of the plurality of active devices are connected to have the same DC gate bias voltage.

Concept 19: The mixer of any one of concepts 12 - 18, wherein half of the gates of the plurality of active devices are connected 180 degrees out of phase with respect to a remaining half of the gates.

Concept 20: The mixer of any one of concepts 12 - 19 is a resistive mixer and wherein a single balanced resistive mixer includes two unit cells, a double balanced resistive mixer includes four unit cells and a double-doubly balanced mixer includes eight unit cells.

Concept 21 : The mixer of any one of concepts 12 - 20 wherein the active devices of different sizes arranged in series have different conductances.

Concept 22: The mixer of concept 21 wherein the active devices of different sizes arranged in series also have different output capacitances.

Concept 23: A method for producing a unit cell used in a mixer, the method comprising: providing a plurality of active devices of different sizes connected in series in said unit cell. Concept 24: The method of concept 23 wherein active devices in said unit cell having differing conductances and capacitances.

Concept 25: A unit cell for a resistive mixer includes a plurality of active devices arranged in series, wherein each of said plurality of active devices having a different output conductance.

Concept 26: A resistive mixer includes a plurality of active devices connected in series with one another to form a unit cell .