BACKGROUND

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).

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.

In a typical mixer, a down-converted 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 (“IIP3”).

IIP3 is a figure of merit used in describing the linearity of many circuits, including mixers. IIP3 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 products are undesirable, so the bigger the ratio, the better (less distorted) the circuit being analyzed. So a higher IIP3 is better than a lower IIP3.

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 has a built in amplifier, then it can exhibit a conversion gain.

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 1-ETs which are utilized to achieve frequency mixing.

To reduce and/or minimize conversion loss and maximize IIP3 (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.

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.

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 IIP3 and conversion loss in this approach, better IIP3 cannot typically be achieved without sacrificing conversion loss.

In some cases, selective drain bias can be applied to utilize device output conductance as described in J. A. 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 IIP3 may still be limited and a more complex layout may be required.

SUMMARY

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.

The unit cell as described above, wherein the plurality of active devices have different size gate widths.

The unit cell as described above, wherein the size ratio of the active devices is 1:2 (IF side:RF side).

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).

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.

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.

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 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.

In 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 side: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 to an intermediate frequency and a second device drain is connected to a radio frequency. The unit cell 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.

In another aspect the present invention relates to an apparatus for reducing a third harmonic generated by a mixer, the mixer comprising a plurality of unit cells, the unit cells being connected in a ring of unit cells having an RF input, a LO input and an IF output, the apparatus comprising: at least a pair of active devices in each of said unit cells, each of said pair of active devices having a pair of current carrying electrodes and at least one control electrode; a first circuit connecting the at least a pair of active devices in each of said unit cells so that said current carrying electrodes of the active devices in each unit cell are coupled in series with each other and also connected to current carrying terminals of neighboring unit cells in said ring of unit cells; a second circuit connecting the at least a pair of active devices in each of said unit cells so their control electrodes are coupled in parallel with each other and also connected to said LO input; and wherein sizes of at least one semiconductor element in each of the pair of active devices in each of said unit cells being differently sized relative to each other in said unit cells.

In another aspect the present invention relates to a method for producing a unit cell in a mixer, the method comprising: providing a plurality of active devices of different sizes connected in series.

