Xudong Wang, RFIC Design Engineer,Bill Beckwith, RFIC Design Engineer,Tom Schiltz, Design Engineering Manager, Weston Sapia, Senior Applications Engineer, and Michael Bagwell, RFIC Design Engineer, Analog Devices
A broadband 3 GHz to 20 GHz SiGe passive mixer requiring only 0 dBm LO drive is presented. A new balun structure is the key innovation that enables the wide RF bandwidth. The same balun topology is used on the IF, enabling a wide, 300 MHz to 9 GHz IF. This high performance double-balanced mixer can be used for upconversion or downconversion. The mixer is packaged in a tiny 2 mm × 3 mm, 12-lead QFN package and delivers 23 dBm IIP3 and 14 dBm P1dB. The mixer consumes 132 mA on a 3.3 V supply.
Wideband mixers have many applications in multifunction wireless transceiv- ers, microwave transceivers, microwave backhaul, radar, and test equipment. A mixer with wide bandwidth allows a single mixer to be used in radio architectures with on-the-fly programmability of various radio parameters.
The advanced silicon-based technologies such as CMOS and BiCMOS have demonstrated the capability for high performance mixers in relatively narrow-band applications. It is highly desirable to have broadband mixers that can be made with lumped elements or other structures compatible with IC fabrication techniques and geometries. Balanced mixers are the preferred topology because of their better overall performance compared to unbalanced mixers with respect to linearity, noise figure, and port-to-port isolation. Baluns are critical components used in single-balanced mixers and double-balanced mixers to convert RF, LO, and IF signals between balanced and unbalanced configurations. It is critical to realize baluns can be integrated in standard IC foundry processes so that broadband inte- grated mixers can be produced.
In this article, an innovative balun structure that can be easily implemented in silicon, GaAs, or any other integrated process is introduced. This balun topology exhibits much wider bandwidth than a traditional balun structure. A 3 GHz to 20 GHz high performance mixer is designed using the wideband balun in a 0.18 μm SiGe BiCMOS process.
The most important performance parameters for a mixer include the conver- sion gain, linearity, noise figure, and operating bandwidth. The baluns used in integrated mixers have significant impact to all these mixers’ performances. The critical performance of an integrated balun includes operating frequency range, insertion loss, amplitude/phase balance, common-mode rejection ratio (CMRR), and physical size.
Two popular balun structures in the integrated circuits applications are traditional planar-transformer baluns1,2 and Marchand baluns.3,4 Both of these baluns have good performance for narrow-band applications. The planar-transformer balun consists of two closely coupled transformers. The self-inductance and the resonant frequency of the inductors are two main bandwidth limiting factors. The self-inductance limits the bandwidth in the lower frequency end, and parasitic capacitance and asymmetry termination on the unbalanced and balanced terminals limit the high frequency end. The Marchand balun consists of four quarter wave transmission lines and usually needs large real estate on the chip. Miniature Marchand baluns have been demonstrated using interleave transformer layout in integrated circuits. The bandwidth of Marchand baluns is limited by the requirement of the electrical length of each line segment. When the electrical length is farther away from the required quarter wavelength, the amplitude and phase balance are degraded. In general, a well-designed transformer balun or Marchand balun can cover a frequency range of 3× to 4× maximum to minimum frequency ratio with reasonable performance.
It is well known that the Ruthroff balun exhibits very wide bandwidth,5,6,7 and many discrete component products have been developed based on the Ruthroff structure. However, no application of a similar structure for a microwave integrated circuit is found.
Figure 1a shows a Ruthroff-style broadband balun schematic that can be easily constructed in a planar semiconductor process using three inductors. One layout example is shown in Figure 1b. In that layout, only two metal layers are needed, with one thick metal layer for three low loss inductors and an underpass metal layer for connections. When additional thick metal layers are available, the L1 and L3 can be vertically coupled, which results in smaller size and possibly better magnetic coupling between them.
The broadband feature benefits from the simplicity of the structure, which results in less parasitic capacitance. The single-ended signal is voltage divided by L1 and L2. As a result, the positive port of the balun is directly half of the voltage of the single-ended signal with the same phase. The negative port of the balun is half of the voltage of the single-ended signal with 180° phase shift due to the negative coupling between L1 and L3.
Excellent amplitude and phase balance over a very wide bandwidth can be achieved. Figure 2 shows the simulated performance of a broadband balun configuration. The amplitude imbalance is the difference between S21 and S31, and the phase error is the phase difference of S21 and S31 away from the desired 180°. The proposed balun has very good amplitude balance and phase difference of close to 180° between 3 GHz and 20 GHz. Common- mode rejection is important for a balun to be used in many applications such as balanced mixers and push-pull amplifiers. The simulated results shown in Figure 5b demonstrate that the 3-inductor balun has better than 20 dB CMRR over the 3 GHz to 20 GHz range.
Like the transformer balun topology, the bandwidth of the 3-inductor balun is limited by the inductance at the low frequency end and by the parasitic capacitance at the high frequency end. When the inductance is lower, the load impedance will have more impact to the voltage division between L1 and L2 for port 3 and the transformed voltage for port 2. Although the ampli- tude balance and phase difference are still acceptable at a low frequency range, the insertion loss is increased. As a result, lower terminal impedance or higher inductance will benefit the low frequency performance. At the high frequency end, the parasitic capacitance between L1 and L2 will degrade the transformer’s performance and results in large phase errors. Careful layout with the consideration of less parasitic capacitance can extend the balun’s high frequency operating range.
The physical size of an integrated balun limits the low end of bandwidth. To explore the feasibility of the proposed balun structure for lower frequency application, a 0.5 GHz to 6 GHz balun is designed and compared with a tra- ditional transformer-based balun, and the performance is shown in Figure 3.
Integrated Broadband RF/Microwave Mixer
A broadband double-balanced passive mixer has been designed in Jazz’s SiGe
0.18 μm process with the 3-inductor balun configuration. The RF, IF, and LO ports of the mixer are 50 Ω single-ended with baluns integrated for the RF and IF ports. The integrated RF balun is optimized to cover the 3 GHz to 20 GHz RF frequency range. The integrated IF balun is optimized to cover a very wide, 500 MHz to 9 GHz, frequency range. The single-ended LO signal is converted to a differential signal internally by an active amplifier circuit to reduce chip size. Two stage broadband amplifiers using high speed NPNs provide enough signal voltage swing to the MOSFET gates of the passive mixer with only 0 dBm input power over the 1 GHz to 20 GHz frequency range.
The mixer is packaged in a tiny 2 mm × 3 mm QFN with flipchip using cop- per pillars for the interconnections. The copper pillar connection has very low additional parasitics to preserve the broadband performance from the silicon. The mixer is biased with 3.3 V supply, and the current consumption is 132 mA at room temperature. The measured conversion loss and IIP3 performance is shown in Figure 5.8 The mixer’s RF, LO, and IF ports are well matched over its wide operating frequency range. Figure 6 shows the return loss of these ports. It should be noted that the RF return loss is dependent on the IF port impedance, and the results in Figure 6a are measured with an IF frequency of 0.9 GHz.
Compared with broadband mixers on the market (such as those in Table 1), the mixer designed with the 3-inductor baluns achieves the widest bandwidth for both the RF and IF range. It requires the lowest LO power with the high- est integration level. The overall performance is superior than any reported product or published broadband mixer product.
A Ruthroff-style broadband balun structure that fits the planar implemen- tation of modern semiconductor process is introduced in this article. A high performance double-balanced mixer using the broadband baluns is designed and measured.
1 Alberto Costantini, Ben Lawrence, Simon Mahon, James Harvey, Gerry McCulloch, and Alexandre Bessemoulin. “Broadband Active and Passive Balun Circuits: Functional Blocks for Modern Millimeter-Wave Radio Architectures.” 2006 European Microwave Integrated Circuits Conference, September 2006.
2 Tin Hao Chen, Kai Chang, Hongmei Wang, G. Samuel Dow, Lu Liu, Stacey Bui, and Tzer Shen Lin. “Broadband Monolithic Passive Baluns and Monolithic Double-Balanced Mixer.” IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 12, December 1991.
3 Chien-Hsiang Huang, Chien-Hsun Chen, and Tzyy-Sheng Horng. “Design of Integrated Planar Marchand Balun Using Physical Transformer Model.” 2009 Asia Pacific Microwave Conference, December 2009.
4 Sheng-Che Tseng, Chinchun Meng, Chia-Hung Chang, Chih-Kai Wu, and Guo-Wei Huang. “Monolithic Broadband Gilbert Micromixer with an Integrated Marchand Balun Using Standard Silicon IC Process.” IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 12, December 2006.
5 Clyde Ruthroff. “Some Broadband Transformers.” Proceedings of the IRE, Vol. 47, August 1959.
6 Richard H. Turrin. “Application of Broad-Band Balun Transformers.” QST, April 1969.
7 Shi-Lang Yang, Dhiren Bhatt, and Wei-Ping Zheng. U.S. patent US6133525. October 2000.
8 LTC5553 Data Sheet. Analog Devices, Inc., 2017.
Xudong Wang, William Beckwith, and Thomas Schiltz. U.S. patent US9312815. April 2016.
About the Authors
Xudong Wang is an RF/microwave design engineer at Analog Devices in Colorado Springs, Colorado. He has a Ph.D. from Northwestern Polytechnical University and 29 years of RF/microwave design experi- ence. He has published more than 50 technical papers in journals and conference proceedings. Xudong holds 12 patents in the U.S. and internationally. He can be reached at firstname.lastname@example.org.
Bill Beckwith is a senior RFIC designer at Analog Devices in Colorado Springs, where his principle focus since 2017 has been the design of microwave and millimeter wave amplifiers and switches. He previously worked at Linear Technology Corporation where he designed high per- formance SiGe and CMOS mixers. Prior to that, he worked at Motorola where he designed GaAs RF switches, mixers, amplifiers, and broadband passive components. He received a B.E.E. degree from Georgia Institute of Technology in 1984 and an M.S.E.E. from Arizona State University in 1990. He can be reached at email@example.com.
Tom Schiltz is an RFIC design manager at Analog Devices in Colorado Springs, Colorado. Tom has a B.S.E.E. and an M.S.E.E. from University of Nebraska and Arizona State University, respectively. He has 32 years of RF/microwave design experience, ranging from deep space transponders to cellular transceivers. He also served on the IEEE’s ISSCC RF and Microwave Subcommittee for seven years. He can be reached at firstname.lastname@example.org.
Weston Sapia is a senior RF applications engineer at Analog Devices in Colorado Springs, Colorado. Originally, he worked at Linear Technology Corporation covering all the RF mixer products. Since becoming part of ADI, he’s mostly worked with upcoming mil- limeter wave imaging products. Weston graduated from California Polytechnic State University, San Luis Obispo in 2010 with a Bachelor of Science in electrical engineering. There, his focus was on analog and RF. He can be reached at email@example.com.
Michael Bagwell works at Analog Devices as an RFIC design engi- neer in Colorado Springs, Colorado. He received an M.S.E.C.E. from Georgia Institute of Technology in Atlanta, specializing in RF and analog design. He has worked in the semiconductor industry for over 20 years designing LNAs, mixers, VCOs, low phase noise amps, and programmable baseband amps for Bluetooth®, WLAN, GSM/ Edge/WCDMA transceivers, and other wireless communications systems, as well as calibration circuits and other supporting designs. He can be reached at firstname.lastname@example.org.