W‐band inductor compensated doubly balanced I/Q mixer
2016; Institution of Engineering and Technology; Volume: 52; Issue: 13 Linguagem: Inglês
10.1049/el.2016.0981
ISSN1350-911X
Autores Tópico(s)Superconducting and THz Device Technology
ResumoElectronics LettersVolume 52, Issue 13 p. 1177-1179 Wireless communicationsFree Access W-band inductor compensated doubly balanced I/Q mixer Zhe Chen, Corresponding Author Zhe Chen zhechen@seu.edu.cn State Key Laboratory of Millimeter Waves, School of Information Science and Engineering, Southeast University, Nanjing, People's Republic of ChinaSearch for more papers by this authorFang Zhu, Fang Zhu Milliway Microelectronics, Nanjing, People's Republic of ChinaSearch for more papers by this author Zhe Chen, Corresponding Author Zhe Chen zhechen@seu.edu.cn State Key Laboratory of Millimeter Waves, School of Information Science and Engineering, Southeast University, Nanjing, People's Republic of ChinaSearch for more papers by this authorFang Zhu, Fang Zhu Milliway Microelectronics, Nanjing, People's Republic of ChinaSearch for more papers by this author First published: 01 June 2016 https://doi.org/10.1049/el.2016.0981Citations: 7AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The development of a W-band inductor compensated doubly balanced I/Q mixer in 0.1 μm GaAs pHEMT technology is presented. The I/Q mixer consists of two ring-diode mixer cells, Marchand balun and Lange coupler for quadrature LO input signal. An inductance is utilised to resonate with the parasitic capacitance of the mixer core. The relationship between the compensating inductance and the diode model parameter is theoretically analysed and derived for the first time. With the compensating inductance, the return loss of the LO port is greatly improved, and thus no need for the matching network of LO port. Measured results show good performance: the typical conversion gain is −10 dB for RF from 75 to 96 GHz, and intermediate frequency of DC to 12 GHz, with the LO power level of +14 dBm. Image rejection is above 21 dB. Due to the doubly balanced topology, high LO-RF isolation is observed with a minimum value of 43 dB across the whole operation frequency band. Introduction Over the past years, the demand for wireless communication systems with large capacity and higher data throughput has promoted tremendous research, such as millimetre-wave radio backhauling. In October 2003, the Federal Communications Commission (FCC) released 13 GHz spectrum in 71–76, 81–86, and 92–95 GHz, dedicated for high speed wireless communications in the United States [1]. The wide frequency bandwidth shows possibility to achieve multi-Gbit/s data rate. For high spectral efficiency, data transmission can use advanced modulation techniques such as quadrature amplitude modulation and orthogonal frequency division multiplexing. These modulation techniques impose particularly high requirements in terms of linearity and I/Q imbalance for the mixer circuit design. Passive doubly balanced mixers show superior rejection of spurious mixing products, good LO-RF isolation, high linearity and simple circuit topology with no need for DC biasing power [2]. At millimetre-wave band, the complex impedance of the diode pairs and the feeding lines should be well matched; otherwise the mixer performance will be strongly affected [3]. Although there are publications relating to millimetre wave I/Q mixers using different transistor technologies [4-7], few of them involved impedance matching for LO port. Li [8] gives a simple way is to compensate mixer core with a lumped inductor. However, the relationship between the compensating inductance and parameters of the equivalent diode model is not provided, which is critical for millimetre-wave mixer design. Fig 1Open in figure viewerPowerPoint Circuit schematic and chip photograph of I/Q mixer In this Letter, the development of a W-band inductor compensated doubly balanced I/Q mixer in 0.1 μm GaAs pHEMT technology is presented. The I/Q mixer consists of two ring-diode mixer cells, Marchand balun and Lange coupler for quadrature LO input signal. An inductance is utilised to resonate with the parasitic capacitance of the mixer core. The relationship between the compensating inductance and the diode model parameter is theoretically analysed and derived. With the compensating inductance, the return loss of the LO port is greatly improved, thus no need for other matching network for LO port. Measured results show that, with the doubly balanced topology, high LO to RF isolation has been observed. The designed doubly balanced I/Q mixer is suitable for high data rate communication system from 75 to 96 GHz. Circuit design The doubly balanced I/Q mixer uses Lange coupler as the quadrature splitter for LO signal and a Wilkinson splitter at the RF port as shown in Fig. 1. With an appropriate off-chip intermediate frequency (IF) quadrature hybrid coupler, the designed I/Q mixer can implement an image-rejection mixer with suppressed upper/lower sideband. Each mixer cell is the doubly balanced ring-diode mixers, which allow the mixer to perform down/up-conversion, including the Marchand balun for LO and RF input, ring-diode core, IF signal extraction network. The I/Q mixer is realised with WIN 0.1 μm GaAs pHEMT process, in which the diodes are realised by connecting the source and drain of the pHMET. Its equivalent circuit is shown in Fig. 2a. The Cj is the gate-to-channel junction capacitance, which is controlled by the voltage across the diode; gj is the non-linear admittance of the junction, which is the main mixing element for the mixer. Rs is the series resistance. At millimetre-wave frequency, Cj would bypass gj and directly degenerate the mixing performance. Hence, special attention should be paid to the design of the ring-diode core, especially at W-band. A simple way is to resonate Cj with a proper inductance. Fig. 2b shows the equivalent circuit of the ring-diode core, with the compensating inductor. Fig 2Open in figure viewerPowerPoint Schematic of a pHEMT diode structure and its equivalent circuit b Equivalent circuit for ring-diode core with compensating inductor In the doubly balanced ring-diode mixer, the ring-diode core is differentially pumped by the LO signal on port 1 and port 2. Due to the symmetry of the circuit, the virtual RF GND exists in the symmetrical plane between port 1 and port 2. For port 1, the single-end admittance is (1)The image part of Yin1 is (2)When the compensating inductor Lcomp resonates with capacitance of the ring core, Im(Yin1) = 0, thus (3)From (3), it is clear that the compensating inductance Lcomp is dependent on the operating frequency ω, the series resistance Rs, the non-linear junction admittance gj, and the junction capacitance Cj. The latter two parameters vary largely as the diode is pumped by LO signals. It is noted that the calculated Lcomp in (3) only takes into account the diode model parameters Cj, gj, and Rs. However, the ring-feeding structure will also contribute parasitic reactance to the ring-diode core, especially at millimetre-wave frequency. Therefore, (3) will provide a reasonable initial value to start, and full-wave simulator helps to find the final optimised value for Lcomp. The doubly balanced ring-diode core is co-simulated with Agilent Advanced Design System (ADS). Fig. 3 shows the simulated S11 of the ring-diode core with and without the microstrip-line inductor, using different LO input power of 10, 12, and 14 dBm. The dash lines denote the mixer core without the inductor. The compensating inductor successfully resonates and reduces the reactance of the ring-diode core from 75 to 96 GHz. The return loss of the LO port is greatly improved, thus no need for other matching network for LO port. Fig 3Open in figure viewerPowerPoint Simulated magnitude of S11 of ring-diode core (NOF = 2, Dw = 10 μm) with and without microstrip-line inductor Fig 4Open in figure viewerPowerPoint Measured results for a Conversion loss of W-band I/Q mixer b Conversion loss with different LO input power at LO of 81 GHz c Image rejection with 25 MHz I/Q input signal (up-conversion) d LO to RF isolation of doubly balanced I/Q mixer The broadband Marchand balun is utilised for differential input LO and RF signals. To increase the coupling coefficients, three-conductor coupled lines are used considering the layout rule for minimum conductor spacing. The Wilkinson power splitter and Lange coupler are important passive parts, which have a strong influence on the magnitude and phase balance of the I/Q mixer. All the design parameters are optimised with ADS and Ansoft High Frequency Structure Simulator (HFSS). Finally, the doubly balanced I/Q mixer is implemented with WIN 0.1 μm GaAs pHMET process. The chip size is 1.48 mm × 1.17 mm. Measurement and results On-wafer measurements are performed for the implemented doubly balanced I/Q mixer with GSG probes. The connector and cable loss have been calibrated for the following measured results. Fig. 4a shows the measured results of the conversion loss of the W-band inductor compensated I/Q mixer, with different LO frequencies. The LO power level is +14 dBm. The conversion loss is between −6.9 and ∼−2.8 dB with a typical value of −10 dB. The corresponding IF is from DC to 12 GHz. Fig. 4b shows the measured conversion loss for different LO input power from +12 to ∼+16 dBm, when LO is fixed at 81 GHz. From the measured results, the input LO power level of +14 dBm is good enough for the mixing operation. Fig. 4c depicts the measured image rejection level with I/Q input signal at 25 MHz, under the up-conversion mode. The input I/Q signals are generated by Agilent signal generator E4438C. With good I/Q balance of the broadband Lange coupler, the measured image rejection level is above 21 dB for RF from 75 to 96 GHz, with a peak value of 37.8 dB around 86 GHz. Fig. 4d shows the measured LO to RF isolation. Due to the doubly balanced mixer topology, measured result shows very high LO-RF isolation above 43.7 dB across the whole operation band, with a peak value of 52.9 dB, which is very useful in the up-conversion application. Conclusion In this Letter, the development of a W-band inductor compensated doubly balanced I/Q mixer in 0.1 μm GaAs pHEMT process is presented. The mixer consists of two ring-diode mixer cells, Marchand balun and Lange coupler for quadrature LO input signal. An inductance is utilised to resonate with the parasitic capacitance of the ring-diode mixer core. The relationship between the inductance and the diode model parameter is theoretically analysed and derived. With the compensating inductance, the return loss of the LO port is greatly improved, thus no need for other LO matching network. Measured results prove the validity of the proposed mixer topology. Due to the doubly balanced topology, high LO-RF isolation has been observed. The designed doubly balanced I/Q mixer is suitable for high data rate communication system from 75 to 96 GHz. Acknowledgments This work was supported in part by NSFC (grant 61501114), and in part by State Key Lab of Millimeter Waves, Southeast University (grant Z201510). References 1Wells, J.A.: 'Faster than fiber: the future of multi-Gb/s wireless', IEEE Microw. Mag., 2009, 10, (3), pp. 104– 112 (https://doi/org/10.1109/MMM.2009.932081) 2Maas, S.A.: ' Microwave mixers' ( Artech House, Norwood, MA, USA, 1992, 2nd edn.) 3Chen, Z., Jiang, X., Hong, W., Chen, J.X.: ' A Q-band doubly balanced mixer in 0.15 μm GaAs pHEMT technology'. IEEE Int. Wireless Symp. (IWS2014), 2014, pp. 1– 4 4Ton, T.N., Chen, T.H., Chang, K.W. et. al.,: ' A W-band monolithic InGaAs/GaAs HEMT Schottky diode image reject mixer'. 14th IEEE Gallium Arsenide Integrated Circuit (GaAs IC) Symp., 1992, pp. 63– 66 5Gavell, M., Ferndahl, M., Gunnarsson, S.E. et. al.,: ' An image reject mixer for high-speed E-band (71–76, 81–86 GHz) wireless communication'. 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