876 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007
2.4/5.7 GHz Dual-Band High Linearity Gilbert
Upconverter Utilizing Bias-Offset
TCA and
LC Current Combiner
Jin-Siang Syu and Chinchun Meng, Member, IEEE
Abstract—A 2.4/5.7 GHz dual-band Gilbert upconversion mixer
is demonstrated using 0.35 m SiGe BiCMOS technology. A bias-offset cross-coupled transconductance amplifier (TCA) is employed in the intermediate frequency port for the linearity im-provement. The dual-band current combiner and the output shunt-shunt feedback buffer amplifier are in the radio frequency (RF) port. The mechanisms of the high linearity upconverter and the design flow of the dual-band current combiner are estab-lished in this letter. The dual-band upconverter has conversion gain of 1.5 0.2 dB,OP1dB of 10.5 9 dBm, andOIP3 of 12/13 dBm for IF=100 MHz, RF= 2.4/5.7 GHz, respectively.
Index Terms—Dual-band, Gilbert mixer, current combiner, shunt–shunt feedback, SiGe heterojunction bipolar transistor (HBT), transconductance amplifier (TCA), WLAN.
I. INTRODUCTION
F
OR wireless communication systems, the current trend is toward multistandards/multiservices, and thus it brings multiband circuit design on a single chip, especially for dual-band WLAN applications. Traditionally, it can be procured by using a dual-band filter cascaded after radio frequency (RF) output stage at the cost of extra loss. In this letter, a dual-band upconversion mixer is demonstrated with a dual-band resonator load followed by the Gilbert mixer core to achieve both the operating frequencies of 2.4/5.7 GHz. Linearity is another important issue in communication systems, especially in transmitters. However, a conventional Gilbert mixer with emitter-coupled (source-coupled) pair input stage suffers from a poor linearity problem that OIP is about 10 dB larger than OP [1], [2] and therefore the signals of adja-cent channels easily interfere with the desired channel. Many ideas are utilized to improve linearity. For example, multitanh approach [3] and class-AB transconductance [4] are designed to provide more linear transfer function of the transconductor by eliminating high order terms of the exponential function of bipolar transistors or the short channel effect of the metal oxide semiconductor (MOS) transistors. Feedback technique is also widely employed by severely suppressing high order distortion at the cost of gain [5], such as emitter degeneration.Manuscript received May 23, 2007; revised August 9, 2007. This work is sup-ported by the National Science Council of Taiwan, R.O.C., under Contracts NSC 96-2752-E-009-001-PAE and NSC 95-2221-E-009-043-MY3, by the Ministry of Economic Affairs of Taiwan under Contract 95-EC-17-A-05-S1-020, and by MoE ATU Program under Contract 95W803.
The authors are with the Department of Communication Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: ccmeng@mail.nctu.edu.tw).
Digital Object Identifier 10.1109/LMWC.2007.910504
Fig. 1. Block diagram of the dual-band Gilbert upconverter.
Fig. 2. Schematic of the SiGe BiCMOS dual-band Gilbert upconverter with the bias-offset cross-coupled TCA and dual-bandLC current combiner.
The block diagram of the dual-band upconversion Gilbert mixer is illustrated in Fig. 1. A Gilbert mixer core is employed to commutate with local oscillation (LO) frequency to produce while is generated by a bias-offset TCA (transconduc-tance amplifier) input stage. The differential current passes through the passive dual-band current combiner with extra gain improvement [6] and then a wideband transimpedance am-plifier (TIA) translates the output current to the voltage signal and achieves output matching simultaneously.
The passive resonator and the Gilbert cell switching mechanism are both inherently linear. Thus, a bias-offset cross-coupled TCA with linear transfer characteristic [7] and a shunt-shunt feedback TIA with strong feedback factor are implemented in the intermediate frequency (IF) input port and RF output port for linearity improvement in this work. Consequently, the upconverter implemented in this letter has 22 dB difference between OIP and OP . Furthermore, the dual-band current combiner is proposed and demonstrated for the first time to the best of our knowledge.
II. CIRCUITDESIGN
The schematic of the 2.4/5.7 GHz dual-band SiGe BiCMOS Gilbert upconversion mixer is shown in Fig. 2. The upcon-verter in Fig. 2 consists of a SiGe HBT LO Gilbert mixer core
SYU AND MENG: 2.4/5.7 GHZ DUAL-BAND HIGH LINEARITY GILBERT UPCONVERTER 877
Fig. 3. Photograph of the SiGe BiCMOS dual-band Gilbert upconverter with the bias-offset cross-coupled TCA and dual-bandLC current combiner.
( , and ), an IF bias-offset CMOS TCA - ), and an RF output dual-band current combiner with a SiGe HBT TIA output buffer ( and ).
The photograph of the dual-band upconverter is shown in Fig. 3. The die size is 1.13 0.97 mm . In the dual-band current combiner, a five-square-turn symmetric inductor is 3 m line width, 3 m line spacing and outer diameter of 120 m while a center-tapped symmetric differential inductor (consisting of and ) is composed of a five-square-turn with line width, spacing and outer diameter of 4, 2.5, and 120 m, respectively.
III. LINEARITYMECHANISMS OF THEGILBERTUPCONVERTER
An upconversion mixer is formed by a TCA, a current switching Gilbert core, a passive resonator and an output TIA. The current commutation is a highly linear process because it only translates the IF signal to the RF signal with every odd order LO frequencies while the passive resonator does not suffer from any nonlinear effect. Consequently, the input TCA and output TIA play important roles for the linearity issue. To achieve high linearity in this work, a bias-offset cross-coupled TCA is implemented in the IF port while the shunt-shunt feedback TIA is in the RF port.
The transconductance of the TCA equals to 2 [7] as long as the NMOS I-V characteristics is in the square-law long channel region, where k is the transconductance parameter of - and is the gate-source dc voltage drop of . The NMOS devices - are biased at a small gate overdrive voltage and the gate lengths of the MOS transistors are 0.5 m to mitigate the short channel effect. By ADS simulation, the output impedance of the dual-band resonator at resonant frequency of 2.4 and 5.7 GHz are both 120 . A shunt-shunt feedback TIA is employed in the output stage to achieve output matching. A wideband TIA with the 3 dB bandwidth of 20 GHz is designed because strong feedback results in high linearity improvement. Passive matching output network is another choice for high lin-earity performance [8].
IV. ANALYSIS OF THEDUAL-BANDLC CURRENTCOMBINER
The schematic of the -shaped dual-band LC current com-biner is illustrated in Fig. 4. At low frequencies, the series branch is dominated by 1/j while the shunt branch is
dominated by 1/j , here . On the
other hand, the series and shunt branches are dominated by and at high frequencies, respectively. Therefore,
Fig. 4. Operational principle of the dual-bandLC current combiner.
Fig. 5. Frequency response of a dual-bandLC current combiner.
the -shaped dual-band current combiner can be treated implicitly as a combination of a low-frequency bandpass , and resonator and a high-frequency band-pass resonator as shown in Fig. 5.
The combination of the LC current combiner and the current source can be represented by its Norton equivalence and as shown in Fig. 4. The can be related to the ABCD matrix elements of the dual-band current combiner.
When the element D of the ABCD matrix equals 1
(1) is achieved, and therefore the equivalent total output current doubles. The roots of (1) and are found as
(2) where
(3)
(4) The design procedure of the dual-band current combiner is as follows: a) Define the dual band frequencies and , and thus the center frequency and difference frequency are specified by (3) and (4). b) Choose proper inductors ( and ) with high qualify factor at the desired frequencies. c) Calculate the value of and by known and .
878 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007
Fig. 6. RF frequency response of the SiGe BiCMOS dual-band Gilbert upcon-verter with the bias-offset cross-coupled TCA and dual-bandLC current com-biner.
Fig. 7. Power performance of the SiGe BiCMOS dual-band Gilbert upcon-verter with the bias offset cross-coupled TCA and dual-bandLC current com-biner. The tone spacing is 1 MHz in two-tone power measurement.
In this work, the two desired frequencies are 2.4 and 5.7 GHz. As a result, the designed values of , and are 4.6 nH, 2.1 nH, 0.4 pF, and 0.88 pF, respectively. The relation of is chosen for a simple design condition.
V. MEASUREMENTRESULTS
The SiGe BiCMOS 2.4/5.7 GHz dual-band upconverter facilitates on-wafer rf measurements. The supply voltage is 3.5 V and the current consumption of the mixer core with the common-drain-configured and input buffers is 6.95 mA.
In Fig. 6, the measured peak conversion gain at 2.4 GHz and 5.7 GHz is 1.5 and 0.2 dB, respectively, with 3 dB band-width of 750 MHz when LO pumping power is 5 dBm. The measured conversion gain varies within 1 dB when LO power changes from 12 dBm to 4 dBm with RF 2.4 GHz, LO 2.3 GHz and IF 100 MHz while it changes from 6 dBm to 5 dBm with RF 5.7 GHz, LO 5.6 GHz and IF 100 MHz. The output RF return loss is better than 13 dB at both 2.4/5.7 GHz. The power performance of the dual-band upcon-verter is illustrated in Fig. 7. It has OP of 10.5 9 dBm, and OIP of 12/13 dBm when input IF 100 MHz, RF 2.4 and 5.7 GHz, respectively. The measured LO–RF isolation is 38/43 dB when LO 2.3 and 5.6 GHz, respectively.
TABLE I
COMPARISON OF THEUP-CONVERSIONGILBERTMIXERS
The difference between OIP and OP can be used to in-dicate the linearity and our work shows excellent linearity of 22 dB difference between OIP and OP . The state-of-the-art Gilbert upconverters are summarized in Table I [1], [2], [6], [9] for performance comparison purpose.
VI. CONCLUSION
A 2.4/5.7 GHz dual-band high linearity Gilbert upconverter is demonstrated using 0.35 m SiGe BiCMOS technology. The input TCA and output TIA dominate the linearity performance of the whole mixer. Hence, a bias-offset cross-coupled TCA and the shunt-shunt feedback buffer are utilized in IF and RF port, respectively. The dual-band upconvesion mixer has conversion gain of 1.5 0.2 dB, OP of 10.5 9 dBm, and OIP of 12/13 dBm for IF 100 MHz, RF 2.4 and 5.7 GHz, respec-tively.
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