10 GHz dual-conversion low-IF
downconverter with microwave and
analogue quadrature generators
J.-S. Syu, C. Meng and Y.-H. Teng
A 10 GHz dual-conversion low-IF downconverter using 0.18-mm CMOS technology is demonstrated. The high-frequency quadrature RF and LO1signals are generated by broadside-coupled quadrature
couplers while a two-section polyphase filter is utilised for the low-frequency LO2quadrature signal generation. As a result, the
demon-strated downconverter achieves a conversion gain of 7 dB, IP1dBof
216 dBm, IIP3of 25 dBm and noise figure of 26 dB at a 1.8 V
supply. The image-rejection ratio of the first/second image signal is 33/42 dB for IF frequency ranging from 10 to 60 MHz, respectively.
Introduction: Both direct-conversion (zero-IF) architectures [1] and low-IF architectures [2] are widely employed in wireless local area network (WLAN) systems. The passive mixer has a low flicker noise corner but the conversion loss forfeits the ability of suppressing noise contributions of the following stages. Thus, a preceding low-noise amplifier with much higher gain is necessary. On the other hand, the conversion gain of an active Gilbert mixer performs good suppression of subsequent noise contributions but the flicker noise issue becomes much serious, especially for high-frequency applications [3, 4]. Although the inductor was placed at the source nodes of the Gilbert core to resonate the parasitic capacitances, the performance was still limited [4]. Therefore, the low-IF architecture is preferred for a 10 GHz receiver because the several tens of megahertz IF band is away from the flicker noise corner. In addition, the DC offset problem can be alleviated by placing a large DC-blocking capacitor at the output. A polyphase filter generates quadrature signals with accurate phase and balanced amplitudes within a narrow bandwidth[5]; however, the parasitic capacitance and the process variation result in the shifting of the centre frequency and the degradation of the phase accuracy, especially at high frequencies. Therefore, a microwave passive quadra-ture coupler[6]is introduced in this work to generate high-frequency quadrature signals.
Circuit design: Fig. 1 shows the block diagram of a 10 GHz dual-conversion low-IF downconverter. The first downdual-conversion system consists of two stacked-LO sub-harmonic mixers with quadrature RF and LO inputs while four Gilbert mixers and a following complex polyphase filter are employed for the second downconversion.
RE_I SHM SHM IF1_I I Q LO1 LO2 polyphase quad. gen. quad. coupler quad. coupler RF C T RF_Q IF1_Q IF2_Q IF2_I (IF_Q) (IF_I) poly phase filter
Fig. 1 Block diagram of 10 GHz dual-conversion low-IF downconverter with microwave and analogue quadrature generators
The frequency relations for the dual downconversions are given by fRF¼2fLO1þfIF1¼2fLO1þfLO2þfIF ð1Þ fIM1¼2fLO1fIF1¼2fLO1fLO2fIF¼fRF2fIF1 ð2Þ fIM2¼2fLO1þfLO2fIF¼fRF2fIF ð3Þ
Because a sub-harmonic mixer is employed, the effective LO1frequency is thus twice the LO1frequency. In this work, fLO1¼ 4.175 GHz and fLO2¼ 1.615 GHz while fIF ranges from 10 to 60 MHz to avoid the effects of flicker noise. As a result, fRF, fIM1and fIM2are about 10, 6.7 and 9.93 GHz, respectively.
The conventional quadrature coupler (QC) is implemented by a quarter-wavelength coupled line[6]. As a result, the 3 dB QC is realised with a coupling factor of 1=pffiffiffi2and a description of the S-parameters is shown inFig. 2. To widen the bandwidth, the coupling factor can be chosen larger if the amplitude imbalance between the coupled and through ports is still tolerable. Note that the phase difference between these two ports of a quarter-wavelength QC under the loseless condition is always 908[6]. f0 usable BW (3dB QC) frequency S -parameter usable BW (high-coupling QC ) fbal1 fbal2 high-coupling QC 3dB QC 1 (0 dB) T C C T tolerable amp. imbalance 2f0
Fig. 2 Description of S-parameters for 3 dB quadrature coupler and high-coupling quadrature coupler
Fig. 3 Die photo of 10 GHz dual-conversion low-IF downconverter with microwave and analogue the quadrature generators
In this work, broadside-coupled 10 and 4 GHz QCs are implemented for RF and LO1quadrature signal generations, respectively. The high coupling factor (0.85) results in a significant amplitude difference at the centre frequency ( f0) and the perfect balanced amplitudes occur at two frequencies, fbal1and fbal2, as shown inFig. 2. For our application of the fixed LO frequency without the criterion of a wide operating band-width, the high coupling factor makes the usable frequency band much below the centre frequency. In other words, for a given operating fre-quency, the coupler can be very compact since the centre frequency of the coupler is several times the operating frequency. The broadside-coupled QC is realised by two spiral inductors using metal 6 and metal 5. The line width, line spacing and outer diameter of each inductor for the 10 GHz QC are 10, 3 and 120 mm, respectively. The dielectric thickness between metal 6 and metal 5 is 0.8 mm, which is much less than the 3 mm line spacing of the inductor; thus, the broadside-coupling is dominant. On the other hand, the 4 GHz QC has 10 mm line width, 3 mm line spacing, 200 mm outer diameter and the same dielectric thick-ness of 0.8 mm. From simulation, the centre frequency of 10/4 GHz QC is 25/13 GHz.
Measurement results: The die photo of the 10 GHz dual-conversion low-IF downconverter is shown inFig. 3and the die size is 1.92 1.7 mm. On-wafer measurement is employed for the RF performance. The power consumption is 80 mW at a 1.8 V supply.
The conversion gain and the image-rejection performance are shown inFig. 4and LO1and LO2power are 10 and 8 dBm, respectively. The image-rejection ratio (IRR) of the first image signal (from 6.675 to 6.725 GHz) is 33 dB covering the IF frequency from 10 to 60 MHz, and the conversion gain of the RF signal is 7 dB. On the other hand, the IRR of the second image signal (from 9.905 to 9.955 GHz) is over 40 dB. The output I/Q waveforms have 0.2 dB amplitude mis-match and 0.158 phase error. IP1dBis 216 dBm and IIP3is 25 dBm when fRF1¼ 10 GHz and fRF2¼ 10.0005 GHz. LO1-to-RF and 2LO1 -to-RF isolations are 40 and 55 dB, respectively. The single-sideband noise figure is 25 dB. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 conversion gain conversion gain, dB IF frequency, MHz 0 10 20 30 40 50
image rejection ratio of first image image rejection ratio of second image
ima
g
e-rejection ratio, dB
Fig. 4 Conversion gain and image-rejection ratio against IF frequency of 10 GHz dual-conversion low-IF downconverter with microwave and analogue quadrature generators
Conclusions: A 10 GHz dual-conversion low-IF downconverter employs microwave couplers for high-frequency quadrature signal
generation. The broadside-coupled coupler with a high coupling factor leads to the size reduction for a narrow-band application. At low fre-quencies, the polyphase filter is still chosen for not only the quadrature signal generator but also the complex filter.
Acknowledgments: This work is supported by the National Science Council of Taiwan, Republic of China, under contract numbers NSC 95-2221-E-009-043-MY3, NSC 97-2221-E-009-171 and NSC 98-2218-E-009-008-MY3, by the Ministry of Economic Affairs of Taiwan under contract number 96-EC-17-A-05-S1-020, and by MoE ATU Program under contract number 95W803. The authors thank the National Chip Implementation Center (CIC) for technical support. #The Institution of Engineering and Technology 2009
9 February 2009 doi: 10.1049/el.2009.0360
J.-S. Syu, C. Meng and Y.-H. Teng (Department of Communication Engineering, National Chiao Tung University, Hsinchu 300, Taiwan) E-mail: ccmeng@mail.nctu.edu.tw
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