模拟与数字 IQ 合成：为您的射频系统选择正确路径

I 和 Q 合成该如何做？模拟还是数字？本文探讨模拟与数字 IQ 方法的基础。

模拟 IQ 调制器（发射端）和 IQ 解调器（接收端）已使用数十年（[1] 到 [3]）。

近期，新型 A/D 和 D/A 转换器问世，可直接对 1–4 GHz 的 IF 进行采样；在第 2、3、4 奈奎斯特区采样（[4] 到 [7]）。这些结合更高速数字逻辑，允许合成（A/D 用）和分离（D/A 用）在数字域完成（[8] 到 [21]）。图 1(a)（调制器）和图 1(b)（解调器）中数据转换器（DAC 或 ADC）位于位置「D」所示。

 

图 1(a). 调制器

 

图 1(b). 解调器

 

另一方面，集成模拟 I、Q 合成器和分离器在 I、Q 通道间有极佳匹配，解决了模拟方式的部分异议。模拟技术虽需双倍数据转换器（A/D 或 D/A），但采样率较低；因此更便宜、功耗更低。图 1(a)（调制器）和图 1(b)（解调器）中数据转换器位于位置「A」所示。

作者开始思考此问题。他在多个 LinkedIn 群组征求意见，获得宝贵回复。经确认同意，致谢如下。他还尽力搜集当代集成电路（IC）相关功能特性，及这些 IC 确定的性能要求。由此尝试得出通用结论，回答：「IQ 调制与解调该用模拟还是数字？」

 

模拟 IQ 方法

模拟 IQ 方法已存在数十年（[1] 到 [3]）。任意 IF 或 RF 信号可表示为

 

R(t) = I(t)cos(2πft) + Q(t)sin(2πft)

 

其中 f 为载波频率，I(t) 称同相分量，Q(t) 称正交分量。模拟 IQ 调制器接收基带信号 I(t)、Q(t) 形成 R(t)。图 1(a) DAC 位置 A 所示。模拟 IQ 解调器以 R(t) 为输入，形成 I(t)、Q(t)。图 1(b) DAC 位置 A 所示。

模拟方法的关键问题是保持两路增益完全一致、相位差精确 90°。常被忽视的是两个低通滤波器。它们应在所有有显著信号能量的频率上精确增益和相位匹配。对这些要求的更精确量化，及偏离造成的损伤，将在后续文章展示。

 

数字 IQ 方法

高速数据转换器（DAC 和 ADC）的最新进展，让人们通过数字化实现 IQ 调制器和解调器功能，规避模拟 IQ 方法节讨论的 IQ 不平衡问题，在数字域增益和相位可零误差产生（[5]、[8] 到 [21]）。调制器情况下，输出端有高速 DAC，如图 1(a) DAC 位置 D 所示。解调器情况下，输入端有高速 ADC，如图 1(b) ADC 位置 B 所示。

这些数字方法常利用混叠效应，使用所谓带通采样（[22] 到 [24]、[24A]、[24B]）。图 2(a) 显示时域采样波形。图 2(b) 显示未采样与采样信号频谱。ADC 采样时钟执行与射频混频器中本振相同的功能。对 ADC，模拟滤波器可仅允许某奈奎斯特区信号通过，此混频作用可将该区信号下变频至基带。

 

图 2(a). 时域采样

 

图 2(b). 未采样与采样信号频谱

 

对 DAC，输出可在时域整形以提升高频性能。

 

图 3(a) 显示「正常」或「非归零」（NRZ）DAC 输出。每采样后输出保持恒定至下一采样。模拟频谱见图 3(b)。

 

图 3(a). 时域采样

 

图 3(b).

 

图 4(a) 显示「归零」（RZ）DAC 输出。每采样后输出保持半个采样周期恒定，再归零。这增加第二奈奎斯特区幅度，见图 4(b)。

 

图 4(a). 时域采样

 

图 4(b).

 

图 5(a) 显示「混合」或「射频」DAC 输出。每采样后输出保持半周期恒定，再变为该值负数。这等同于使用本振波形两极性的混频器操作。模拟频谱图 5(b) 在第二奈奎斯特区有更大幅度。波形经上述任一方法生成后，需用低通或带通滤波器滤出所需频率，移除不需要的混叠和杂散响应。

 

图 5(a). 时域采样

 

图 5(b).

 

数字方法避免正交不平衡问题。但所有数据转换器因量化和采样效应有自身非期望损伤。部分效应将在下文展示。这些高速数据转换器的成本和功耗也常高于模拟 IQ 网络。

 

致谢

当本报告涉及的问题初现作者脑海时，他通过 LinkedIn 群组征询意见。收到多条有用回复。允许使用个人信息者：Gary Kaatz、Khaled Sayed (Consultix-Egypt)、Dieter Joos (ON Semiconductor)、Jaideep Bose (Asmaitha Wireless Technologies)。作者也感谢妻子 Elizabeth，她大概纳闷丈夫在家庭办公室关起门来做明显没报酬的工作。

 

参考文献

以下参考文献将用于本系列各文章。

 

模拟 IQ 调制器和解调器：一般描述

[1] Shou-Hsien Weng; Che-Hao Shen; Hong-Yeh Chang, "A wide modulation bandwidth bidirectional CMOS IQ modulator/demodulator for microwave and millimeter-wave gigabit applications," Microwave Integrated Circuits Conference (EuMIC), 2012 7th European , vol., no., pp.8,11, 29-30 Oct. 2012

[2] Eamon Nash; "Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity "; Analog Devices Application Note AN-1039; 2009

[3] Anon; "An IQ Demodulator-Based IF-to-Baseband Receiver with IF and Baseband Variable Gain and Programmable Baseband Filtering "; Analog Devices Circuit Note CN-0320; 2013

 

高速数据转换器（DAC 和 ADC）；一般信息

[4] Justin Munson; "Understanding High Speed DAC Testing and Evaluation"; Analog Devices Application Note AN-928; 2013

[5] Engel, G.; Fague, D.E.; Toledano, A, "RF digital-to-analog converters enable direct synthesis of communications signals," Communications Magazine, IEEE, vol.50, no.10, pp.108, 116, October 2012

[6] Chris Pearson; "High Speed, Digital to Analog Converters Basics"; Texas Instruments Application Report SLAA523A; 2012

[7] Alex Arrants, Brad Brannon and Rob Reeder; "Understanding High Speed ADC Testing and Evaluation"; Analog Devices Application Note AN-835, 2010.

 

数字 IQ 调制器和解调器

[8] Samueli, H.; Wong, B.C., "A VLSI architecture for a high-speed all-digital quadrature modulator and demodulator for digital radio applications," Selected Areas in Communications, IEEE Journal on, vol.8, no.8, pp.1512,1519, Oct 1990

[9] Wong, B.C.; Samueli, H., "A 200-MHz all-digital QAM modulator and demodulator in 1.2-nm CMOS for digital radio applications," Solid-State Circuits, IEEE Journal of, vol.26, no.12, pp.1970, 1980, Dec 1991

[10] Ken Gentile; "Digital Quadrature Modulator Gain "; Analog Devices Application Note AN-924; 2009

[11] Lou, J. H.; Kuo, J.B., "A 1.5-V CMOS all-N-logic true-single-phase bootstrapped dynamic-logic circuit suitable for low supply voltage and high-speed pipelined system operation," Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on, vol.46, no.5, pp.628,631, May 1999

[12] Vankka, J.; Sommarek, J.; Ketola, J.; Teikari, I; Halonen, K. A I, "A digital quadrature modulator with on-chip D/A converter," Solid-State Circuits, IEEE Journal of, vol.38, no.10, pp.1635, 1642, Oct. 2003

[13] Yanlin Wu; Dengwei Fu; Willson, A, "A 415 MHz direct digital quadrature modulator in 0.25-nm CMOS", Custom Integrated Circuits Conference, Proceedings of the IEEE 2003, vol., no., pp.287,290, 21-24 Sept. 2003

[14] Sommarek, J.; Vankka, J.; Ketola, J.; Lindeberg, J.; Halonen, K., "A digital modulator with bandpass delta-sigma modulator," Solid-State Circuits Conference, 2004. ESSCIRC 2004. Proceeding of the 30th European, pp.159, 162, 21-23 Sept. 2004

[15] Lin, P.F.; Lou, J. H.; Kuo, J.B., "A CMOS quadrature modulator for wireless communication IC," Circuits and Systems I: Fundamental Theory and Applications, IEEE Transactions on, vol.44, no.6, pp.559, 561, Jun 1997

[16] Parikh, V.K.; Balsara, P.T.; Eliezer, O.E., "All Digital-Quadrature-Modulator Based Wideband Wireless Transmitters," Circuits and Systems I: Regular Papers, IEEE Transactions on, vol.56, no.11, pp.2487, 2497, Nov. 2009

[17] Alavi, M.S.; Staszewski, R.B.; de Vreede, L.C.N.; Long, J.R., "A Wideband13-bit All-Digital I/Q RF-DAC," Microwave Theory and Techniques, IEEE Transactions on, vol.62, no.4, pp.732, 752, April 2014

[18] Inkol, Robert and Saper, Ron; "Digital Quadrature Modulator for Radar ESM Applications" Canadian DEFENCE RESEARCH ESTABLISHMENT OTTAWA TECHNICAL NOTE 92-10; 1992

[19] Ziomek, C.; Corredoura, P., "Digital I/Q demodulator," Particle Accelerator Conference, 1995., Proceedings of the 1995 , vol.4, no., pp.2663,2665 vol.4,1-5 May 1995

[20] Ho, K.C.; Chan, Y.T.; Inkol, R., "A digital quadrature demodulation system," Aerospace and Electronic Systems, IEEE Transactions on , vol.32, no.4,pp.1218,1227, Oct 1996

[21] Bravo, A; Cruz-Roldan, F., "Digital quadrature demodulator with four phases mixing for digital radio receivers," Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on, vol.50, no.12, pp.1011,1015, Dec. 2003

 

带通采样（Rev .04 将「次谐波采样」改为「带通采样」）

[22] Parssinen, A; Magoon, R.; Long, S.I; Porra, Veikko, "A 2-GHz subharmonic sampler for signal downconversion," Microwave Theory and Techniques, IEEE Transactions on, vol.45, no.12, pp.2344, 2351, Dec 1997

[23] Jensen, B.S.; Schmidl Sobjaerg, S.; Skou, N.; Krozer, V., "Compact front-end prototype for next generation RFI-rejecting polarimetric L-band radiometer," Microwave Conference, 2009. EuMC 2009. European, vol., no., pp.1626, 1629, Sept. 29 2009-Oct. 1 2009

[24] Ahmed, S.; Saad El Dine, M.; Reveyrand, T.; Neveux, G.; Barataud, D.; Nebus, J. M., "Time-domain measurement system using Track & Hold Amplifier applied to pulsed RF characterization of high power GaN devices," Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International , vol., no., pp.1,4, 5-10 June 2011

[24A] Akos, D.M.; Stockmaster, M.; Tsui, J. B Y; Caschera, J., "Direct bandpass sampling of multiple distinct RF signals," Communications, IEEE Transactions on , vol.47, no.7, pp.983,988, Jul 1999

[24B] Ching-Hsiang Tseng; Sun-Chung Chou, "Direct downconversion of multiple RF signals using bandpass sampling," Communications, 2003. ICC '03. IEEE International Conference on , vol.3, no., pp.2003,2007 vol.3, 11-15 May 2003

 

IQ 不平衡效应，无补偿或利用提议

[25] Lopez-Martinez, F.J.; Martos-Naya, E.; Paris, J.F.; Entrambasaguas, J.T., "Exact Closed-Form BER Analysis of OFDM Systems in the Presence of IQ Imbalances and ICSI," Wireless Communications, IEEE Transactions on, vol.10, no.6, pp.1914, 1922, June 2011

[26] Yaning Zou; Valkama, M.; Renfors, M., "Performance Analysis of Space-Time Coded MIMO-OFDM Systems Under I/Q Imbalance," Acoustics, Speech and Signal Processing, 2007. ICASSP 2007. IEEE International Conference on, vol.3, no., pp.III-341, III-344, 15-20 April 2007

[27] Chia-Liang Liu, "Impacts of I/Q imbalance on QPSK-OFDM-QAM detection," Consumer Electronics, IEEE Transactions on, vol.44, no.3, pp.984, 989, Aug 1998

[28] Heung-Gyoon Ryu, "Diversity Effect of OFDM Communication with IQ Imbalance in the Rayleigh Fading Channel," Communication Software and Networks, 2010.ICCSN '10. Second International Conference on, vol., no., pp.489,493, 26-28 Feb. 2010

[29] Stroet, P.; "Measuring Phase and Delay Errors Accurately in I/Q Modulators"; Linear Technology Application Note 102; AN102-1; October 2005

7.6 IQ 不平衡效应，有补偿或利用提议

[30] Tarighat, A; Sayed, AH, "Joint compensation of transmitter and receiver Impairments in OFDM systems," Wireless Communications, IEEE Transactions on, vol.6, no.1, pp.240, 247, Jan. 2007

[31] Marey, Mohamed; Steendam, Heidi, "Novel Data Detection and Channel Estimation Algorithms for BICM-OFDMA Uplink Asynchronous Systems in the Presence of IQ Imbalance," Wireless Communications, IEEE Transactions on , vol.13, no.5, pp.2706,2716, May 2014

[32] Narasimhan, B.; Narayanan, S.; Minn, H.; Al-Dhahir, N., "Reduced-complexity baseband compensation of joint Tx/Rx I/Q imbalance in mobile MIMO-OFDM," Wireless Communications, IEEE Transactions on , vol.9, no.5, pp.1720,1728, May 2010

[33] Ozdemir, O.; Hamila, R.; Al-Dhahir, N., "I/Q Imbalance in Multiple Beamforming {OFDM} Transceivers: SINR Analysis and Digital Baseband Compensation," Communications, IEEE Transactions on, vol.61, no.5, pp.1914, 1925, May 2013

[34] Inamori, M.; Bostamam, AM.; Sanada, Y.; Minami, H., "IQ imbalance compensation scheme in the presence of frequency offset and dynamic DC offset for a direct conversion receiver," Wireless Communications, IEEE Transactions on , vol.8, no.5, pp.2214,2220, May 2009

[35] Tarighat, A; Sayed, AH., "MIMO OFDM Receivers for Systems With IQ Imbalances," Signal Processing, IEEE Transactions on , vol.53, no.9, pp.35833596, Sept. 2005

[36] Hai Lin; Yamashita, K., "Subcarrier allocation based compensation for carrier frequency offset and I/Q imbalances in OFDM systems," Wireless Communications, IEEE Transactions on, vol.8, no.1, pp.18,23, Jan. 2009

7.7 基带 DAC 和 ADC 要求

[37] Suno-Won Chung; Seung-Yoon Lee; Kyu-Ho Park, "An energy-efficient OFDM ultra-wideband digital radio architecture," Signal Processing Systems, 2004. SIPS 2004. IEEE Workshop on, vol., no., pp.211, 216, 13-15 Oct. 2004

 

射频 DAC/ADC 要求；射频非线性要求

[38] de Mateo Garcia, J.C.; Armada, AG., "Effects of bandpass sigma-delta modulation on OFDM signals," Consumer Electronics, IEEE Transactions on , vol.45, no.2, pp.318,326, May 1999

[39] Maurer, L.; Schelmbauer, W.; Pretl, H.; Springer, A; Adler, B.; Boos, Z.; Weigel, R., "Influence of receiver front end nonlinearities on W-CDMA signals," Microwave Conference, 2000 Asia-Pacific, vol., no., pp.249, 252, 2000

[40] Kitaek Bae; Changyong Shin; Powers, E.J., "Performance Analysis of OFDM Systems with Selected Mapping in the Presence of Nonlinearity," Wireless Communications, IEEE Transactions on , vol.12, no.5, pp.2314,2322, May 2013

[41] Mahim Ranjan; Larson, L.E., "Distortion Analysis of Ultra-Wideband OFDM Receiver Front-Ends," Microwave Theory and Techniques, IEEE Transactions on, vol.54, no.12, pp.4422, 4431, Dec. 2006

 

7.9 LTE-Advanced 载波聚合；宽带频谱要求。

[42] Pedersen, K.I; Frederiksen, F.; Rosa, C.; Nguyen, H.; Garcia, L.G.U.; Yuanye Wang, "Carrier aggregation for LTE-advanced: functionality and performance aspects," Communications Magazine, IEEE , vol.49, no.6, pp.89,95, June 2011