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AD9765AST 数据表(PDF) 32 Page - Analog Devices |
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AD9765AST 数据表(HTML) 32 Page - Analog Devices |
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32 / 44 page  AD9763/AD9765/AD9767 Data Sheet Rev. G | Page 32 of 44 QUADRATURE AMPLITUDE MODULATION (QAM) EXAMPLE USING THE AD9763 QAM is one of the most widely used digital modulation schemes in digital communications systems. This modulation technique can be found in FDM as well as spread spectrum (that is, CDMA) based systems. A QAM signal is a carrier frequency that is modulated in both amplitude (that is, AM modulation) and phase (that is, PM modulation). It can be generated by independently modulating two carriers of identical frequency but with a 90° phase difference. This results in an in-phase (I) carrier component and a quadrature (Q) carrier component at a 90° phase shift with respect to the I component. The I and Q components are then summed to provide a QAM signal at the specified carrier frequency. A common and traditional implementation of a QAM modulator is shown in Figure 83. The modulation is performed in the analog domain in which two DACs are used to generate the baseband I and Q components. Each component is then typically applied to a Nyquist filter before being applied to a quadrature mixer. The matching Nyquist filters shape and limit each component’s spectral envelope while minimizing intersymbol interference. The DAC is typically updated at the QAM symbol rate, or at a multiple of the QAM symbol rate if an interpolating filter precedes the DAC. The use of an interpolating filter typically eases the implementation and complexity of the analog filter, which can be a significant contributor to mismatches in gain and phase between the two baseband channels. A quadrature mixer modulates the I and Q components with the in-phase and quadrature carrier frequency and then sums the two outputs to provide the QAM signal. QUADRATURE MODULATOR DAC 10 10 DAC CARRIER FREQUENCY NYQUIST FILTERS TO MIXER DSP OR ASIC Σ 0° 90° Figure 83. Typical Analog QAM Architecture In this implementation, it is much more difficult to maintain proper gain and phase matching between the I and Q channels. The circuit implementation shown in Figure 84 helps improve the matching between the I and Q channels, and it shows a path for upconversion using the AD8346 quadrature modulator. The AD9763 provides both I and Q DACs a common reference that improves the gain matching and stability. RCAL can be used to compensate for any mismatch in gain between the two channels. The mismatch can be attributed to the mismatch between RSET1 and RSET2, the effective load resistance of each channel, and/or the voltage offset of the control amplifier in each DAC. The differential voltage outputs of both DACs in the AD9763 are fed into the respective differential inputs of the AD8346 via matching networks. AD9763/ AD9765/ AD9767 IOUTA IOUTB IOUTA IOUTB DCOM1/ DCOM2 DVDD1/ DVDD2 AVDD VPBF BBIP BBIN BBQP BBQN LOIP LOIN VOUT WRT1/IQWRT ACOM + SPECTRUM ANALYZER AD8346 CLK1/IQCLK I DAC WRT2/IQSEL CFILTER VDIFF = 1.82V p-p RL RL RB RB RB RL RL RL RL LA LA LA LA RL CA CA RB RA RA RA RL RB RA 0 TO IOUTFS AD8346 AVDD AD976x AVDD TEKTRONIX AWG2021 WITH OPTION 4 I DAC LATCH Q DAC LATCH Q DAC NOTES 1. DAC FULL-SCALE OUTPUT CURRENT = IOUTFS. 2. RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE FROM OHMTEK ORNXXXXD SERIES OR EQUIVALENT. VMOD VDAC DIFFERENTIAL RLC FILTER RL = 200Ω RA = 2500Ω RB = 500Ω RP = 200Ω CA = 280pF CB = 45pF LA = 10µH IOUTFS = 11mA AVDD = 5.0V VCM = 1.2V RL CB 0.1µF RA CB PHASE SPLITTER ROHDE & SCHWARZ FSEA30B OR EQUIVALENT ROHDE & SCHWARZ SIGNAL GENERATOR SLEEP FSADJ1 FSADJ2 MODE REFIO 2kΩ 20kΩ 0.1µF 256Ω 22nF 2kΩ 20kΩ 256Ω 22nF Figure 84. Baseband QAM Implementation Using an AD9763 and an AD8346 |
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