radio transmitter architectures trends for infrstructure'

a Radio Transmitter Architectures Trends for Infrastructure Analog Devices, Inc. January 1999 a Key Issues for Tx Architecture • Channel Filtering (Pulse Shaping) requirements – standards specific • Transmit mask – filters must clean up any weaknesses in the DAC or mixers (SFDR, Adjacent Channel Noise) • In-band quality of transmitted signal – EVM, CNR • Frequency Plans – IF and RF frequencies and image problems • Partitioning of chips and boards – digital vs. analog?, baseband vs. IF, or RF?, which board?. • Changing System goals – Multi-mode radio? Phased-Array system? Micro- or Pico- cells? a Transmit Architectures “I” DATA “Q” DATA F c LPF LPF “I” DAC “Q” DAC Serial/Parallel Encoder Serial Input Data + x x 0 90 Quadrature Upconverter Symbol Clock LO I(t) Q(t) Q LPF (t) I LPF (t) Baseband, Analog Up-conversion Digital Upconverter Digital Modulation & Up-conversion with Direct IF Out Digital I & Q, Multi-carrier TX I Q w c LPF LPF “I” DAC “Q” DAC Serial/Parallel Encoder Serial Input Data 0100 1000 + 0 90 Quadrature Upconverter N x Symbol Clock LO I(t) Q(t) Q LPF (t) I LPF (t) Digital Interpolation Filter x N Baseband with Digital Interpolation, Analog I & Q X X Serial Input Data 0100 1000 Serial/ Parallel Encoder + x x 0 o NCO N x 90 o N x DAC LPF Pulse Shape Pulse Shape + CH 1 CH 2 CH 3 CH ‘N’ DAC LPF DUC Format, Pulse Shape DUC Format, Pulse Shape DUC Format, Pulse Shape DUC Format, Pulse Shape a Traditional TX Architecture Nyquist Baseband Quadrature Modulator • Serial input data grouped into “Symbols” of M bits • M data bits of symbol split evenly into separate I and Q data paths with N bits of resolution ( i.e. N=M/2) – QPSK - 2 bits/symbol requires a 1-bit D/A (i.e. comparator) – 64-QAM - 6 bits/symbol requires 3-bit D/A • Each DAC produces PAM output (i.e. stepped output response) which is band-limited by matching Low Pass Filters (LPF) • Quadrature upconverter translates to high IF or directly to RF “I” DATA “Q” DATA F c LPF LPF “I” DAC “Q” DAC Serial/Parallel Encoder Serial Input Data 0100 1000 + x x 0 90 Quadrature Upconverter Symbol Clock LO I(t) Q(t) Q LPF (t) I LPF (t) QAM at IF or RF is DSB-SC F C Main Lobe Side Lobe 2 x F SYMBOL a Nyquist Baseband, Analog I & Q: Simplest, conventional approach: • pulse shaping may be done with multi-pole, complex analog filters • need good gain match between I and Q converters and all post filters: pole mismatch creates constellation distortion. • need good offset match between converters and post filters to avoid LO feedthrough. • analog filters clean up harmonics and out-of-band problems from converters: converter sampling rate and analog performance requirements are modest. • Most of the work is done in the analog signal processing domain. + x x a Dual DACs for TX FEATURES • 14-Bit Dual Transmit DAC • 125 MSPS Update Rate • SFDR and IMD: 75 dB • Gain and Offset Matching: <0.5% • Dual port or interleaved data • On-chip 1.2 V Reference • Single +5 V or +3 V Supply Operation • Power Dissipation: <400 mW @ 5 V • Power Down Mode: 50 mW @ 5 V • 48-Lead QFP • Can be used with AD8346 Quadrature Modulator for direct quadrature up-conversion to up to 2.5 GHz AD9767 + x x I Q a What is Interpolation? •The synthesis of “new” intermediate data samples via a digital filter based on weighted average of previous data samples FUNDAMENTAL FREQUENCY DOMAIN TIME DOMAIN FUNDAMENTAL DIGITAL FILTER RESPONSE 1 st IMAGE "NEW" 1 st IMAGE "SURPRESSED" 1 st IMAGE 2f CLOCK 4f CLOCK 2f CLOCK 4f CLOCK 1/4f CLOCK 1/f CLOCK 2f CLOCK 4f CLOCK f CLOCK 4 X f CLOCK 4 X 4 x INTERPOLATION FILTER INPUT DATA LATCH DAC DAC'S SIN(X)/X RESPONSE 3 NEW SAMPLES CREATED FOR EACH ORIGINAL SAMPLE NOTE DAC'S STEPPED OUTPUT RESPONSE 5 vs. 20 Samples per Sinewave Cycle a Comparing 14-bit TxDAC’s w/ and w/o Interpolation in the TIME DOMAIN AD9774 TxDAC w/ 4X Interpolation AD9764 Note: AD9774 produces four samples for every one sample of the AD9764! a Comparing 14-bit TxDAC’s w/ and w/o Interpolation in the FREQUENCY DOMAIN -90 -80 -70 -60 -50 -40 -30 -20 0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 2.50E+07 3.00E+07 3.50E+07 4.00E+07 Frequency(Hz) AD9774 w/ 4X Interpolation AD9764 w/o Interpolation AFTER -90 -80 -70 -60 -50 -40 -30 -20 0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 2.50E+07 3.00E+07 3.50E+07 4.00E+07 Frequency(Hz) BEFORE Original Sin(x)/x Response “Shaped” by Digital Filter Sin(x)/x Response to random input data 9-Pole LPF 3-Pole LPF a Interpolated Baseband with analog I &Q Modulator • Digital Interpolation Filter simplifies analog low pass filters (LPF) • Complex “Nyquist Filters” best implemented via digital filter • DAC resolution as well as its AC and DC performance now critical in meeting specified spectral mask requirement • Resolution of Digital Filter and DAC determined by Spectral Mask requirements (i.e. suppression of sin(x)/x sidebands) • DACs with on-chip interpolation filters (AD9761, AD9774) I Serial/Parallel Encoder Serial Input Data 0100 1000 Q w c LPF LPF “I” DAC “Q” DAC + x x 0 90 Quadrature Upconverter N x Symbol Clock LO I(t) Q(t) Q LPF (t) I LPF (t) Digital Interpolation Filter x N [...]... each channel before summing (redundant) • Equalizer occurs in analog domain a Σ AD6622 I&Q DAC or Real DAC • AD6622 Filters all channels after summing • Equalizer performed digitally in AD6622 29 Next Generation TxDACs: AD975X • Improved SFDR Performance – >6dB Improvement Over First Gen (+3V) TxDACs at 20MHz Output – Needed to support multi-carrier TX architecture • Improved INL/DNL Performance (14-bit)... but we use the NCO to do the frequency hopping: • no more complicated than for a fixed IF, HOWEVER, IF filtering must now be broadband, so it can no longer clean up the close in DAC spurs: DAC performance must meet the Tx mask requirement across the hopping band • A very attractive way of realizing the frequency hop IF the DAC performance is good enough a Spectral Output of Wideband Tx Architecture -20... even more of the work digitally, but analog performance requirements for the DAC becoming more severe a Digital Modulation & Up-conversion with Direct IF out “I” DATA Serial Input Data 0100 1000 • • • • Serial /Parallel Encoder Pulse Shape Quadrature Upconverter “I” DATA X Nx 0o “Q” DATA Pulse Shape NCO “Q” DATA 90o Nx + DAC LPF X Quadrature mix is now performed in digital domain I & Q Match is “perfect”... data PLL Clock Generator 1X clk 14 EdgeTriggered Latch 14 2X clk 4X clk 14 2 14 2 AD9774 14-Bit DAC AD9774 vs Stand-Alone DACs Note: Improved SFDR performance at high Fout Due to aliased harmonics falling out-of-band AD9774, AD9764, and HI5741 0dBFS SFDR Performance at 32 MSPS 85 80 SFDR-dBc 75 AD9774 AD9764 HI5741 70 65 M u ltito n e P e r fo r m a n c e A D 9 7 6 4 a n d A D 9 7 7 4 v s H I5741 60 80.0... TX The “big win”, one radio replaces many: • multiple digital upconverters put each carrier at a slightly different IF frequency • digital IFs are summed and put through a single DAC • DAC IF can be up to around 1/3 the master clock • DAC must now handle multi-carriers: more Gaussian energy distribution • As with agile narrowband, no ability to clean up DAC spurs in band: DAC performance must be excellent... Interpolation Filter (FIR, L=128), (interpolation rate 1, 4, 5, 6 and 8) – High speed CIC Interpolation Filter (2- 8,192) – 32 bit NCO – Supports on board Look-Up Table pulse shaping for BPSK, QPSK & GMSK – Phase Equalization for meets IS-95 a 26 Digital Up-Converter AD6622, Four Channels 1 2 + 3 4 SDIN SDFS SPort RCF SCLK CCI Filter SDIN SDFS SPort RCF SCLK CCI Filter NCO QAM SDIN SDFS SPort RCF SCLK... replaces two • analog filters are now IF, but can still clean up the DAC spurs • Highest IF frequency is roughly 1/3 the DAC clock • The first big step to processing digitally, but analog performance requirements for the DAC are becoming more severe a Digital Up-Converter with on-chip DAC x + DAC x • AD9856 Features – – – – – – – a Single Channel Device Up to 70 MHz output bandwidth Integrated 12-bit... Supply Operation • Pin-compatible with Other TxDAC Family Products • Highest Performance DACs on the Market! a Next Generation TxDACs Improvements AD9754XR vs AD9764 SFDR vs FOUT & AOUT @ 50MSPS 90 9754 85 80 0dBFS 75 dB -6dBFS -12dBFS 70 0dBFS -6dBFS 65 -12dBFS 9764 60 55 50 0 5 10 15 FOUT (MHz) a 20 25 IMPROVED NOISE PERFORMANCE SNR=50 dBFs AOUT=0 dBFS FOUT=10 MHz FCLK=100 MSPS AD9764 -103dBm SNR=59dBFs... MSPS AD9764 -103dBm SNR=59dBFs AD9754 a Note: -128dBm is Ideal Noise Floor Spectral Mask Requirement Determines DAC Resolution/AC Performance 10 dBm -30 dBm -50 dBm -60 dBm fCARRIER f-30dBm f-50dBm • “Spectral Mask” at I/Q DAC outputs often 8 dB or better than antenna output for system error budget purposes • Dynamic error sources typically dominate in degradation of spectral mask beyond what is predictable... que ncy(Hz) Fre que ncy(Hz) Compare Noise Floor! • Based on datasheet specifications, HI5741, has superior DC linearity performance (DNL, INL) when compared to AD9764 • Yet, AD9764 has lowest noise floor! • Remember…Since outputs of I/Q DAC is “noise-like”, DAC’ s small signal AC performance is important a W-CDMA Adjacent Channel Inteference 14 vs 10 Bits TxDACs Noise limited by Analyzer AD9754 14-bits . a Radio Transmitter Architectures Trends for Infrastructure Analog Devices, Inc. January 1999 a Key Issues for Tx Architecture • Channel Filtering (Pulse. x DAC LPF Pulse Shape Pulse Shape + CH 1 CH 2 CH 3 CH ‘N’ DAC LPF DUC Format, Pulse Shape DUC Format, Pulse Shape DUC Format, Pulse Shape DUC Format, Pulse Shape a Traditional TX Architecture Nyquist Baseband. vs. IF, or RF?, which board?. • Changing System goals – Multi-mode radio? Phased-Array system? Micro- or Pico- cells? a Transmit Architectures “I” DATA “Q” DATA F c LPF LPF “I” DAC “Q” DAC Serial/Parallel Encoder Serial