radio transmitter architectures trends for infrstructure'

Key Issues for Tx Architecture• Channel Filtering Pulse Shaping requirements – IF and RF frequencies and image problems • Partitioning of chips and boards – digital vs.. Nyquist Baseband

Radio Transmitter Architectures

Trends for Infrastructure

Analog Devices, Inc

January 1999

Key Issues for Tx Architecture

• Channel Filtering (Pulse Shaping) requirements

– 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?

Serial Input

Data

+ x

x

0 90

Quadrature Upconverter

Symbol Clock

LO I(t)

Q(t) QLPF(t)

ILPF(t)

Baseband, Analog Up-conversion

Digital Upconverter

Digital Modulation & Up-conversion

with Direct IF Out

Digital I & Q, Multi-carrier TX

I

Q

wcLPF

Serial Input Data

0100 1000

+

0 90

Quadrature Upconverter

N x Symbol Clock

LO I(t)

Q(t) QLPF(t)

ILPF(t) Digital

Interpolation Filter

x N

Baseband with Digital Interpolation,

Analog I & Q

X X

Pulse Shape

Pulse Shape

DUC Format,

Pulse Shape

DUC Format,

Pulse Shape

DUC Format,

Pulse Shape

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)

Data

0100 1000

+x

x

0 90

Quadrature Upconverter

Symbol Clock

LO I(t)

2 x FSYMBOL

Nyquist Baseband,

Analog I & Q:

Simplest, conventional approach:

• pulse shaping may be done with multi-pole, complex

• analog filters clean up harmonics and out-of-band

problems from converters: converter sampling rate andanalog performance requirements are modest

• Most of the work is done in the analog signal processingdomain

+ x

x

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

• Can be used with AD8346 Quadrature

Modulator for direct quadrature up-conversion

to up to 2.5 GHz

A D 9 7 67

+ x

x I

Q

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!

Fre que ncy(Hz)

AD9774 w/ 4X InterpolationAD9764 w/o Interpolation

AFTER

-90 -80 -70 -60 -50 -40 -30 -20

Frequency(Hz)

BEFORE

Original Sin(x)/x Response

“Shaped” by Digital Filter

Sin(x)/x Response

to random input data

Interpolated Baseband with

analog I &Q Modulator

specified spectral mask requirement

(i.e suppression of sin(x)/x sidebands)

I

Serial/Parallel Encoder

x

0 90

Quadrature Upconverter

N x Symbol Clock

LO I(t)

Q(t) QLPF(t)

ILPF(t) Digital

Interpolation Filter

x N

Interpolated Baseband,

Analog I & Q:

Improved, conventional approach:

• pulse shaping done digitally, may be re-configurable

• interpolation moves DAC images out: reconstruction filtersbecome easier

• still need good gain match between I and Q converters, filtermatch is easier, since poles aren’t critical

• need good offset match between converters and post filters toavoid LO feedthrough

• analog filters still clean up harmonics and out-of-band

problems from converters: converter sampling rate is

increased, but analog performance requirements are modest

• Most of the work still done in the analog signal processing

domain, pulse shaping done digitally

Dual DAC with 2X

COMP1 COMP2 COMP3

QOUTA QOUTB WRITE INPUT

DAC DATA INPUTS (10 BITS)

DUAL DAC FUNCTIONAL BLOCK DIAGRAM

Baseband Interpolating DAC:

Target Specs, Features

• Full Scale Settling Time

2X

Triggered Latch

Edge-1 X clk

14 14

14 14

14-Bit DAC

AD9774

AD9774 vs Stand-Alone DACs

AD9774, AD9764, and HI5741 0dBFS SFDR Performance at 32 MSPS

A D 9 7 6 4 a n d A D 9 7 7 4 v s H I 5 7 4 1

30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0

O u t p u t S c a l e F a c t o r

AD9764 AD9774 HI5741

Note: Improved SFDR performance at high Fout Due to aliased harmonics falling out-of-band

AD9772 2X Interpolating DAC

• 14-bit TxDAC+ with 2x

Interpolation Filters

• 250 MSPS DAC Update Rate

• 125 MSPS Input Data Rate

14/12 BIT DAC (0.35)

PLL

EXTERNAL FILTER

BASEBAND - 50MHz

IF

AD9772

Interpolated Bandpass,

Analog I & Q:

The analogy to ADC undersampling:

• similar to interpolated baseband, except the digital

interpolation filter selects one of the Z domain images

• Channel Modulation occurs prior to DAC

• LO feedthrough is now separated from IF band: can be

filtered at the IF, offset match no longer as critical

• analog filters still clean up harmonics and out-of-band

problems from converters: converter sampling rate is

increased, as well as analog output frequency from the DAC

• Highest IF frequency is roughly 1/3 the DAC clock

• Now doing even more of the work digitally, but analog

performance requirements for the DAC becoming more

severe

Bandpass Filters DAC Image frequency

Serial Input

Data 0100 1000

Serial /Parallel Encoder

Digital Modulation & Up-conversion

with Direct IF out

Quadrature Upconverter

X

X +

NCO

N x

Pulse Shape

“perfect” Excellent sideband suppression.

to “frequency hop” in digital domain.

distortion.

Digital Up-conversion,

Direct IF

The first “big step” is to mix first IF digitally:

• tuning is a simple matter of loading a new frequency

to the NCO

• quadrature, done digitally, is virtually perfect

• one DAC, directly generating IF frequency, replacestwo

• 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 analogperformance requirements for the DAC are becomingmore severe

+ x

x

– Integrated 12-bit D/A converter.

– Programmable sample rate interpolation filter

– Programmable reference clock multiplier

– Internal sin(x)/x compensator filter

– > 48 dB SFDR at 70 MHz Analog DAC Out

+ x

x

DAC

AD9856 QDUC

Product highlights:

•True 12-bit data path and DAC enables device

to target upstream and downstream HFC applications.

•Highly programmable ref CLK PLL & interp.

filters support a wide range of data rates/applications.

+ x

x

DAC

SPI Interface to AD8320/21 Programmable Cable Driver Amplifier

INV.

SINC X

Complex

Data In

DC - 80 MHz Output

DAC

Bi-directional SPI Control Interface:

32-bit frequency tuning word Frequency Update

Interpolation filter rate Reference clock multiplier rate Spectral phase inversion enable Cable driver amplifier control

Reset Master

TX Enable

4X/8X Selectable Interpolating Halfbands

4X/8X Selectable Interpolating Halfbands

2X to 63X Selectable Interpolator

DDS and Control Functions

Profile Select 1-2

Profile Select 3-4

Prog Clock Multiplier

(I/Q Sync)

12 12 12

12

12 12

12 12

Digital Up-conversion

“Agile” IF Output

Same as previous example, 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 DACspurs: DAC performance must meet the Tx maskrequirement across the hopping band

• A very attractive way of realizing the frequency

hop IF the DAC performance is good enough

+ x

x

TX mask

Wide Filter

Spectral Output of Wideband Tx

Architecture

-100 -90 -80 -70 -60 -50 -40 -30 -20

Digital Modulation & Up-Conversion:

Pulse Shape

Pulse Shape

Pulse Shape

SIGNAL AT HIGH FREQUENCY

N CARRIERS UP AT IF FREQUENCY

Digital Up-conversion,

Multi-carrier TX

The “big win”, one radio replaces many:

• multiple digital upconverters put each carrier at a slightlydifferent 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

The ultimate in flexibility, very powerful for phased array

arrangements etc , but are the analog components (DACs,mixers, power amps, etc.) good enough?

Four Channel Digital

a 27

CCI Filter

NCO QAM RCF

NCO QAM RCF

SPort

CCI Filter

NCO QAM RCF

SPort

CCI Filter

NCO QAM RCF

Digital Up-Converter

AD6622, Four Channels

+

1 2

3

4

AD6622 NCO and QAM

spur 0 1 2 3 4 5 6 7 8 9

1ST IF

DAC LPF

• CSM Filters each channel

before summing (redundant)

• Equalizer occurs in analog

Next Generation TxDACs: AD975X

• Improved SFDR Performance

– >6dB Improvement Over First Gen (+3V) TxDACs at20MHz Output

– Needed to support multi-carrier TX architecture

• Improved INL/DNL Performance (14-bit)

– 14-bit AD9754 calibrated to 14-bit +/-1LSB DNL

• Single +5V Supply Operation

• Pin-compatible with Other TxDAC Family

Products

• Highest Performance DACs on the Market!

Next Generation TxDACs

Improvements

AD9754XR vs AD9764 SFDR vs F OUT & A OUT @ 50MSPS

50 55 60 65 70 75 80 85 90

F OUT (MHz)

0dBFS -6dBFS -12dBFS 0dBFS -6dBFS -12dBFS

9754

9764

• Dynamic error sources typically dominate in degradation

of spectral mask beyond what is predictable from theeffects of quantization!!!

Dynamic Errors Prevail Over Static Error

Sources Limiting Spectral Mask

0.00E+00 2.00E+06 4.00E+06 6.00E+06 8.00E+06

Fre que ncy(Hz)

-90 -70 -50 -30 -10

0.00E+00 2.00E+06 4.00E+06 6.00E+06 8.00E+06

Fre que ncy(Hz)

• Based on datasheet specifications, HI5741, has superior

DC linearity performance (DNL, INL) when compared toAD9764

• Yet, AD9764 has lowest noise floor!

• Remember…Since outputs of I/Q DAC is “noise-like”,

AD9764 - 14 bit CMOSTxDAC HI5741 - 14 bit BiCMOS DAC

Compare Noise Floor!

-102dBm w/ RSB=30 kHz

Direct IF WCDMA w/ 100% Channel Loading

AD9754

w/ BW=4.1 MHz Spacing=5 MHz

Direct IF WCDMA w/ 10% Channel Loading

AD9754

@ 65.536 MSPS

ACP=61.5 dB w/ BW=4.1 MHz Spacing=5 MHz

Multi-tone Reveals In-band CNR Performance

100 Ω

MINI-C I R MINI-C U I T S T1-1T

T o H P 3 5 89 A Spectrum/ Network Analy zer

50 Ω i n put

50 Ω 20 pF

50 Ω 20 pF

Tektronix AWG-202 1

T o H P 3 5 89 A Spectrum/ Network Analy zer