wideband modem design using fpgas

© 2004 Altera Corporation

Wideband Modem Design Using FPGAs

Wideband Modem Design Using FPGAs

Rob PeltAltera

Rob PeltAltera

Proceeding of the SDR 04 Technical Conference and Product Exposition. Copyright © 2004 SDR Forum. All Rights Reserved

2© 2004 Altera Corporation - Confidential

OutlineOutlineAnalog Up ConversionDigital Up Conversion (DUC)Polyphase Digital Up ConversionConclusionQuestions



Analog Up ConversionAnalog Up ConversionCurrent Modems− Digital Baseband− Digital Intermediate Frequency (IF) Possibly− Analog RF (Possible Direct Up Conversion)

PowerAmplifier

Filter

VCO

NCO

LowNoise

AmplifierFilter

VCO

NCO

Switch

FPGA-based Digital IF & Baseband Processing

Digital-to-

Analog

Analog-to-

Digital

InterfaceBasebandProcessing

Antenna



Analog Digital Up ConversionAnalog Digital Up ConversionImpairments are Introduced− High Order Mixing Products

3rd, 5th, 7th, . . . Order Terms− Error Vector Magnitude (EVM) Impairments

I/Q Phase & Magnitude Imbalance− Carrier Feed-through− Harmonics− Sideband Noise

Analog Filter Characteristics− Non-Linear Phase− Group Delay Variation



Moving from Analog to DigitalMoving from Analog to DigitalAdvantages− Higher Performance− Lower Implementation Loss− Time to Market− Flexibility

Disadvantages− Incremental Cost− Higher Power



Digital Up ConversionDigital Up Conversion

Quadrature Digital IFEliminates I/Q Imbalance (EVM Distortion)Single D/A Converter

ModulatorDigital-to-

Analog

VCO

PowerAmplifierNCO

CICFIR

CICFIR

FPGA

Anti-Aliasing

Filter



Digital Up Conversion in FPGADigital Up Conversion in FPGACurrent FPGAs are Feature Rich− Arithmetic Operators− Embedded Memory− High-Speed I/O− PLL

FPGA Clock Speeds are >300 MHz− Bandwidths up to 150 MHz



Altera’s Stratix II FPGA FamilyAltera’s Stratix II FPGA Family

123849,383,040179,400EP2S180

121924,520,44890,960EP2S90

6,747,840

2,544,192

1,369,728

419,328

Total Memory

Bits

252

144

64

48

18x18Multipliers

EP2S130

EP2S60

EP2S30

EP2S15

Device

12132,540

1260,440

633,880

615,600

PLLEquivalent Logic

Elements



Digital Up ConverterDigital Up Converter

314 MHzFFT (256 point)

404 MHzNCO

339 MHzFIR Filter

Maximum SpeedComponent

Stratix II Common Building BlocksSpeeds at >300 MHzNyquist Bandwidth of ~150 MHz



Polyphase ApproachPolyphase ApproachCurrent D/A converters Have Sample Rates of >1 GSPS− Bandwidth of >500 MHz

Standard DUC Approaches Limit Bandwidth to ~150 MHZPolyphase DUC can Take Advantage of D/A Sample Rates− Bandwidths up to DAC Nyquist



Polyphase ApproachPolyphase ApproachFamiliar Polyphase DecompositionEach Sub-Filter Processes a Different Phase of the Input SignalOutput Commutator Supplies Up-Sampling

Polyphase Sub-Filter



x(t)

y(t)

Commutator



Polyphase Digital Up ConversionPolyphase Digital Up ConversionPolyphase Decomposition of Up-ConversionSub-DUC Replaces Sub-FiltersLow Voltage Differential Signaling (LVDS)− Serializer Mux− Replaces Commutator

Digital-to-Analog

Converter

NCO

Sub-Filter

LVDSSerializer

LVD

S

Sub-DUC 0

Sub-DUC 1

FPGA-based Polyphase Modem

Sub-DUC N

BasebandProcessing



Sub-DUC ConstructionSub-DUC ConstructionMixerFilter− Normal Polyphase Decomposition

NCO− Frequency− Initial Phase− Spectral Inversion Compensation



NCO FrequencyNCO FrequencySet frequency Based on Desired Alias

( )gen s_nco

__ gen s_nco

for F < F /2 * for F > F /2

gena gen

gen s nco

FF

F F alias = −

Where:Fa_gen is alias frequency setting for the NCOFgen is the desired output frequencyalias is the Nyquist zone number



NCO PhaseNCO PhasePhase Set for Alias Frequency

Where:N is the number of sub-DUCsFs_dac is the clock speed of the DACFgen is the desired output frequency

_

360* genN

s dac

FF N

φ =∗



ConclusionConclusionFPGAs are Well Equipped for Digital Up ConversionD/A Converters with GHz Sampling Rates are AvailableUsing Polyphase Decomposition & Aliasing, FPGAs Can Be Used to Interface to the GHz D/A Converters


© 2004 Altera Corporation

Thank YouThank YouFor More Information Visit www.altera.comFor More Information Visit www.altera.com


wideband modem design using fpgas

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