seminario_idetic_120120

www.idetic.eu

Seminar in

Practical Application ofDirect Digital Synthesis (DDS)

Speaker: Baltasar Pérez Díaz <[email protected]>

Author: Baltasar Pérez Díaz

Multimedia Room, Polivalente II, 2nd floor

Campus de Tafira, Las Palmas 20th January, 2012

2IDeTIC Seminar20th January

Practical Application of Direct Digital Synthesis (DDS)

BACKGROUND INFORMATION

Wattmeter

Driver

BroadbandTransceiver

1KW Poweramplifier

DUC/DDC multichannel

(FPGA)

Broadband digital HF transceiver



TXRX OL

ANT

Control

PS

19’’ subrack 3U form board (100x160mm) with connector DIN41612Rear: digital signalsFront: analog signalsPower Supply: 220Vac and 12V DC

BACKGROUND INFORMATION



PRESENTATION INDEX

1. DDS Theorya. Block diagram, spurs, topologies

2. Example: AD9954a. Datasheet

3. DDS in radio subsystems

4. Summary

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954

DDS IN RADIOSUBSYSTEM

SUMMARY



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

• DDS is a method of producing an analog waveform,usually a sine wave,• by generating a time-varying signal in digital form

• converted into analog signals using a DAC

• NCO (Numeric Controlled Oscillator) also called

• Advantages

• Capable of generating a variety of waveforms (sine, triangle, square)• Preferred form of signal generation nowadays• Fast switching capability (freq. hopping systems (phase-continuous))• High precision sub Hz (mHz) and sub degree phase tuning• Digital circuitry

• Small size (single chip) fraction of analog synthesizer size• Fewer components per system - low cost• Small low-powered devices – portability• Easy implementation (no Barkhaussen criterion, PLL (LPF design))• Fewer assembly operations / reduced product reject rates



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Typical DDS Architecture

1/1/ffoutout1/1/ffoutout

1/1/ffclkclk

1/1/ffoutout

1/1/ffclkclk

1/1/ffoutout

1/1/ffclkclk

ffoutout==ffclkclkFFrr

22NNAccumAccum--ulatorulator

NN24 to24 to48bits48bits

FrequencyFrequencyWordWord WW

14 to14 to16bits16bits

SineSineLookupLookupTableTable

RR10 to10 to14bits14bits

LowLowPassPassFilterFilter

SineSineWaveWave

FFrr

Ref clkRef clk

Digital CircuitsDigital Circuits

DD--toto--AAConvConv..



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

DDS THEORY

Phase Accumulator as a digital phase wheel

• The phase accumulator is actually a modulo-M counter.• Increments its stored number each time it receives a clock pulse. Magnitude determined

by word (M).• This word forms the phase step size between reference-clock updates; it sets how many

points to skip around the phase wheel.• The larger the jump size, the faster the phase accumulator overflows and completes its

equivalent of a sine-wave cycle.• The number of discrete phase points contained in the wheel is determined by the

resolution of the phase accumulator (n), which determines the tuning resolution of the DDS.

0 pi 2pi



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

• Sampling theory (sinc(x)) • Nyquist: Fundamental signal <= Fclk/2 (1/3 better)• Filter required to eliminate unwanted products• Let’s increase the fundamental signal frequency

DDS THEORY

Digital to Analog Converter Output



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

DDS THEORY

Digital to Analog Converter Output

• Sampling theory (sinc(x)) • Nyquist: Fundamental signal <= Fclk/2 (1/3 better)• Filter required to eliminate unwanted products



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Super-Nyquist Operation

• SNR, SFDR and power reduction • SAW filter (narrowband)• Change Fs to center sinc envelope peak

DDS THEORY



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

• History

• Earlier designs – 1970s – for audio signal applications –from sampled values of sine wave in ROM later driving DAC

• Modern approaches – much improved and mostly derivative of the classical approach

• Practical use in Comms System by 1990s • 1980s – Highest freq. - < 10 MHz– limitation of DAC tech.• Current DDS systems – 1GHz (new 2.7GHz)

• DDS vs PLL

• DDS generates lower frequency than PLL• DDS frequency can be controlled in very fine increments.• The frequency of a DDS synthesizer can be changed much

faster than that of the PLL.• DDS occupies a single chip, and PLL several• Easy oscillator design in DDS, complex in PLL



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Not only waveforms generator, also modulations

Square-, triangular-, and sinusoidal outputs from a DDS

Also Sweep, Chirp, RAM profiles, Amplitud control FSK



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Not only waveforms generator, also modulations

FSKIQ generator capability

AD9958 (2 DDS cores)AD9959 (4 DDS cores) (beamforming)

BPSK (2 phases) and QPSK (4 phases)



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs !!! Worst problem in DDS

DDS THEORY

• AD9835 (fclk=50MHz)• Spurs situation and quantity depend on output frequency



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources

DDS THEORY

• A DDS have four principal sources of spurs



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: Reference Clock

DDS THEORY

- A DDS functions like a high-resolution frequency divider with reference clock

- The spectral characteristics of the reference clock directly impact those of the output, but at a reduced magnitude due to the frequency division.

- The improvement is 20 log(N), where N is the ratio of input to output frequencies.

- Tip: Use the highest frequency clock

Figure: 300MHz clock down to 80MHz and 5MHz



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: Reference Clock

DDS THEORY

- Modulating the clock amplitude generates spurs in its output spectrum.

- The limiter stage converts the sine wave to a square wave, and the AM spurs are thus converted to PM (phase modulation) spurs.

- The quality of the reference clock imposes limits on DDS performance in ways that are often recognizable (those DDS spurs that maintain their relationship to the carrier as you change the output frequency).



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: Phase truncation

DDS THEORY

- Consider a 32-bit phase accumulator

- If we maintain all 32 bits throughout, the DDS core would occupy a large die area and dissipate significant power.

- Truncating the value from the phase accumulator (passing only the accumulator's most significant bits to the angle-to-amplitude mapper) reduces area and power

- The phase-truncation spur mechanism models as a noise source summed

Example: 20bits accum to 8 bits

As the value in the discardedsection accumulates, it eventuallyoverflows into the truncatedphase word (effect is phase-modulation spurs)



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: Phase truncation

DDS THEORY

Spurs level= -6.02P

P (phase truncatedword)

Example:

-6.02x14=-84dBc

Note: if no bits in thediscarded portionare set to logic one,then nophase-truncationspurs occur



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: DAC error terms

DDS THEORY

- A DAC's quantization noise and distortion determine its SNR

- SNR is proportional to the DAC resolution in bits (SNR=6.02N+1.76(dB))

- SNR calculation describes an ideal DAC. Real DACs also have nonlinearities due to process mismatches and imperfect bit-weight scaling.

- Harmonic alias because the DAC is a time-sampled system. Well-defined relationship



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Spurs sources: DAC error terms

DDS THEORY

Example:

- DDS tuned to 25.153 MHz with a reference clock of 100M samples/sec generates low-order odd harmonics close to the fundamental

- DDS has a 14-bit DAC. The SFDR within the 4-MHz bandwidth is better than –73 dBc

- Raising Fref to 400M samples/sec eliminates the alias products of the third, fifth, and seventh harmonics within the first Nyquist zone.



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

SpurKiller Technology: The Results on a DDS Output Spur

OUTPUT FREQUENCY = 166 MHzFclk = 500 MSPS

500 kHz / DIVISION 500 kHz / DIVISION

BEFORE AFTER



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

SpurKiller Technology

• Use an auxiliary DDS channel to add in a signal at the same frequency andamplitude as the spur, but 180° out of phase with the highest spur…

AD9911 DDS core

COS(X)

FTW

FrequencyAccumulator

PhaseOffset

143216 10

DAC

DDS Channelfor spur reduction

DDS Channelfor amplitudemodulation

DDS Channelfor phase

modulationRegister Register Register

• The frequencies at which spurs appear are simple functions of the sampling rate and the programmed output frequency.• Spurs are therefore predictable. • In addition, the relative phase of each spur does not change.

It’s all in the Digital Domain!



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Important parameters for RF: Phase Noise

- Phase noise is a measure (dBc/Hz) of the short-term frequency instability of the oscillator (jitter in time-domain)

- Jitter in oscillators is caused by thermal noise, instabilities in the oscillator electronics, external interference through the power rails, ground, and even the output connections.

- DDS introduces some jitter/noise (phase truncation/DAC) but overall is reference clock jitter. It’s critical! Also take slew-rate into account.

- Dividing down the frequency of a high-frequency clock is one way to reduce jitter

Typical output phase noise plot for the AD9834.

Output frequency is 2 MHz and M clock is 50 MHz.

Very good phase noise comparable toprofesional equipment!!

DDS THEORY



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Important parameters for RF: SFDR (Spurious-Free Dynamic Range)

- It’s the ratio (measured in decibels) between the highest level of the fundamental signal and the highest level of any spurious.

- For the very best SFDR, it is essential to begin with a high-quality oscillator.

- SFDR is an important specification in an application where the frequency spectrum is being shared with other communication channels and applications.

- If a transmitter’s output sends spurious signals into other frequency bands, theycan corrupt, or interrupt neighbouring signals.

AD9834 (10-bit DDS) with a 50MHzmaster/reference clock

Figure (a)fout=16.667MHz (i.e. MCLK/3)SFDR=80

Figure (b)fout=4.8MHzSFDR=50

span=Nyquist

DDS THEORY



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Synthesizer topologies

PLL General Architecture

RF

÷N

Loop Filter VCO

Phase/ Frequency Detector

Fref

refRF FNF



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY


DDS Used as PLL Reference

nref

RF 2FNMF

RF

÷N

Loop Filter

VCO


Fref DDS

M



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY


DDS Used in Fractional-N Loop

RF

Loop Filter VCO


Fref

DDS

refRF

n2FM

F

M



DDS THEORY

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY


DDS Used in Translation Loop

RF

Loop Filter VCO


Fref

DDS

÷N

n2clkFMrefRF FNF

Fclk



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

DDS THEORY

Main website: http://www.analog.com/dds

Analog Devices DDS product list

ADISim DDS

DDS configuration assistant

RSonline DDS Prices

DDS Tools



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

EXAMPLE: AD9954

400 MSPS internal clock speed Integrated 14-bit DAC Programmable phase/amplitude dithering 32-bit frequency tuning accuracy 14-bit phase tuning accuracy Excellent dynamic performance

>80 dB narrowband SFDR

Phase noise better than –120 dBc/Hz Serial I/O control Ultrahigh speed analog comparator Automatic linear & nonlinear freq sweeping 4 frequency/phase offset profiles 1.8 V power supply



INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

EXAMPLE: AD9954

400 MSPS internal clock speed Integrated 14-bit DAC Programmable phase/amplitude dithering 32-bit frequency tuning accuracy 14-bit phase tuning accuracy Excellent dynamic performance

>80 dB narrowband SFDR

AD9954 datasheet

Phase noise better than –120 dBc/Hz Serial I/O control Ultrahigh speed analog comparator Automatic linear & nonlinear freq sweeping 4 frequency/phase offset profiles 1.8 V power supply

Freq. resolution = 400MHz/2^32 = 0,093HzPhase resolution = 360/2^14 = 0,02º



AD9833 Evaluation Board(Simple board)

EXAMPLE: AD9954

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY



AD9959 Evaluation Board(Complex board)

EXAMPLE: AD9954

INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY



Broadband Transceiver oscillator board (v0)INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

DDS IN RADIO SUBSYSTEM



Comparison: frequency output purity 400MHz Clock vs Agilent GeneratorINDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY




INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

3-30 MHz ADC

BW = 1.0 MHz112 MHz

122.7 MHz

82-109 MHz

BW 1 MHz10.7 MHz

Image-rejection Filter 3-30MHz

Band Pass Filters 3-30MHz RF Amp

20 dB

DDS

Low Pass10.7 MHz

AGC

75 dB

RS232

Antenna

ControllerCARDS12

3-30 MHz ADC

BW = 1.0 MHz112 MHz

BW = 1.0 MHz112 MHz

122.7 MHz

82-109 MHz

BW 1 MHz10.7 MHzBW 1 MHz10.7 MHz

Image-rejection Filter 3-30MHzImage-rejection Filter 3-30MHz

Band Pass Filters 3-30MHz

Band Pass Filters 3-30MHz RF Amp

20 dB 20 dB

DDS

Low Pass10.7 MHz

AGC

75 dB

RS232

Antenna

ControllerCARDS12

Broadband receiver block diagram

Problem: 112MHz (F.I.1) & 10.7MHz (F.I.2) spursSolution: Narrow filtering




Broadband Transceiver oscillator board (v1) (Manolo version)INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY




Comparison: oscillator board v0 (green) vs v1 (blue)INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY




INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

SDR (Radio Definida por Software)

- It moves F.I. (12KHz)inside soundcard bandwidth(96KHz) to avoid spurs- Changes DDS oscillator freq.




INDEX

INTRODUCTION

DDS THEORY

EXAMPLE: AD9954


SUMMARY

Remember:

• DDS can be used to obtain a variety of precision waveforms• Compared to other frequency generating techniques, a DDS has many

advantages.• DDS has well known error characteristics

• Spurious frequency components in the output signal (bad)• Bandwidth of the output signal (bad)

• Output spectrum purity/quality depends largely on reference clock quality

Future Trends

• Higher clock speeds (digital technology also DAC conversion speed)• Lower spur levels

SUMMARY

www.idetic.eu

Seminar in

Practical Application ofDirect Digital Synthesis (DDS)

Speaker: Baltasar Pérez-Díaz <[email protected]>

Author: Baltasar Pérez Díaz

Multimedia Room, Polivalente II, 2nd floor

Campus de Tafira, Las Palmas 20th January, 2012

Thanks for your attention!!Any question?

En españó, plis!!

seminario_idetic_120120

Documents

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