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Manish Abraham 11033698

COMPUTER CONTROL OF A DIRECT DIGITAL SYNTHESIZER 1. Aims and objectives.

2. Function Generator .

3. Direct Digital Synthesis .

4. Theory (why & how it works).

5. Direct Digital Synthesizer .

6. Software.

7. Errors.

8. Conclusion.

AIMS AND OBJECTIVES AIM

This project is a design engineering project which has been undertaken in order to use a direct digital synthesizer understanding working of DDS, look at types of DDS, choosing from the specifications and to set accurate frequency in the range of 7.2 MHz to 9.2 MHz’s with a step size of 10 KHz.

OBJECTIVES

Understand the Output", Frequencies.

• Collect the Background Information about a DDS.

• To Understand the applications of a DDS.

• Determine the required data for select output Frequency..

•"Workout the output frequency", changes of a direct digital synthesizer.

• Understand the sources of spur and design a filter

.

FUNCTION GENERATOR

To produce different types of wave forms (sine, square, triangle, etc.) with a particular frequency bandwidth and for different analysis work, we use a function generator.

Normally we used to use a Manuel Function generator. Potentiometers are used to adjust the waveforms frequency and amplitude.

The feedback regarding the provision of the output & the equipment is not delivered to the user. Hence, we never know whether the desired waveform is available at the output or not unless it is calculated by using appropriate equipment’s.

It is often difficult to adjust the frequency and magnitude of the waveforms to the precise value needed because with potentiometers we face tuning problems. The function generator does not recognize distant control of the waveform. Hence we have to physically regulate the function generator every time we need to change the waveform. A standard function generator does not allow interfacing and collaboration with other devices. However by using a computer as a main station, multiple different devices can be joined together to form a complete testing and experimentation setup. The above weaknesses of a manual function generator can be overcome using a computer controlled function generator like the AD9834

PHASE Μ TIME

360 = q

360t*Fout

TIME

Fout

TIME0

PHASE

DIRECT DIGITAL SYNTHESIS

Direct digital synthesis (DDS) is a process of creating an Analog waveform normally a sine wave by producing a time-varying signal in the digital form and after that it performs a digital-to-analog conversion.

DISCRETE PHASE VS DISCRETE TIME

PHASE

TIME

TIME

2n-1

0

nclk

out 2

FF

Fout

clkF

1

HOW DO YOU BUILD THIS?

PHASE

TIME

2n-1

0

nclk

out 2

FF

nPHASE REGISTER

CLOCK

n

PHASE ACCUMULATOR

n

1

n = 24 - 48 BITS

Fclk

CHANGING FREQUENCY

PHASE

TIME

2n-1

0

nclk

out 2

FMF

nPHASE REGISTER

CLOCK

n

PHASE ACCUMULATOR

nn = 24 - 48 BITS

M

FREQUENCY CONTROL M = TUNING WORD

DELTA PHASE

REGISTER M

n

M

Fclk

WE NEEDED A SINE WAVE AS PER OUR OBJECTIVE

AMPLITUDE

TIME0

nclk

out 2

FMF

nPHASE REGISTER

CLOCK

n

PHASE ACCUMULATOR

nn = 24 - 48 BITS

FREQUENCY CONTROL M = TUNING WORD

DELTA PHASE

REGISTER M

n

M

Fclk

PHASE-TOAMPLITUDECONVERTER

p

SIGNAL FLOW THROUGH THE DDS

2n=fo

M • fc

M = JUMP SIZE

REFERENCECLOCK

PHASEACCUMULATOR

(n-BITS)

PHASE-TO-AMPLITUDECONVERTER

DACM

TUNING WORD SPECIFIESOUTPUT FREQUENCY AS AFRACTION OF REFERENCECLOCK FREQUENCY

DIGITAL DOMAIN ANALOG

N

DDS CIRCUITRY (NCO)TO

FILTER

2n=Fo M

Fclk

Fclk

n

AN OVERALL WORKING OF A DDS

6-bitphasewheel

01234

63

…

…

024

3129…

… 5-bitamplituderesolution

vector dataraw DDS or DAC outputfiltered output compared output

DIRECT DIGITAL SYNTHESIZER

DDS devices are extremely small and draw little power.

• Currently available DDS devices can generate frequencies between 1 Hz and 400 MHz (based on a 1-GHz clock), with a time resolution to 48 bits.

• The new devices which is used, are of low cost as they use the new process technologies they are combined with DDS’s inherently excellent performance and the ability to digitally reprogram the output waveform which make the DDS system particularly appealing compared to more discrete and less flexible standard solutions.

A basic Direct Digital Synthesizer comprises of a frequency reference which may be crystal or SAW oscillator, a digital to analog converter and a numerically controlled oscillator (NCO).

APPLICATIONS OF A DIRECT DIGITAL SYNTHESIZER

Waveform generation in communication.

• Enhanced tunability reference frequencies for phase-locked loops (PLLs).

• Generating pilot signals for WDM optical-channel identification.

• Local oscillators.

• Direct transmission.

• Signal research in industry and biomedicine.

i) Digitally creates programmable waveforms with easily adjustable frequency and phase.

ii) Locate resonances or compensate for temperature drifts.

iii) Generate pulse width modulated signals for micro actuators.

iv) Examine attenuation in LANs or telephone cables.

v) Flexible frequency boost in measuring sensor impedance.

Electronic surveillance

• Generating pilot signals for WDM optical-channel identification.

• Maritime applications in sonobuoy systems.

A TYPICAL APPLICATION

DDS in transmit section of sonobuoy

DDS in receive section of sonobuoy.

AD9834

The outputs of AD9834 generate sinusoidal, triangular, and square-wave in nature which is produced by using the DDS (direct digital synthesis) architecture.

The AD9834 is a 75MHz, low power DDS device capable of producing high performance sine and triangular outputs. Since it consumes only 20mW of power at 3V, it makes the AD9834 an ideal candidate for power sensitive applications.

Both phase modulation and frequency modulation can be performed with the AD9834.

The frequency registers are 28 bits; with a 75 MHz clock speed, resolution of 0.28 Hz can be achieved. The AD9834 evaluation board includes a 75 MHz oscillator that provides the MCLK(75MHz) for the AD9834.

Digital and Analog are free of each other & is powered between 2.3 V to 5.5 V.

THE SOFTWARE THAT WE USE From this window, we can control all the functionality of the AD9834. The main features are listed below and described in detail in the sections that follow. • Access to both Frequency Register 0 and Frequency Register 1 • Access to both Phase Register 0 and Phase Register 1 • Access to the RESET bit • Ability to sweep through a frequency range using either the Frequency Register 0 or Frequency Register 1 • Access to the programming method • Access to the sleep options • Access to sign bit output options • Access to the IOUT output options

ADI SIM DDS DESIGN TOOL

The advantage of this tool is to assist a user to select and evaluate Analog Devices; Direct Digital Synthesis (DDS) IC's which also included AD9834 DDS. The tool uses mathematical equations to approximate the overall performance of the selected device & calculates possible errors.

PROCEDURE

The Experiment was carried out in Microwave electronics lab where firstly the dc convertor was connected and a voltage was kept between 2.3V and 5.5V. The output wire was connected with the two devices which we would be discussing ahead in the discussion.

The evaluation board was set up as under two stages:

1. Time Domain.

2. Frequency Domain.

TIME DOMAIN

We know that f(t) = sin(wt) Here w is frequency component and in this case we observe a sine wave as a DDS produces a sine wave

FREQUENCY DOMAIN Fourier theory (WANG, T. T. & FAN, H. Y. 2009) tells us that the time-

domain is made up of few sine waves of appropriate frequency, amplitude, and phase. We can transform a time-domain signal into its frequency domain equivalent. Measurements in the frequency domain tell us how much energy is present at each particular frequency. Fourier theory says that the constituent sine waves are separated in the frequency domain by 1/T where T is the time period of the signal.

Frequency-domain graph shows how much of the signal lies within each given freq u

People who work in wireless communications are interested in out-of-band and spurious emissions. For e.g., cellular radio systems must be checked for harmonics of the carrier signal that might interfere with other systems operating at the same frequencies as the harmonics. Engineers are also very interested about the distortion of the message modulated onto a carrier. Spectrum monitoring is another important frequency-domain measurement activity. The Spectrum monitoring can be done with the help of a spectrum analyser over a range of frequencies.

The time domain signal is digitalised using a Fourier analyser, then uses digital signal processing (DSP) techniques to perform a fast Fourier transform (FFT) and display the signal in the frequency domain.

This photo shows when we were able to set an exact frequency of 7.2 MHz the clock frequency is 75MHz hence we observe second harmonic was observed in the clock frequency of 75-7.2 = 67.8MHz.

THE SPURIOUS EMISSIONS WERE OBSERVED

ERRORS

nclk

out 2

FMF

PHASE ACCUMULATOR

Fclk

nn

FREQUENCY CONTROL M = TUNING WORD

PHASE REGISTER

DELTA PHASE

REGISTER M

CLOCK

n n

n

PHASE TRUNCATION 12-19 BITS N-BITS

(10-14)

n = 28 BITS

PHASE-TOAMPLITUDECONVERTER

SYSTEM CLOCK

Fout

AMPLITUDE QUANTIZATION

pM

DAC ERRORS

DAC

REFERENCE CLOCK

In case of an AD9834 evaluation board the DDS acted like a high resolution frequency divider with the reference clock in input and the DAC as the output.

When we try modulating the clock amplitude it generates spurs in the output spectrum,

The attenuation calculation of 20 log (75Mhz/7.2MHz) predicted a 20.35dB improvement.

Whereas when we calculated it was noted that there was additional spur attenuation.

This was because the modulating sine wave of the reference clock encounters a limiter which converted the AM spurs to PM.

A high rate reference clock spends less time to traverse the region where noise is jittered.

As per Intel (1999) Lower clock frequencies allowed higher resolution and better spectral purity for any given number of bits in the accumulator (N) and a given ROM size

PHASE TRUNCATION

The power dissipation and die area is reduced. The o/p is divided in to two sections the phase word p which sends to the mapper and discards bits M= T-P where T is the phase accumulators tuning word width. This is the reason for the production of phase modulation spurs. When we do practical experiments DDS drives a PLL where the spur within the loop is amplified by 20 log (N) dB, where N is the multiplication factor.

Due to the phase truncation we observed that the first harmonic is generally the strongest , It causes phase modulation with a periodic waveform.

ANGLE TO AMPLITUDE MAPPING

The phase to amplitude conversion of a time sampled sine wave is

Where Vai is the sample amplitude, Vp is the half of DACs full scale voltage and oi is the value of samples phase word.

It was observed that retaining more bits in the phase word reduces these errors, i.e. set the phase word to a minimum of 3 bits wider than DAC.

QUANTIZATION NOISE, DAC NONLINEARITIES

We can calculate the quantization noise as it is directly proportional to the DAC resolution

SNR = 7.21 N +1.3 (dB)

Where N is the DAC resolution in bits.

In the end we came to a conclusion that we can reduce the spurs caused by aliases by increasing the reference frequency fref

Solution –

– Spurs generally occurs due to uniform stepped periodic sequences.

We can introduce a few jitters (minimal noise) which should be able to destroy the periodicity.

This would convert the spur energy in to broad band phase noises.

As per Chao Huang (2009) frequency planning is the only way that works in eliminating the spurs of DDS in real circumstances.

ALIASED DISTORTION TERMS

Freq

Distortion in an analog system

FreqFs 2Fs

Distortion in an sampled system

BAND-PASS FILTERS

As per J. Vankka (2005) DDS has constraints to reach higher frequencies due to large frequency ratio between clock and output signal frequencies. Finateu, T (2007) said that meeting RF communication system requirements are strongly affected by this drawback

The band pass filters can also be used to generate waveforms above Nyquist frequency the replicated images can be filtered to extract the desired image and the sampled signals duplicate at multiples of the sampling frequency .

In this band pass filter f1 is the desired output freq i.e. . 7.2 MHz f2 is the clock frequency i.e. 75MHz It was observed that the AD9834 used digital signal processing to construct output wave forms which were sinusoidal in nature. A frequency sweep was done between 7.2 and 9.2 MHz.

SUMMARY OF FEATURES IN AD9834 ARE AS FOLLOWS:

(i) A high resolution frequency of three different types of waveform is generated: sine wave, square wave and triangular wave. We were concerned about the sine wave. The output frequency the chip is changed by changing the value of the phase accumulator.

(ii)Due to crystal locked output frequency, there is a stable output frequency since there is no time & temperature drift for the frequency.

(iii) Theoretically, the AD9834 is able to generate a maximum output frequency of fMCLK/2. The maximum clock frequency that can be used by the AD9834 is 75 MHz. Therefore, the chip is able to generate a highest frequency of 37.5MHz.

(iv) It had been used for digital communication system , spectrum systems which are spread.

(v) In future the synthesizers will have higher clock speeds and lower spur levels.

(vi) Appropriate in terms of size, switching frequency, resolution, fixedness and accuracy

Background information was collected effectively and there was a deep understanding about the direct digital synthesis and applications of direct digital synthesizers.

The Evaluation board was used to test the effectiveness of obtaining the precision wave forms between the range of 7.2 MHz and 9.2 MHz with a step size of 10 KHz.

There was precise phase control without affecting the output frequencies.

The basic functioning of a direct digital synthesizer was understood, the required input sources for the output frequencies were kept in consideration.

Got specific output frequency since Fout = FTW * MCLK / 2^N where FTW is the register value for setting the output frequency and N is the size of the register. With the AD9834 N = 28. To solve for a FTW for a desired output frequency, FTW = (Fout / MCLK) * 2^N. The changes in an output frequency were established.

Both the time domain as well as the frequency domain was taken into consideration getting both oscilloscopes as well as spectrum analyzer in to usage. There was a detailed study and practical experiment done to find out the usage of the evaluation board.

Prototype development on the sources of spur and errors in a direct digital synthesizer and practically observation with the help of spectrum analyzer. Solutions were researched and developed to reduce the spur. Basic designing of a lab view programme which explained the functioning of a direct digital synthesizer. Design considerations for a filter was taken into account and a designing of a band pass filter.201 different readings were taken and the design tool ADI Sim DDS was used effectively to evaluate the general simulated performance of the AD 9834 DDS chip. The tool used mathematical equations to approximate the overall performance of AD9834 and calculated all possible errors. Two different interactive design tools were used effectively (refer appendices).

THANK YOU