Clock Jitter Effects on DDS Waveforms

Jonathan Owen

Project Purpose

• Explain the following concepts:– Theory of DDS operations– Nyquist zones– Basics of clock jitter– DAC output modes– Clock jitter effects on DDS Waveforms

Acronyms

• DDS – Direct Digital Synthesis• DAC – Digital-to-Analog Converter• LUT – Look Up Table• RZ – Return to Zero• NRZ – Non-Return to Zero

The DDS Block Diagram

http://www.analog.com/media/en/training-seminars/tutorials/MT-085.pdf

Matlab Demonstration of DDS

See Code For DDS DemonstrationDDSdemo.m

Nyquist Zones

http://analog-eetimes.com/en/basics-of-adcs-and-dacs-part-1.html?cmp_id=7&news_id=207801388&page=6

Clock Jitter TheoryThe following clock jitter is very exaggerated for demonstration purposes [ 10% Jitter (1σ) ]

Reading Clock Jitter Values• Clock jitter values are typically listed in data

sheets with these forms:– In terms of time [50 us with 1σ std. dev.]– In terms of percentage [0.1% with 1σ std. dev.]– In terms of PPM (Parts per million) [1000 ppm]

• Hardware producers will always try to make their product seem better (be cautious!).

• It is important to be aware of the standard deviation for the given clock jitter values.

Translating Clock Jitter Values to Discrete Simulation

• Assume a given jitter value of 1% with 1σ standard deviation.

• The jitter percentage value is translated into a time value: Tjit = 0.01*Tclk

• With known sample period Ts, the number of samples to represent Tjit is calculated:

SampleJit = Tjit / Ts• Each individual value of SampleJit is multiplied by a

value return from the randn() function to achieve the 3σ value of the symbol jitter.

RZ Waveform (Without Jitter)

NRZ Waveform (Without Jitter)

RZ Spectrum (Without Clock Jitter)

NRZ Spectrum (Without Clock Jitter)

RZ Spectrum (With Clock Jitter 0.1%)

RZ Spectrum (With Clock Jitter 1%)

NRZ Spectrum (With Clock Jitter 0.1%)

NRZ Spectrum (With Clock Jitter 1%)

Nyquist Zones 28-35 (RZ 0.1%)

Nyquist Zones 28-35 (RZ 1%)

Nyquist Zones 28-35 (NRZ 0.1%)

Nyquist Zones 28-35 (NRZ 1%)

Citations[1] Analog Devices. (2009, 10 08). Fundamentals of Direct Digital Synthesis (DDS), Rev.0. Retrieved from

AnalogDevices: http://www.analog.com/media/en/training-seminars/tutorials/ MT-085.pdf[2] http://www.tek.com/dl/76W_30631_0_HR_Letter.pdf[3] http://analog-eetimes.com/en/basics-of-adcs-and-dacs-part-1.html?cmp_id=7&news_id=207801388&page=6