Designing Synchronization Sources in a Network Element

1. Designing Synchronization Sources in a Network Element Dejan Habic Raltron Electronics Corp. dhabic@raltron.com

2. Agenda • A generic timing system solution in NE • Clock cards architecture • Line cards synchronization sources • Performance Evaluation and test results examples

3. HF PLL PLL Framer (OCx) PLL Framer (OCx) Framer (DSx) HF PLL Framer (DSx) Clock Clock Host mP Input Input Host mP Line Card 1 Line Card 2 System Clock Card 2 System Clock Card 1 Line Card n+1 Line Card n A Generic Timing Solution in NE Back plane DS1,E1,Bits In

4. A Generic Timing Solution – attributes • Functionality • Performance • Redundancy or overall robustness of the system. • Automatic respond to all external or internal events at all levels. • Alarm and status signal messages. • Control interface to the all elements of the system.

5. Clock Card Architecture - general issues • Number of external reference inputs • Number of output signals required • Selecting the local oscillator (LO) for required quality of the clock • Filtering algorithms • Reference qualification • Phase transient suppression • Holdover performance • Switching between the modes • Master-slave operation of the two clocks

6. t Phase Detector & MUX VCXO DAC Loop Filter & Control Clock Architecture – SPLL type 1

7. Clock Architecture – SPLL type 1 • Supports all modes of operation (locked, holdover and free-run) • Loop parameters can be optimized in software and changed dynamically during operation • Easy to achieve low bandwidth PLL • Easy to change in software for different requirements (timing; switching) • Temperature and other stability compensation of the LO can be implemented. • Digital noise in the loop • High-pull VCXO and DAC are issues to consider.

8. T,U,... Phase Detector & MUX Free Running Oscillator DDS Loop Filter & Control Output Synthesizer Clock Architecture – SPLL type 2

9. Clock Architecture – SPLL type 2 • Supports all modes of operation (locked, holdover and free run) • Easier to achieve higher accuracy of the clock (e.g. Stratum 3E). • Oscillator is free running reference • Easier to control the loop • Loop parameters can be optimized in software and changed dynamically • Easy to achieve low bandwidth • Easy to change in software for different requirements (timing; switching) • Various disciplining algorithms for control of the LO can be implemented. • Digital noise in the loop • Phase noise and spurious due to DDS should be carefully considered.

10. Line Cards Timing – General Issues • Generate highest possible frequency required by system with the cleanest output signal – crystal or SAW based oscillators. • The output oscillator should be with very good jitter performance – avoid signal multiplication if possible, using high frequency fundamental crystal technologies • The PLL should be optimized with respect to in/output frequencies, possible transients during switching etc… • Should provide smooth switching between two references. • Should provide control interface, statuses and alarm messages.

11. MUX & Control VCXO PLL Line Cards Timing – Switching + PLL Reference 1 To Framer Reference 2 Statuses, Alarms and Control

12. Line Cards Timing – Switching + PLL • Simple analog implementation of PLL with low jitter frequency source at the output. • Provide automatic switching between two references. • Possible to implement a “hit-less” switching. • Interface for status, control and monitoring. • Different frequency sources require new optimizations of the loop. • Influence of temperature, aging and other environmental effects on the frequency source should be considered.

13. Characterizing the behavior of the system • Parameters and behavior are well defined and specified by international recommendations (BellCore, ANSI, ITU-T, ETSI). • Important to ensure that the system complies with the recommendations. • Prove system performance.

14. Measurement and Analysis • Determine measurement equipment • Determine set up of the equipment for specific measurements • Follow the measurement procedures • Measurements • Analysis and interpreting the results

15. The types of measurements • Frequency accuracy – (ppm) • Pull-in, Hold-in… – (ppm) • Jitter and wander generation (TIE, MTIE, TDEV) • Wander tolerance – (TIE, MTIE, TDEV) • Holdover performance – (TIE, MTIE, f(T,t)…) • Switching and transient – (TIE, MTIE) • Other tests such as environmental tests…

16. Measurement – wander generation • Wander generation – the amount of noise produced at the output when an ideal input is applied. • Signal with white noise applied at the input. • Measured TIE for a given observation time period • Calculate MTIE and TDEV

17. Measurement – wander tolerance • Wander tolerance – the tolerable level of noise at the input, while keeping the output within the recommended limits. • signal with known/specified noise distribution applied at the input • Measure TIE for a given observation time period • Calculation of TDEV • The system should operate without any alarm, switch reference or to holdover mode.

18. Measurement – reference switching • Behavior of the output during reference switching - output phase transient. • The unit is locked to one reference and then switched to another one. • Measure TIE during that period • Calculate MTIE

19. Measurement – 1ms transient • The response of the output phase when a 1ms phase transient is applied at the input. • The unit is locked to the reference when the transient is generated. • Measure TIE during the event. • Calculate MTIE. • Evaluate PLL parameters - damping factor, peaking…

20. Measurement – 1ms transient with phase build-out implemented • The response quite different when phase build-out feature is implemented.

21. Measurement – holdover performance • The whole set of test that characterize holdover operation. • Initial frequency offset. • The frequency offset due to temperature changes. • The frequency offset due to aging. • The additional frequency offset.

22. Measurement – environment testing • Parameters’ testing over extreme environmental conditions (e.g. wander generation as a function of temperature). • TIE measurement while exposing the DUT to temperature variations. • External variations of environmental conditions may effect power supply stability, performance under vibrations, humidity, etc…

23. Summary • Understand all the requirements of synchronization for a particular product. • Determine specifications and performances to be achieved. • Determine the role of each element in the system and integration impact amongst them. • Select and evaluate each components in the system. • Integrate a system. • Test and characterize it.