Slide 1: How to Measure Clock Jitter – Part I: Principle and Practice
SiTime University Turbo Seminar Series How to Measure Clock Jitter – Part I Principle and Practice April 8-9, 2013
Slide 2: Agenda
Agenda • Jitter definitions and terminology • Who cares about jitter • How to measure clock jitter www.sitime.com 2
Slide 3: What is Clock Jitter
What is Clock Jitter • Jitter is, “The deviation of an event timing relative to its ideal value” • Event? - defined by specific type of jitter • Ideal value? - event timing on an ideal clock, often estimated from average value of the event • Jitter definitions: • Period Jitter • Deviation of the clo
Slide 4: Who Cares About Clock Jitter
Who Cares About Clock Jitter • Digital applications (Time Domain) • Computers • Data Servers • Network Interface Cards (NICs) • Telecom Equipment • Embedded Systems • Industrial and appliance controllers • Graphics and Video Displays • Consumer Electronics • Digital cameras, camcorders, game console
Slide 5: Jitter Definitions and Terminology: Period Jitter (PerJ)
Jitter Definitions and Terminology Period Jitter (PerJ) Oscilloscope view t0 t1 t2 t3 tk T1 T2 T3 Tk • Period jitter: variation of period from ideal period (typically average period) • Event: Two consecutive rising or falling edges • Ideal value = average period PerJit (k ) Tk Tc PerJit RMS ST
Slide 6: Clocks in High Speed Digital Systems
Clocks in High Speed Digital Systems CLOCK CLOCK CPU Interface Interface Parallel Parallel GPU CPU Memory etc Serial Transmit Serial Receive CLOCK CLOCK CLOCK Serial Receive Serial Transmit Peripheral CLOCK www.sitime.com 6
Slide 7: State Machine (Processor) Clocking
State Machine (Processor) Clocking • Processors, microcontrollers • Application Processors (graphics, network, etc) • State-Machines, DSP Parameter Application Requirements MEMS Timing Solutions Frequency Range 1 MHz to 600 MHz 1 MHz to 800 MHz Frequency Stability 25 ppm to 50 ppm 5 ppm to 50 ppm Pe
Slide 8: Jitter Measurement
Jitter Measurement • Jitter can be measured with the following equipment • Real-time oscilloscope • Equivalent-time oscilloscope • Time interval analyzer (TIA) • Bit error rate tester • Specialized, under-sampling instrument (often used in automatic test equipment) • Phase noise analyzer (for phase
Slide 9: Jitter Measurement with Real-Time Sampling Oscilloscope
Jitter Measurement with Real-Time Sampling Oscilloscope • Uses interpolation to estimate the threshold crossing time of the edges 50% crossing 50% crossing time - rising time - falling Sampling Points www.sitime.com 9
Slide 10: Real-Time Scope Measurement Errors
Real-Time Scope Measurement Errors • Amplifier Noise • Slew Rate Quantization Noise JMEASURED = JSUT2 + JTBJ2 + JVJC2 Trigger Jitter • Time Base Jitter (negligible) • Sampling Rate • Jitter measurement is subject to two types of oscilloscope errors 1. Voltage Noise Converted to Jitter (JVJC): • Osci
Slide 11: Voltage Noise to Jitter Conversion
Voltage Noise to Jitter Conversion Tn V n / SlewRate Tn Vn / SlewRate (Vn ) (Vn ) Faster rise/fall time reduces voltage noise impact on jitter (Tn ) (Tn ) • Minimize voltage noise to jitter conversion • Sharpen the edges Increase SlewRate • Reduce scope noise Decrease Vn www.sitime.com 11
Slide 12: Reduce Voltage Noise to Jitter Conversion (Decrease Scope Noise)
Reduce Voltage Noise to Jitter Conversion (Decrease Scope Noise) • Select the right front-end bandwidth • Amplifier noise is proportional to its bandwidth • Optimal bandwidth to reduce the scope amplifier noise while avoiding slowing down the signal edge • Example: T_rise = 1ns Amp_BW ~ 1GHz 1 Amp
Slide 13: Reduce Voltage Noise to Jitter Conversion (Increase Sampling rate)
Reduce Voltage Noise to Jitter Conversion (Increase Sampling rate) • Select maximum sampling rate • The higher the sampling rate, the more accurate edge interpolation • Minimum sampling rate required 10 times higher than the inverse of rise/fall time 5 Sampling _ ratemin min(t rise _ 20 / 80 , t f
Slide 14: Reduce Voltage Noise to Jitter Conversion (Increase Signal Slew Rate)
Reduce Voltage Noise to Jitter Conversion (Increase Signal Slew Rate) • Choose smaller vertical setting (volt/div) • Smaller setting may cause the signal to go outside the screen. This is OK on most scopes • Vertical zoom reduces the scope quantization noise • 50mV/div is sufficient for most LVCMOS
Slide 15: Oscilloscope Time Base Jitter
Oscilloscope Time Base Jitter • Time-base jitter • The oscilloscope time-base jitter directly impacts the jitter measurement • Ensure the scope time-base jitter is less than 0.5ps RMS for accurate jitter measurements in the range of 1ps or higher. Jitter Measurement Error Due To Time Base Jitter 45%
Slide 16: Consistent Period Jitter Measurements Across Different Instruments
Consistent Period Jitter Measurements Across Different Instruments Other Jitter Error Sources: • Test Probe: • Do not use active probes for jitter measurement. The probe amplifier adds to the instrument jitter floor significantly. • Test Fixture: • Use good lab power supplies to reduce POWER SUPPLY
Slide 17: Recommended Period Jitter Measurement Setup
Recommended Period Jitter Measurement Setup Setup 1: Connection to scope through 15 pF comparator. Maintains 15pF High loading to the oscillator. slew rate SiTime jitter characterization setup Setup 2: Direct connection to oscilloscope. Causes 50Ω loading to the oscillator output. 50 Ω www.sitime.co
Slide 18: Summary
Summary • Jitter is the result of edge uncertainty in clock signals • Using default oscilloscope auto-setups is not optimal for accurate and consistent jitter measurement • Tips for measuring period jitter with real-time digital oscilloscope: • Use oscilloscope with low time base jitter (less than 0
Slide 19: Contact Information
Contact Information • For Questions, contact SiTime Technical Support Technicalsupport@sitime.com • For Turbo Webinar pdf Downloads on SiTime’s Web Site www.sitime.com/support/sitime-u/turbo-webinars •All new webinars will be posted within 24-hours • For app notes and other info, visit http://www.si