Application Report
SNAA285 – October 2015

Time-Domain Jitter Measurement Considerations for LowNoise Oscillators
Arvind Sridhar and Gabe Ayala
ABSTRACT
Phase Noise and Jitter are important metrics that describe the quality of a low-noise oscillator. Jitter is atime-domain representation of the timing accuracy of a signal and Phase Noise is the frequency domain
representation of noise spectrum around the carrier tone. Equipment for measuring jitter in time-domain,such as real-time oscilloscopes, excel in their ability to directly measure period jitter, cycle-to-cycle jitter,TIE and data-dependent jitter which makes them an ideal choice for characterizing high speed serial links.
Frequency domain equipment, such as a Phase Noise Analyzer, on the other hand, measures the RMS
power of a signal in a frequency band of interest. Phase Noise Analyzers typically have a much lower
noise floor than most real-time oscilloscopes making them an ideal choice for characterizing ultra-low jitter
oscillators such as LMK61E2.

Trademarks

SNAA285 – October 2015
Submit Documentation Feedback

Time-Domain Jitter Measurement Considerations for Low-Noise Oscillators
Copyright © 2015, Texas Instruments Incorporated

1

www.ti.com

1

When measuring jitter performance of oscillators, it is important to understand the limitations of themeasurement equipment used. This application note will primarily focus on highlighting some of thelimitations of real-time oscilloscopes used for time-domain jitter analysis.
Time-domain jitter specifications of interest are period jitter, cycle-to-cycle jitter, time interval error (TIE)
and Total Jitter (TJ).
Per JEDEC Standard JESD65B, period jitter is defined as the deviation in cycle time of a signal withrespect to the ideal period over a random number of cycles. The number of cycles selected for thismeasurement should be significant (1000 to 10,000) to have sufficient confidence in the measurement.
Peak-to peak period jitter is defined as the difference between the largest and the smallest period value
from the sample size used for the measurement. Period jitter can also be specified as a root means
squared (RMS) value in lieu of peak-to-peak. This jitter specification is primarily used for guaranteeing
setup and hold times for logic in CPUs and FPGAs.
Cycle-to-cycle jitter is defined as the variation in cycle time of a signal between adjacent cycles over arandom sample of adjacent cycle pairs. Once again, the number of cycles selected for this measurement
should be significant (1000 to 10,000) to have sufficient confidence in the measurement. Cycle-to-cycle
jitter is typically specified as the greater of the absolute values of the worst case positive or negative
variation in cycle time between adjacent cycles. As an example, if the largest positive cycle-to-cyclevariation is 62ps and the largest negative cycle-to-cycle variation is -65ps, the overall cycle-to-cycle jitter iscommonly specified to be 65ps.
TIE, also known as accumulated jitter is the deviation of a clock period from the ideal clock period
measured over a significant number of cycles. It includes jitter contribution due to high and low jittermodulation frequencies. This specification of jitter is commonly used in SONET and Optical Transport
Networking (OTN) equipment.
Total Jitter (TJ) is another time-domain jitter specification commonly used in Serial IO applications that isderived from the TIE measurement. It is measured at a specified Bit Error Rate (BER) for the serial IO link
and combines the effect of Random and Deterministic jitter components. It predicts a peak-to-peak jitterthat will only be exceeded with a probability equal to the BER. Real-time oscilloscopes typically include
jitter analysis software tools that decompose TJ into random (RJ) and deterministic jitter (DJ) components.
The relationship between TJ, RJ and DJ is shown in Equation 1.
where•

n is the application BER multiplier(1)

Table 1 gives a complete list of multipliers for required BER.
Table 1. BER Multipliers

2BERMultiplier

10-3

6.582

-4

10

7.782

10-5

8.834

10-6

9.784

-7

10

10.65410-8

11.462

10-9

12.218-101012.934

10-11

13.614

10-12

14.26010-13

14.882-141015.47810-15

16.028Time-Domain Jitter Measurement Considerations for Low-Noise Oscillators
Copyright © 2015, Texas Instruments Incorporated

SNAA285 – October 2015
Submit Documentation Feedback

www.ti.com

The jitter specifications described above were measured using a real-time digital phosphor oscilloscopefrom Tektronix (DPO70804C) on a 312.5 MHz output frequency clock generated using the LMK61E2 highperformance programmable oscillator from Texas Instruments. The measurement results are shown inFigure 1.

Figure 1. Time-Domain Jitter Measurement Results
The sample size for the above measurements was > 10000 samples and the sampling rate was set to25GS/s (maximum allowable sampling rate for the oscilloscope). The acquisition interval for themeasurement was 40us. A Bit Error rate of 1E-12 was selected for the Total Jitter (TJ) measurement anda 20 MHz low pass filter was applied to the TIE, TJ, RJ, and DJ measurements.
Jitter measurements on real-time oscilloscopes are affected by factors inherent to the oscilloscope. The
major sources of noise within the oscilloscope are sampling jitter, vertical noise, timing stability and signal
interpolation error.
Signal interpolation error due to linearly interpolating between voltage samples can be minimized by using
sin(x)/x (or sinc) interpolation setting on the oscilloscope (if available) and by maximizing signal amplitude
to input full scale.
Vertical noise in an oscilloscope is intrinsic to the input attenuators, preamplifiers and A/D converters. This
noise varies with the front-end attenuator and amplifier settings, bandwidth limiting on the scope andsample size used for the measurement. Oscilloscope user manuals specify the vertical noise at aparticular V/div setting or as a percentage of the V/div setting.
Timebase stability is another potential source of error in real-time oscilloscopes. The timebase stability is
usually specified in the oscilloscope datasheets. ADC aperture uncertainty and quantization error are othersources of error in the oscilloscope that manifest as amplitude and timing noise.

SNAA285 – October 2015
Submit Documentation Feedback

Time-Domain Jitter Measurement Considerations for Low-Noise Oscillators
Copyright © 2015, Texas Instruments Incorporated3www.ti.com

The slew rate of the signal being measured affects the jitter. If the edge rate of the signal measured isslow relative to the system bandwidth, any amplitude noise at the threshold crossing of the signal canresult in increased jitter. Slew rate amplifiers can potentially be used to increase the slew rate of the signal
before it is connected to the oscilloscope. In general, the faster the edge rate of the signal being
measured, the lower will be the impact of any amplitude noise. The use of a limiting amplifier(ONET1191P) for total jitter measurement has been covered in detail in the applications note SCAA120A
from Texas Instruments. It should be noted that any limiting amplifier or slew rate amplifier device willcontribute some additive jitter to the signal being measured. Also, for measuring signals with fast slew
rates, an oscilloscope with adequate sampling rate should be used. The measurements in this applicationnote were not made using any limiting amplifiers or slew rate amplifiers.
The above sources of error are factored in a parameter called JNF for the Tektronix DPO70804C real-time
oscilloscope. Jitter Noise Floor is defined as the intrinsic noise portion of a jitter measurement. This sets
the lower limit on the amount of jitter that the oscilloscope can detect. If the input signal has jitter lower
than the jitter noise floor of the oscilloscope, the measured jitter on the oscilloscope will not accurately
represent the true jitter of the input signal.
JNF is described in Equation 2 (referred from Tektronix application note 61W_18786_2).

where•••••

A = input signal amplitude (volts)
trm = 10% to 90% measured rise time (sec)
FS = full scale range of input
tj = short/medium term aperature uncertainty (sec rms)
N = input-referred noise (volts rms)(2)

The details of this specification are beyond the scope of this application note.

4Time-Domain Jitter Measurement Considerations for Low-Noise OscillatorsCopyright © 2015, Texas Instruments Incorporated

SNAA285 – October 2015
Submit Documentation Feedback

www.ti.com

2Tektronix uses time interval error to measure JNF. The typical value of JNF reported in the oscilloscopedatasheets from Tektronix correspond to the longest record length and maximum sample rate setting. The
typical Jitter Noise Floor (RMS TIE) for DPO70804C is 450fs.
The unfiltered TIE measurement result for LMK61E2 using DPO70804C oscilloscope is shown in Figure 2Figure 2. Unfiltered TIE Measurement Results
The total measured RMS TIE is approximately 757 fs RMS. The oscilloscope contributes roughly 450 fs
RMS TIE. The RMS TIE due to the device (LMK61E2) can be calculated to be approximately 608 fs RMS.
Please note that the above jitter specification applies to Tektronix real-time oscilloscopes. Agilent lumps all
the jitter sources into a similar parameter called Jitter Measurement Floor. From the Agilent 90000 XSeries datasheet, the jitter measurement floor is specified by Equation 3.

(3)

The first term in Equation 3 corresponds to the noise influence due to the slew rate of the signal being
measured. The second term represents the oscilloscope intrinsic noise. These parameters can be found in
the oscilloscope specifications from the manufacturer website. Other scope manufacturers have their own
specification that factors in the sources of jitter discussed above.

SNAA285 – October 2015
Submit Documentation Feedback

Time-Domain Jitter Measurement Considerations for Low-Noise Oscillators
Copyright © 2015, Texas Instruments Incorporated

5

www.ti.com

3

The impact of the jitter noise floor of the oscilloscope can also be seen in the random jitter measurement
result (RJ). The filtered RJ from Figure 1 is ~287fs. A low pass filter of 20 MHz was applied for thismeasurement. Since the acquisition interval was set to 40us, effect of noise frequencies down to 1/40us =25 kHz are included in the measurement. In contrast, the integrated RMS phase jitter (12 kHz – 20 MHz)measured using the Agilent 5052 Phase Noise Analyzer (superior noise floor when compared to real-timeoscilloscopes) is ~87fs (as shown in Figure 3). Filtered RJ in time-domain is equivalent to the Integrated
RMS phase jitter in frequency domain.

Figure 3. LMK61E2 312.5 MHz Phase Noise
In summary, when performing jitter measurements in time-domain using real time oscilloscopes fromdifferent oscilloscope vendors, it is important to consider all the sources of jitter that are intrinsic to theoscilloscope. The impact of some of these errors can be minimized by careful action on part of theoperator. Ensuring fast slew rate for the signal being measured, use of the highest sampling rate available
on the oscilloscope for the measurement, setting the input signal amplitude to full scale and use of sincinterpolation or enhanced bandwidth setting (if available) on the oscilloscope are some of the steps thatthe operator can follow to minimize the sources of error in the jitter measurement. The oscilloscope withthe lowest jitter noise floor will represent the true jitter of the input signal most accurately.

6Time-Domain Jitter Measurement Considerations for Low-Noise Oscillators
Copyright © 2015, Texas Instruments Incorporated

SNAA285 – October 2015
Submit Documentation Feedback

IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATA SHEETS), DESIGN RESOURCES (INCLUDING REFERENCEDESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANYIMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRDPARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicablestandards, and any other safety, security, regulatory or other requirements.
These resources are subject to change without notice. TI grants you permission to use these resources only for development of anapplication that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license
is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you
will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of theseresources.
TI’s products are provided subject to TI’s Terms of Sale or other applicable terms available either on ti.com or provided in conjunction withsuch TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for
TI products.
TI objects to and rejects any additional or different terms you may have proposed. IMPORTANT NOTICE
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2022, Texas Instruments Incorporated