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Techniques to Measure and Avoid Jitter in PCBs

Author Profile img: Poulomi Ghosh

By Poulomi Ghosh

June 14, 2023 | 0 Comments

techniques-to-measure-and-avoid-clock-jitter-in-pcbs.jpg

Contents

A signal’s deflection from its ideal rising and falling edges is called a jitter in PCB. The jitter in clock signals can impact the timing synchronization and consequently disrupt the overall signal integrity of the system. Clock signals with less jitter are essential to meet high-speed data communication.

Non-uniform impedance, crosstalk, interference, and power supply noise are some prevalent factors that reduce the signal-to-noise ratio (SNR) and cause jitter in communication channels. Clock jitter in your designs can be detected using an eye diagram.

In this article, you will learn:

  • How to measure clock jitter using an eye diagram
  • Design techniques to eliminate jitter in circuit boards
  • Various types and sources of jitter in PCBs

3 ways to measure clock jitter in your PCB

Primarily, there are three ways to assess jitter in time domain:

Period jitter

Period jitter is the deviation of the clock cycle from its ideal position. It compares each period length with the average clock period of the ideal signal. According to JESD65B and JEDEC standards, you need to assess the signal over 10000 clock cycles to have enough data to evaluate period jitter.

Cycle-to-cycle jitter

Cycle-to-cycle jitter indicates the maximum variation between any two adjacent clock periods. It is represented using an absolute magnitude instead of positive or negative values.

pcb-clock-jitter-classification.jpg
PCB clock jitter classification

Timing interval error (TIE)

It refers to the difference between the observed and the ideal clock edges. You can measure the TIE jitter by subtracting the actual clock edge from the ideal clock edge. TIE relates to clock edges, whereas period jitter corresponds to the clock period.

 

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How to measure clock jitter with an eye diagram

An eye diagram, also known as an eye pattern, is a type of oscilloscope display to visualize the quality of a digital signal. You can analyze the pattern to evaluate the signal integrity parameters, such as clock jitter, inter-symbol interference, bit error rate (BER), and SNR. You can also capture the eye diagram in a software tool and compare it with the same specifications to determine if your signal meets the required performance criteria.

eye-diagram-at-transmitter-and-receiver-in-pcb.jpg
Eye diagram captured in an oscilloscope measures clock jitter in PCB

You can create an eye diagram by sampling the digital signal repetitively and applying the samples to the vertical input of the oscilloscope. A wider and higher eye-opening indicates superior signal performance with low clock jitter. Here, various eye diagram parameters are explained below:

Eye amplitude

An eye amplitude refers to the difference between the one and zero levels. Based on the eye amplitude, the logic circuits in the data receiver will decide if a received data bit is a 0 or 1. The one and zero levels are measured in the 40% to 60% range of the horizontal scale of the eye diagram.

Eye height

The vertical opening of an eye diagram refers to the eye height. An ideal eye-opening should be equal to the eye amplitude. When noise introduces into the system, the eye will close based on the noise level. Furthermore, the height of the eye-opening indicates the signal-to-noise ratio, which is a measure of how much noise is present in the signal.

eye-pattern-to-measure-pcb-jitter.jpg
Various eye diagram parameters

Eye-crossing percentage

It is a measure of the amplitude of the crossing points. It indicates duty cycle distortion in the high-speed signal and provides a clear indication of the system’s data pulse symmetry. To determine the eye crossing percentage, you have to find out the one level, zero level, and crossing level on the oscillator waveform. Use the following equation to calculate the crossing percentage:

Eye crossing % = 100 ✖ [(crossing level – zero level)/(one level – zero level)]

eye-crossing-percentage-in-pcb.jpg
Eye-crossing percentage

Bit period

Refers to the horizontal opening of an eye diagram including the crossing points.

Eye width

It is a measure of the horizontal opening of an eye diagram excluding the crossing points of the eye.

Rise time

Transition time from 10% to 90% of the signal level.

Jitter

The time deviations in rising and falling edges from their ideal positions at the crossing point of an eye diagram represent jitter. The width of the eye-opening indicates the amount of clock jitter present in the signal.

Intersymbol interference

ISI is a phenomenon that occurs when the signal from one symbol overlaps with the other. The presence of ISI distorts the shape of the eye.

 

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6 design rules to fix jitter in PCBs

1. Terminate the trace correctly

Terminate the trace correctly using series, parallel, Thevenin, and ac termination strategies. Implement series termination to terminate the trace at the driver’s end. The other three mitigate the signal reflection at the receiver end.

a. Series termination: Place the resistor as close as possible to the driver. Here, the sum of the transmitter and termination impedances will be equal to the transmission line impedance.

value of the resistor makes the trace impedance equal to the sum of the impedance of the termination resistor and the output.

series-trace-termination-in-pcb.jpg
Series trace termination

b. Parallel termination resistor:  In this method, the resistor is connected in parallel with the receiver as shown in the circuit below.

parallel-trace-termination-in-pcb.jpg
Parallel trace termination

c. Thevenin termination: This technique uses a Thevenin equivalent circuit to terminate the transmission line. The circuit consists of two resistors and a voltage source.

Here, Thevenin’s resistance is equal to the characteristic impedance of the transmission line. The source voltage is equal to the voltage at the end of the transmission line without termination.

thevenin-trace-termination-in-pcb.jpg
Thevenin trace termination

d. AC termination: This method uses a capacitor and a resistor in series to terminate the transmission line. Here, the equivalent impedance will be equal to the characteristic impedance of the signal at the clock frequency.

ac-trace-termination-in-pcb.jpg
AC trace termination

2. Include decoupling capacitors

Use decoupling capacitors near clock drivers and receivers. They help to filter clock jitter in circuit design.

3. Incorporate guard traces

Place guard traces around clock signals with 3W to 5W spacing. They protect clock signals from noise and interference.

guard-trace-in-pcb.jpg
Guard traces around clock signal

4. Employ ground planes

Apply ground planes wisely to distribute ground currents evenly, which helps to reduce clock jitter and improve signal integrity.

5. Avoid clock signal routing near critical signal traces

Refrain from routing clock signals near high-current traces and sensitive analog signals, which might introduce EMI in your system.

6. Employ clock buffer

Use a clock buffer with low-impedance output to trim down clock jitter in PCB design.

Types of jitter in time domain

Clock jitter affects the system’s performance, regardless of whether it is correlated or uncorrelated with the interference source. Jitter can be classified into three types based on its characteristics and sources:

Random jitter

Random jitter, also known as Gaussian or thermal noise jitter, contributes the most to the overall system timing uncertainty. This cannot be detected and eliminated easily and is characterized by a Gaussian probability distribution function.

In the waveform below, two tails extend away from the center. Although, the probability at some points becomes negligible and they don’t reach zero, implying random jitter presence in the system.

random-jitter-in-pcbs.jpg
Gaussian probability distribution function of random jitter

Deterministic jitter

Deterministic jitter, also called non-Gaussian jitter, arises due to clock skew, crosstalk, signal reflections, impedance discontinuity, or EMI. Most deterministic jitter is periodic and repetitive, therefore, this narrowband timing variation is easier to detect and eliminate.

Deterministic jitter is sub-classified into:

  • Periodic jitter: It occurs at regular intervals due to cyclic timing deviations (explained in the upcoming section) in the system. It is mainly caused by power supply noise and has the same periodic frequency as the noise source.
  • Data-dependent jitter (DDJ): Data-dependent clock jitter dynamically modifies system duty cycles and produces irregular clock edges. Duty cycle distortion occurs when a clock generates a positive pulse width unequal to the negative pulse width. DDJ in PCB will arise due to intersymbol interference and channel attenuation.

To learn more, read how to reduce signal attenuation in high-speed PCBs.

Frequency domain jitter or phase noise

An instance of a clock jitter in the frequency domain is phase noise. It is the noise of an oscillator deduced from the ratio of the power at the given offset frequency to the total power of the carrier.

Phase noise is significant in RF PCBs where one RF signal can interfere with adjacent signals at the transmitter and receiver sides. Additionally, a spectrum analyzer with a near-field probe and a low-noise amplifier can analyze the phase noise in an oscillator.

Once the power spectrum is obtained, you can estimate the phase noise at a specific offset within a 1 Hz bandwidth from the carrier frequency. Use the formula below to determine the phase noise at each center frequency in a 1 Hz bandwidth for a particular offset:

Phase noise, L(f)= 10 log [ P(fm) / P(f0) ]

phase-noise-in-pcb.jpg
Phase noise measurement

By integrating the phase noise at predetermined frequency offsets from the carrier signal, you can calculate the phase jitter in a PCB. Phase jitter refers to the energy present at the specified offset frequency compared to the energy of the carrier signal.

phase-jitter-in-pcb.jpg
Compute phase jitter by integrating phase noises calculated in different offset frequencies

 

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What causes deterministic jitter in PCB design?

Power supply voltage fluctuations

Voltage fluctuations in the power supply sources cause clock jitters. Placing decoupling capacitors away from the power supply, common-mode noise due to inadequate grounding, and linking high-frequency noise into power tracks are responsible for this kind of interference.

Crosstalk shifts the phase of the signal

It is the unwanted electromagnetic coupling of signals between adjacent traces on a circuit. Crosstalk can shift the phase of the waveform and the signal can arrive at the destination at a different time than the expected one.

crosstalk-causes-clock-jitter-in-pcb.jpg
Crosstalk between two traces causes clock jitter

Reflection and ringing induce delay

Signal reflections that travel back and forth on a transmission line are one of the primary contributors to jitter. The actual signal combines with the reflected signal and arrives at the receiver with a delay. This results in timing errors or jitter in PCB.

 

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Dielectric material with high-loss tangent

PCB material with a high-loss tangent behaves like a low-pass filter and attenuates the clock signal. This attenuation increases with the frequency and thereby escalates the clock jitter.

pcb-signal-attenuation.jpg
Signal attenuation caused by lossy PCB material

What contributes to random jitter?

Shot noise

The random movement of charge carriers in a semiconductor causes shot or white noise. This movement depends on the fluctuation of the current from its average value and the random distribution of electrons and holes in a semiconductor.

Flicker noise

Flicker or pink noise occurs when trapped charge carriers are randomly released between the interfaces of two materials. It is inversely proportional to the frequency.

Thermal noise

Even when there is no average current flowing through the circuit, thermally excited charge carriers cause random voltage fluctuations. This is referred to as thermal noise or Nyquist noise.

Key takeaways

  • Period jitter, cycle-to-cycle jitter, and time interval error are the 3 ways to measure jitter in PCBs.
  • Distorted eye width in an eye diagram indicates the presence of a clock jitter in your design.
  • Implement the right controlled impedance, trace termination, and grounding strategies to avoid jitter.

 

With technological advancement, clock oscillators must provide stable clock pulses in a circuit design. Jitter in the time and frequency domains can disrupt synchronization and jeopardize the signal integrity in the circuit.

Comment below if you have any queries on jitter and eye diagram analysis. We will be glad to help you.

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