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22 DESIGN007 MAGAZINE I JANUARY 2023 are attenuated, rollingoff the signal rise time, distorting the signal, reducing bandwidth, and closing the eye. Reflections are reduced dramatically by elim- inating the stub. Back-drilling the via stub is a common practice on thick PCBs to minimize stub length for bit-rates greater than 3Gbps (1.5GHz). However, at transmission rates >10Gbps (5GHz), back-drilling alone may not be adequate to reduce jitter and bit error rate. Eye diagram analysis is not the same as a bit error rate (BER) analysis, but the two tech- niques are oen used in conjunction. e bit error rate percentage is calculated as the num- ber of bit errors per unit of time. Bit synchroni- zation errors are a factor, as well as distortion, interference, and noise. e eye pattern is a composite signal that indicates the channel bandwidth, attenuation, jitter, reflections, comparative delay, and rise/ fall time variations. Eye pattern measurements can show the overall signal integrity of a data path. ey provide instant visual data that digi- tal designers can use to check the signal integ- rity of a design and uncover problems early in the design process. Key Points • An eye diagram provides a visual indica- tion of the voltage and timing uncertainty associated with the signal. • e stimulus is normally a pseudo-random bit stream. Each cycle waveform is aligned to a common timing reference, typically a clock. • Overlaying many bits produces an eye diagram. • e quality of a high-speed digital signal can be quickly determined by using a compliance mask overlay. • e eye mask is made up of the setup/ hold time and stable voltage threshold specifications. • Jitter arises when a rising or falling edge occurs at times that differ from the ideal. • e impact of termination is clearly visible in the eye diagrams generated. • A practical way to analyze DDR signals is via the composite eye diagram. • A composite eye diagram can tell the exact jitter content and distribution in your memory interface, as the DQ signals are referenced to the strobe signal. • Reflections at the peaks on the waveform indicate inappropriate termination. • Eye diagrams can also pick stubs on high-speed serial links. • Back-drilling the via stub is a common practice on thick PCBs to minimize stub length for bit-rates greater than 3Gbps (1.5GHz). Figure 5: High-speed 10Gbps. Ethernet differential signal (left), and with via resonance (right).