Intersymbol interference, or ISI, occurs when a channel spreads a symbol’s response into the times used to read other symbols. The receiver’s decision then depends on neighboring data as well as the intended symbol. This makes the data pattern part of the measurement.
Use this guide to plan a channel comparison or understand why a serial link becomes difficult to receive. Start with the signaling mode, rate, transmitter, receiver, fixture and a known test pattern. The worked model below uses binary symbols; multilevel systems need their corresponding decision levels and test requirements.
See how earlier symbols change a sample
Changing a long run of identical bits to an alternating pattern can change the observed result because the channel sees a different history. Preserve the pattern and its length when comparing two fixtures or settings.
Read an eye diagram with its acquisition conditions
An eye diagram overlays repeated symbol intervals. Its shape helps reveal amplitude and timing variation, but the display depends on the clock reference, triggering, equalization, bandwidth and number of observations. Tektronix’s Jitter Map explanation treats ISI and other pattern-related mechanisms as part of data-dependent behavior.
An apparently open eye from a limited acquisition can miss rare events. A closed unequalized waveform may also be expected at the input of a receiver designed to apply equalization. Tektronix’s SerDes debug discussion explains why receiver functions and bit-error measurements matter alongside waveform inspection.
- Identify the observation point: transmitter, channel output or an accessible receiver/test point.
- Record whether the displayed eye is raw or processed and which clock recovery/equalization was applied.
- Keep pattern, rate, sampling depth, bandwidth and fixture correction with the image.
- Use the protocol’s actual test definition when making a compliance claim.
Make the channel change the controlled variable
A channel coupon can introduce a documented interconnect response into a test. Trace length alone is an incomplete description: material, geometry, discontinuities, connectors and return paths affect the result. Characterize or obtain the response of the complete fixture over the needed bandwidth.
- Save a baseline with the reference channel, fixed transmitter settings and documented receiver configuration.
- Insert the candidate channel or stress fixture and confirm the intended connections and port mapping.
- Preserve pattern, rate, voltage settings, measurement bandwidth and temperature for the comparison.
- Record both waveform observations and error-count behavior where the equipment supports them.
- Change equalization separately, then repeat the comparison so its contribution is visible.
- Return to the reference channel to check that the setup itself has remained consistent.
Use the board-connector guide to identify which launches and mating interfaces belong in the channel definition. If a fixture correction removes them mathematically, make that boundary explicit.
Give a zero-error run a denominator
When errors occur, keep their count, locations if available and the test duration. Pattern-related clusters can suggest a different cause from disturbances associated with cable movement, temperature or power events. Choose the next experiment to separate those possibilities.
Use compensation with a repeatable acceptance test
Transmitter de-emphasis and receiver equalization can reduce the effect of channel memory, within their supported range. They also have interactions and costs that depend on the link. Use the de-emphasis guide for amplitude conventions and a controlled setting comparison.
Define acceptance around the actual product: required error behavior, operating conditions and repeatability after restart or reconfiguration. Save the channel identity, firmware, settings and fixtures so another engineer can reproduce the result. If a change improves one pattern but degrades another, report both rather than selecting only the most attractive eye.
The useful conclusion names the condition resolved: for example, a documented channel and supported equalization setting met the planned test across the stated conditions. That evidence is much more actionable than “the eye looks better.”
Keep a working record
Download the intersymbol interference worksheet (editable text). Save a copy for each comparison or test. It includes the example assumptions, fields for source references and space for your results.