ISI Filter Emulating Transmission Lines for SerDes Testing
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Solution Overview
Problem
Economic production testing of high-speed SerDes devices is challenging due to costly and slow instrument-based solutions, and existing loop-back methods fail to adequately screen for faults caused by intersymbol interference (ISI) in data signals transmitted via transmission lines or cables.
Innovation Solution
A highly adjustable ISI filter using a combination of pin diodes and varactor diodes in a multi-stage resonance circuit is employed to emulate the effects of varying transmission line lengths and frequencies, allowing for precise injection of data-dependent jitter for testing transceivers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If instrument based solutions are used for testing SerDes devices, then measurement precision is improved, but productivity deteriorates due to costly and slow testing
Solution Approach 1:
The patent implements Built-In Self Test (BIST) functionality within the SerDes device itself, allowing the device to autonomously perform BER measurements without external instrument intervention. The device includes internal pattern generators, pattern checkers, and error counters that enable self-testing, thereby eliminating the bottleneck of external instrument-based testing and improving productivity while maintaining measurement precision.
Solution Approach 2:
The patent extracts the essential BER testing functionality from complex external instruments and implements it as integrated circuits within the SerDes device. By taking out the core measurement functions (pattern generation, comparison, error counting) and embedding them in the device under test, the system eliminates the need for slow external instrumentation while preserving accurate BER measurement capabilities.
2Ease of operation
If simple loop back testing is used between transmit and receive portions of SerDes, then ease of operation is improved, but reliability deteriorates because it does not cover faults from intersymbol interference
Solution Approach 1:
The patent applies preliminary anti-action by intentionally introducing artificial intersymbol interference (ISI) into the test signal path before the receiver processes it. The test system includes ISI injection circuits that pre-distort the signal with realistic channel impairments, thereby preventing the simple loopback method from missing critical faults related to intersymbol interference and enabling comprehensive reliability testing.
Solution Approach 2:
The patent introduces an intermediary component - the ISI injection circuit - between the transmitter and receiver in the loopback path. This intermediary adds controlled signal degradation that mimics real transmission channel effects, allowing the test to detect faults related to intersymbol interference while maintaining the simplicity of loopback configuration.
3Reliability
If external signal conditioning such as ISI injection is used to verify Rx input eye and equalization, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent merges the ISI injection functionality with the existing SerDes device structures by integrating the injection circuits directly into the transmit or receive paths. Rather than using separate external signal conditioning equipment, the test functions are combined with the device under test, reducing overall system complexity while maintaining reliable device characterization capabilities.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The filter effectively measures a device's ability to reject ISI effects, achieving up to 28% eye closure at 1 Gbps and 100% at 6.4 Gbps, emulating the characteristics of transmission lines from 6 to 90 inches in length, thereby improving the efficiency of SerDes device testing.
Implementation Method 1
A filter includes a pin diode, an inductive element, and a varactor diode coupled as a resonance circuit. The filter injects data dependent jitter onto a digital data signal with a given data rate for testing a transceiver.
Data Source
AI summary
A filter includes at least a pin diode, an inductive element, and a varactor diode coupled as a resonant circuit. The filter injects data dependent jitter into a digital data signal with a given data rate for testing a transceiver.