Characterization of Voltage and Freqency Margin

Characterizing voltage margin

Let us assume that we have a DDR bus operating at a voltage of 1.8V. Now assume that the operating voltage is reduced to 1.7V. The DDR bus may fail or may keep working. If it keeps working, it indicates robustness of the system. If it keeps working, we may want to further reduce the voltage to see if it fails at 1.65V. We iterate this process and find the range of voltage in which it keeps working. This gives margin of the system.

The purpose of voltage margin test is to check the range of voltage over which the IO bus continues to operate without errors. The methodology involves running a software to read, write and verify patterns on data bus while the voltage level is reduced in steps. If the margin is too low the system is likely to create failure condition in the field. A high margin indicates a robust system with respect to noise in amplitude domain caused by power supply noise, crosstalk or any other reason affecting the amplitude of the signal. In another form of voltage margining the reference voltage is shifted around the center point till failure condition is observed.

At the circuit design level, at least the first iteration of the PCB should contain ability of change the required voltages for which margin tests are required to be conducted. Depending upon the application there may be more that one voltage levels that may need to be changed for voltage margin characterization.

Characterizing frequency margin

Characterizing frequency margin is similar to the voltage margin. The frequency of the link under observation is increased while a software to write, read and verify the data keeps running. The frequency is raised till the failure condition is observed. High frequency margin indicate robustness of operation in time and frequency domain. The designer should continue to make efforts to improve frequency margin by continuously upgrading the design.

The only way to differentiate a robust system from marginal system is to conduct the margining test. If I have to do only one thing to ensure that the system is designed well, I will do the margin tests. It is the final test against which the capability of a signal integrity expert will be judged.

Bus characterization involves use of some special instruments - Oscilloscope, Time Domain Reflectometer (TDR), Scalar Network Analyzer, Vector Network Analyzer.

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