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Accurate Measurements On High-Speed Rambus Traces Present Challenges
Using time-domain-reflectometry normalization and fast oscilloscopes, measurements can be made on very short traces.
Date Posted: March 20, 2000 12:00 AM
Coupon Measurements Validate It
An experiment was performed to validate the accuracy of the TDR measurement on progressively shorter Rambus traces. A 6-in.-long, 28-Ù microstrip test coupon fabricated from FR4 material was used. The characteristic impedance was measured with the 10-GHz TDR probe. The resultant TDR measurement can be seen in Figure 8 as the longest trace waveform.
The board was then cut in half to measure the remaining 3-in. section (middle waveform), and cut one more time to yield a 1.5-in. section. A measurement marker was placed 26.4 mm from the probe-tip reference plane, marking the physical location within the microstrip structure where the measurements were taken. At the 26.4-mm marker location, the three measurements were within 300 mÙ of each other.
When performing characteristic-impedance measurements of Rambus motherboards and SO-RIMM modules, the errors that are introduced by test fixturing and probes must be removed. The critical nature of Rambus signal-integrity measurements calls for calibration procedures beyond the standard TDR test methodologies. Effects of ground-lead inductance, cable attenuation, and connector reflection increase measurement error outside of the acceptable limits for short Rambus traces. When working with Rambus motherboard and SO-RIMM signals, get the necessary confidence with time-domain-reflectometry normalization and verification with a precision calibration substrate.
*Normalization is firmware built into the HP 54750A that's based on the Bracewell transform originally licensed from Stanford University. A detailed normalization measurement technique is described on Intel's web site at the URL http://developer.intel.com/design/chipsets/memory/pcbtest.htm.
R&D Equipment List:
HP 54750A digitizing oscilloscope
HP 54754A differential TDR plug-in module
HP N1020A TDR probe kit
HP 54006 6GHz resistive divider probe
HP 54701A 2.5 GHz Active probe
HP 54121-68701 RF accessory kit
Manufacturing Equipment List:
HP 83480A-K16 Rambus RF switch matrix
HP 83480A-K17 Rambus RIMM test fixture