Frequency dependent effects are becoming more prominent with the increasing data rates of digital systems. Differential circuit topology is proliferating throughout design laboratories with the goal of enhancing the data carrying capable of the physical layer. Simple impedance and delay measurements of copper transmission lines on backplanes are not sufficient to ensure accurate analysis of gigabit interconnects. The challenge to push design rules to the limit now requires the use of concurrent time and frequency domain analysis. This paper will discuss methods to achieve proper characterization using a Time Domain Reflectometer (TDR) oscilloscope and Vector Network Analyzer (VNA). Measurement accuracy and error correction techniques will be discussed for both time domain and frequency domain instrumentation. It will be demonstrated that accurate 4-port frequency dependent models can closely simulate performance of a differential channel.
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