Mixed Mode Versus True Mode

When defining a TDR Setup for a differential DUT, you can select to use Mixed Mode or True Mode method of deriving S-parameters. This selection is made in the Differential Step Setup dialog. This topic explains the differences of these two measurement methods. The following measurement algorithms are used for each measurement mode:

Mixed Mode (default method)

Positive stimulus steps are input to both the DUT's (+) and (–) port inputs and the response from the (–) port is mathematically converted as if a negative step had been applied.

True Mode

A positive stimulus step is input to the DUT's (+) port input and a negative stimulus step is input to the DUT's (–) port input.

Designing perfectly matched sources for both positive and negative stimulus steps is impossible. To ensure accurate measurements, the two input stimulus steps must be identical. Since True Mode uses different step sources (negative and positive), they cannot be identical. Mixed Mode solves this problem by using the same step generator for both steps and then mathematically converting the response as if a negative step had been applied. The mathematical conversion is extremely accurate. This can be tested by comparing the S-parameters measured using both methods to the S-parameters measured on a high-performance network analyzer, such as an Keysight N5225A PNA Microwave Network Analyzer.

Since almost all TDR/TDT measurements are performed on passive devices, Mixed Mode provides the most accurate measurements. In rare cases, the DUT may be non-linear which may cause step compression. This compression can degrade performance. In this case, selecting True Mode may provide better results.

For these reasons, Mixed Mode provides the most accurate S-parameters for virtually all differential TDR measurements. Mixed Mode is the default mode in FlexDCA. Most Vector Network Analyzers (VNAs) stimulate in Mixed Mode.