Fig 2. This figure shows the impedance profile of an 1169A probe and a differential browser measured with a network analyzer. Note how closely this profile matches Figure 1. Yet PrecisionProbe software was able to make this measurement without the need of extra equipment.
When maximizing design margins, a large contributor to the error in a measurement is the medium through which the signal travels to the measurement system. Cables are inherently lossy, and probes add excess loading. Typically, probes will not be flat in their frequency response across their entire bandwidth range. In addition to the loss and loading, every piece of hardware is different, which means that the same signal traveling through different media could yield entirely different answers.
Oscilloscope vendors have added software recently to compensate for this loss. For instance, oscilloscopes have recently added waveform transformation software, which accounts for cable and probe characteristics and corrects for the loss and nonlinearities of the medium through de-embedding. The result has been an evolution in standards definitions as many now assume application of de-embedding to the waveform to adjust for cable/fixture/probe losses.
While this method will work in many cases, it does have a few drawbacks. The most noticeable is the fact that most correction across all cables and probes stems from a single S-parameter file (characterization model). Cables, fixtures, and probes do vary, which can lead to significant inaccuracies. In addition, actually measuring the characterization of the medium takes expensive equipment and time, which makes it prohibitive to model each individual cable or probe, which would ensure maximum margins in the design.
Agilent’s recently introduced PrecisionProbe software, which runs on its 90000 X-Series and 90000A Series oscilloscopes, solves both of these issues (Fig. 1). The software will characterize every probe or cable in the link and does not require extra pieces of equipment such as a vector network analyzer, or VNA (Fig. 2). The result is a simpler answer that enables users to maximize margins in the design.
