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For high-speed design and analysis, Tektronix recently introduced wide bandwidth DSA/DPO70000 oscilloscopes. To ensure signal integrity and accurate measurements when verifying, debugging and testing high-speed signals with these new-generation instruments, probes play a crucial role. Also, finer circuit dimensions and higher circuit density with increasing use of differential signaling are placing new demands on probe attachment performance. To address these stringent requirements of circuit designers, Tektronix has developed a new probe architecture, called Z-Active. This probe architecture preserves high bandwidth while providing improved connectivity with low loading. In essence, the Z-Active architecture is a hybrid approach composed of a distributed attenuator topology feeding an active probe amplifier (Figure 1). In fact, it is a combination of compensated attenuator and Z₀ probes. Hence, it acts like a compensated attenuator for signal frequencies from dc to 160 MHz, and for frequencies above 160 MHz, it acts like a Z₀ probe.

As shown in Figure 1, the buffer amplifier is at the heart of the active probe. This silicon germanium (SiGe)-based buffer amplifier, which drives the 50 Ω coaxial cable, offers enough high-frequency gain to compensate for skin loss effect in coaxial cables.

In reality, the Z-Active probes use a tiny passive probe tip element that is separate from the amplifier, extending the usable reach of the probe. In traditional active probes, adding this much length can introduce signal fidelity problems. However, this architecture maintains high dc input resistance and presents a higher ac impedance than previous probe architectures. It accomplishes this while providing significant length between the probe body and the probe attachment point to the device under test (DUT). This architecture provides the best of both worlds: high dc impedance like existing active probes and the stable high-frequency loading of Z₀ probes.

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