High-impedance, high-bandwidth oscilloscope probe captures serial bus signals

The Infiniimax 4 active probe from Keysight reaches 52 GHz, enough to see signals on PCIe 6/7, 800 Gb/sec Ethernet, DDR 5/6, and other high-speed serial buses.

Oscilloscopes with 100 GHz and higher bandwidths can display signals on 224 Gb/sec PAM4 serial buses and they’ve been available for several years. Without probes of high enough bandwidth, your signals will attenuate and distort before they ever reach the oscilloscope’s front end. That’s where the Infiniimax 4 active probe from Keysight can help.

Working in conjunction with Keysight’s Infiniium UXR–B series oscilloscopes, the Infiniimax 4 probes feature bandwidth up to 52 GHz (Brickwall filter front end), 40 GHz (Bessel-Thomson filter). The plot shows that the probe maintains an impedance of 500 Ω out to its rated bandwidth. The probe’s input impedance is, of course, greater at lower frequencies. PAM4 signals at 224 Gb/sec data rates need that 52 GHz bandwidth because they produce two bits per symbol clocked on both rising and falling edges, hence the need for bandwidth that exceeds 50 GHz.

The Infiniimax 4 probe consists of a flexible probe tip with solder points. Four 100 kΩ resistors just behind the solder tips provide a high input impedance while minimizing the distance to the solder tips. The term “high impedance” refers to the probe’s input impedance relative to the transmission channel’s impedance, to minimize the energy that goes into the probe.

The flexible probe tip attaches to a base using three screws so you can easily disassemble the base and replace the flexible tip. A rigid cradle holds the base and the amplifier module that connects to the probe. From there, a cable brings the signal to the oscilloscope.