Unlock High-Frequency Oscilloscope Signals with James Wilson's 2GHz Active Probe

Oscilloscopes are invaluable tools for electrical engineers when debugging or characterizing a circuit. However, you need to get a signal into them to get their full benefit. Creative technologist James Wilson developed a single-ended 2GHz active probe and has open sourced the design. The nearly flat frequency response is fantastic, especially considering the DIY-friendly components. Moreover, this open source design offers an affordable alternative to the market's high-priced (and proprietary) active probes.

Passive probes are great for general-purpose applications. They tend to have a few hundred megahertz of bandwidth, but that range comes with 10-15 picofarads of loading. Active probes offer much higher bandwidth with much lower capacitive loading, but they are expensive and are usually locked to a specific vendor's oscilloscope.

Wilson's 2GHz active probe design defies those trade-offs. It is open sourced and uses relatively common components. For example, the core amplifier IC is a 3GHz JFET amplifier from TI that comes in a SOIC package, the PCB design uses OSH Park's four-layer service — which is not impedance controlled, and the passives are no smaller than 0402! Overall, the probe achieves the following specifications:

• DC–2GHz analog bandwidth
• 10:1 attenuation
• Input impedance of 1 MΩ // 1 pF
• 50 Ω output impedance

The probe's design has three fundamental blocks: an input network, a composite amplifier, and a power supply. Each block has at least one trick to accomplish the overall design.

For example, the input network has a frequency compensation capacitor. On paper, a trimmer capacitor seems perfect for this function. However, its parasitics are less than ideal. Wilson toyed with the idea of a plate capacitor and trimming away copper. Of course, that has a downside: once the copper is removed, it is gone.

So far, Wilson has produced three revisions of the 2GHz active probe. On the third revision, Wilson's probe achieved a relatively flat response from DC to a -3 dB bandwidth of 2GHz. Since it outputs to a 50 ohm SMA and uses a barrel jack for its power supply, it can work with almost any oscilloscope (or other 50-ohm test tool.) This versatility empowers you to use the probe in various applications, making it a valuable addition to your toolkit.

You can download the Altium design files and PCB gerbers from this GitHub repository. Wilson also included simulation files from an open-source field solver called openEMS, which provided the simulations for the transmission line. Check out the 2GHz active probe blog post for detailed explanations of the entire circuit.