Circuit: DIY 1-GHz Active Probe Circuit

Measuring a specific point (node) in an RF circuit with a regular oscilloscope probe can disrupt the circuit's performance, even when the probe is set to a less intrusive mode. A solution is a specialized probe designed to minimize interference. Check out this 1-GHz active probe design.
 

Active Probe Circuit

The active probe circuit is fairly simple. Refer to the circuit diagram of the do-it-yourself active probe. A dual-gate MOSFET guarantees light, uniform loading of RF signals over a frequency range extending well beyond the 1-GHz mark.
 
"A dual gate MOSFET, T1, is used in a source-follower configuration," the designer explains. "This provides a low output impedance to drive the coax cable and test equipment. The signal at the probe tip is applied to gate 1. The impedance at gate 1 is a very high resistance shunted by a few picofarads of capacitance. The choice of MOSFET used in the circuit is not critical ...  Be sure however to steer clear of ‘-R’ suffix devices because they have a different pinout and will not work on the proposed PCB."

The PCB was designed with compactness and low input capacitance in mind. Hence, the use of SMD parts.
 

The Original Project

The article, “Poor Man’s 1-GHz Active Probe,” appeared in Elektor April 2004. You can download the article for free during the two weeks following the publication of this news item. Enjoy the article!

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Editor's Note: The article was first published in a 2004 edition of ElektorMag. Some of the components, products, PCBs, and/or links may no longer be available. Still, we feel the educational content is valuable, and we think it will inspire you to start new projects of your own.