Oscilloscopes are invaluable instruments for electrical engineers when debugging or characterizing a circuit. Nonetheless, you should get a sign into them to get their full profit. Inventive technologist James Wilson developed a single-ended 2GHz lively probe and has open sourced the design. The practically flat frequency response is unbelievable, particularly contemplating the DIY-friendly elements. Furthermore, this open supply design affords an reasonably priced various to the market’s high-priced (and proprietary) lively probes.
Passive probes are nice for general-purpose functions. They have an inclination to have a number of hundred megahertz of bandwidth, however that vary comes with 10-15 picofarads of loading. Lively probes provide a lot larger bandwidth with a lot decrease capacitive loading, however they’re costly and are normally locked to a particular vendor’s oscilloscope.
Wilson’s 2GHz lively probe design defies these trade-offs. It’s open sourced and makes use of comparatively frequent elements. For instance, the core amplifier IC is a 3GHz JFET amplifier from TI that is available in a SOIC package deal, the PCB design makes use of OSH Park’s four-layer service — which isn’t impedance managed, and the passives are not any smaller than 0402! General, the probe achieves the next specs:
- DC–2GHz analog bandwidth
- 10:1 attenuation
- Enter impedance of 1 MΩ // 1 pF
- 50 Ω output impedance
The probe’s design has three basic blocks: an enter community, a composite amplifier, and an influence provide. Every block has no less than one trick to perform the general design.
2GHz probe response (📷: James WIlson)
For instance, the enter community has a frequency compensation capacitor. On paper, a trimmer capacitor appears good for this operate. Nonetheless, its parasitics are lower than perfect. Wilson toyed with the concept of a plate capacitor and trimming away copper. After all, that has a draw back: as soon as the copper is eliminated, it’s gone.
To this point, Wilson has produced three revisions of the 2GHz lively probe. On the third revision, Wilson’s probe achieved a comparatively flat response from DC to a -3 dB bandwidth of 2GHz. Because it outputs to a 50 ohm SMA and makes use of a barrel jack for its energy provide, it could work with virtually any oscilloscope (or different 50-ohm take a look at software.) This versatility empowers you to make use of the probe in numerous functions, making it a invaluable addition to your toolkit.
You may obtain the Altium design recordsdata and PCB gerbers from this GitHub repository. Wilson additionally included simulation recordsdata from an open-source subject solver referred to as openEMS, which supplied the simulations for the transmission line. Try the 2GHz lively probe weblog submit for detailed explanations of the whole circuit.