Open-Source 2 GHz Oscilloscope Probe

If you do any work with high-speed signals, you quickly realize that probing is an art unto itself. Just having a fast oscilloscope isn’t enough; you’ve got to have probes fast enough to handle the signals you want to see. In this realm, just any old probe won’t do: the input capacitance of the classic RC probe you so often see on low-bandwidth scopes starts to severely load down a circuit well below 1 GHz. That’s why we were really pleased to see [Andrew Zonenberg’s] new open-source design for a 2 GHz resistive probe hit Kickstarter.

The design of this new probe looks deceptively simple. Known as a Z0-probe, transmission-line probe, or resistive probe, the circuit works as a voltage divider, created from the 50-Ohm input impedance of a high-speed oscilloscope input and an external resistor, to reduce loading on the circuit-under-test. In this case, the input resistance has been chosen to be 500 Ohms, yielding a 10x probe. In theory, building such a probe is as simple as soldering a resistor to the end of a piece of coaxial cable. You can do exactly that, but in practice, optimizing a design is much more complex. As you can see in the schematic, just choosing a resistor of the right value doesn’t cut it at these frequencies. Even the tiny 0402-size resistors have parasitic capacitance and inductance that affect the response, and choosing a combination of parts that add to the correct resistance but reduce the overall capacitive loading makes a huge difference.

2 GHz Passive Probe Schematic

Don’t be fooled: the relatively simple schematic belies the complexity of such a design. At these speeds, the PCB layout is just as much of a component as the resistors themselves, and getting the transmission-line and especially the SMA footprint launch correct is no easy task. Using a combination of modeling with the Sonnet EM simulator and empirical testing, [Andrew] has ended up with a design that’s flat (+/- 1 dB) out to 1.98 GHz, with a 10-90% rise time of 161 ps. That’s a fast probe.

The probe comes in a few options, from fully assembled with traceable specs to a DIY solder-it-yourself version. You probably know which of these options you need.

We really like to see this kind of knowledge and thoroughness go into a project, and we’d love to see the Kickstarter project reach its goals, but perhaps the best part is that the design is permissively open-source licensed. This is a case where having the board layout open-sourced is key; the schematic tells you maybe half of what’s really going on in the circuit, and getting the PCB right yourself can be a long and frustrating exercise. So, have a look at the project, and if you haven’t got probes suitable for your fastest scopes, build one, or better yet, support the development of this exciting design.

We’ve seen [Andrew’s] oscilloscope work before, like glscopeclient, his remote oscilloscope utility program.

8 thoughts on “Open-Source 2 GHz Oscilloscope Probe

  1. Related: 1ghz active probe for $20 in parts, with a bunch of theory, as long as you have access to some good characterization and calibration equipment:
    1ghz passive probe with no PCB (so no issues with characterizing the pcb impedance):
    I’ve built and tested the latter and it worked quite well. It also requires use of a signal analyzer to trim. I want to use a probe that has close to 2x the bandwidth of the scope it’s on, so the probe rolloff isn’t the limiting factor, and this did well.

  2. Every scope company has such a probe, though 2ith higher bandwidth, typically 8-10GHz and with flatter response.

    I recently published an article in Signal Integrity Journal about such probes.

    The downside of several series resistors is that it increase non-linearity due to stray capacitance

    1. Yes, better probes exist – but none even close to this price range. Of course stuff like the Picoconnect 900 series, LeCroy PP066, Tek 54006A, R&S RT-ZZ80 will do better, but those are all $1000+.

      This probe is intended to compete with the likes of the Pico TA061, Keysight N2874A, etc for a fraction of the price. I’ve benchmarked it against the TA061, which I believe is the same OEM probe design as the N2874A, and got substantially flatter frequency response and close to 1 GHz more bandwidth. If you know of a <$600 commercial probe that outperforms my design I'd love to hear about it.

  3. Very cool. Have tinkered with HV probes a bit, using a balance of capacitance and resistance to attain reasonable frequency response. But pushing past 1 MHz was hard enough (HV probes are huge – 200 kV +…). Maybe a higher bandwidth one could be the next lock-down project.

  4. Just an Idea.
    If putting a resistor on the end of a piece of coax cable “sorta” works, then what about soldering a piece of resistance wire to the core of a piece of coax cable, then heating the core of the coax with a high current through the core and then pulling the resistance wire partially through the coax cable to replace the core.
    Or with some calculation, use a metal tube and a piece of resistance wire, both of the correct diameter and make a coaxial arrangement from that.

    Wire can be made very straight by putting one end in a vice, and the other end in a (accu) drill, and then pull on the wire and rotate the drill simultaneously to torque the wire.

    I would not now it this works. My scope stops at 50MHz…

    1. I think that most commonly available resistance wire, in gauges the same size as a coax core, would need to be several meters long to get the right resistance. By the time you’re into exotic alloys and special construction, you’re back to the $$$ a probe set problem again.

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