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DIY 1.3 GHz FET Probe

An active scope probe with high-ish bandwidth and very low input capacitance

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I wanted to see if an oscillator in one of my circuits was actually oscillating - so I needed a probe with low capacitance. That's why I was looking for such a thing, and I found the design that this probe is based on.

I needed a scope probe with low input capacitance for the following use case:

In one of my projects, I wasn't sure if a crystal oscillator was actually oscillating. Probing with the stock probes that came with my Rigol DS1054Z didn't seem like such a good idea. The probe has about 10 pF of capacitance, and the crystal's nominal load capacitance is about 7 pF - that's a problem. If it's loaded correectly, attaching the probe might stop it. If the load is too little, attaching the probe might allow the oscillator to oscillate at all. So a probe with low input capacitance was needed (this is not a sound assessment of the situation, I just guessed that).

Looking for ready designs online revealed a design from a bachelor's thesis, which I adapted a bit. In fact, it had already been picked up by Wolfgang:

https://electronicprojectsforfun.wordpress.com/rf-measurement-techniques/high-frequency-probes/

After I (re-)designed the PCB, I sent one to him for some measurements. Here are the results:

https://electronicprojectsforfun.wordpress.com/solder-in-probes/

  • Design Files!

    Christoph04/19/2022 at 20:49 0 comments

    Repo for the initial version which was just an eyeballed copy of Wolfgang's design: https://github.com/crteensy/FETProbe_v0

    Repo for the "tiny" version with RF shield and power supply/signal through u.Fl: https://github.com/crteensy/FETProbe_tiny

    The build is pretty simple, but there are two things to point out:

    The input capacitor is not a component you can pick up and place, but it's formed by copper features on the PCB. Here's the top part, a large pad next to the FET's gate 1 (pin 4):

    On the bottom we have copper as well, but connected to the through-hole input pad and configurable by some little extra copper pads:

    We haven't tried it but theoretically the probe can be tuned a bit with this.

    The second thing to note is that the RF shield must be modified prior to soldering, because it would otherwise provide too much stray capacitance and wouldn't really fit either. So one sidewall has to be removed:

    I did this by filing the edge off a bit and then wiggling it until it fell off entirely.

    However, before the shield can be soldered on, all the other parts have to be soldered. It's not much:

    I'm thinking about ways to make it smaller, but that would probably require a custom RF shield. And it works.

  • The beginning

    Christoph04/18/2022 at 22:22 1 comment

    Wolfgang designed his PCB in PCBExpress (I think), so I recreated his layout in KiCad. This was the result:

    This was pretty small already, but was quite inconvenient for a couple of reasons:

    • The power supply didn't have a connector, it was done through two soldered wires. That's not a problem per se, but the signal output is done through an SMA jack. It was only partially "pluggable".
    • Without an RF shield, touching the probe changes its output. That is indeed bad.

    I sent it to Wolfgang anyway, and it performed well. The next step was to improve the design a bit towards a tool that can be handled well, and that can be solderen into a circuit (for when lead inductance really counts). The SMA was swapped for a u.Fl, and so was the power supply input. Adding an RF shield wasn't so easy because the common, small, off-the-shelf parts all have four walls. I picked a small one that looked easy to modify, and came up with this:

    This is the probe characterized on Wolfgang's page:

    https://electronicprojectsforfun.wordpress.com/solder-in-probes/

    Next log will be about assembly and will also contain a link to a repo with layout files and all that.

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