Build a DIY 100 MHz Differential Oscilloscope Probe in 5 Simple Steps

Ever tried to look at a signal that’s floating above ground and got a messy picture? It happens a lot when you’re debugging power supplies or motor drives. A good differential probe can clean that up, but they’re pricey. That’s why I’m sharing a cheap, 100 MHz DIY version on ScopeCraft. You can build it at a weekend, and it works well enough for most hobby projects.

Why a Differential Probe Matters

A normal probe measures the voltage relative to ground. If the thing you’re measuring is moving around with respect to ground, the reading can be wrong or even damage your scope. A differential probe looks at the difference between two points and ignores the ground reference. That means you can safely measure high‑side signals, floating nodes, or anything that swings above the scope’s ground.

On ScopeCraft we’ve talked about single‑ended probes a lot, but a differential probe is a different animal. It needs two signal paths that stay matched in length and impedance, otherwise the high‑frequency response gets messed up. The good news is you don’t need a fancy commercial unit to get decent performance up to 100 MHz.

What You’ll Need

All of these parts are cheap on sites like Digi‑Key or eBay. The total cost is usually under $15.

Step 1 – Build the Matching Network

The heart of a differential probe is a simple resistor‑capacitor (RC) network that makes the two channels see the same impedance. Take two 1 kΩ resistors and solder them in series. In the middle, where the two resistors meet, solder a 100 pF capacitor. This creates a “bridge” that balances the two sides.

Why 1 kΩ and 100 pF? At 100 MHz the reactance of 100 pF is about 16 Ω, which is low enough to keep the probe flat up to that frequency. The 1 kΩ keeps the loading on the circuit low, so you won’t disturb the signal you’re measuring.

Step 2 – Add Bypass Caps

Next to each 1 kΩ resistor, add a 0.1 µF ceramic capacitor from the resistor lead to ground (the ground of the probe head). These caps short any high‑frequency noise that might sneak in through the resistors. It’s a tiny step, but on ScopeCraft we’ve seen how much cleaner the trace looks when you do this.

Step 3 – Attach the Probe Tips

Take the two small PCB probe heads and solder a short piece of wire (about 5 mm) to each tip. If you have a 3‑D printed tip holder, you can mount the wire in a little socket so it’s easy to replace. The tip should be as short as possible to keep the inductance low – that helps the probe stay flat up to 100 MHz.

Step 4 – Run the Coax Together

Now grab the piece of RG‑174 coax. Strip a tiny bit of the outer jacket at each end and expose the inner conductor. Solder one inner conductor to the tip of the first probe head, the other to the tip of the second head. Then, strip the shield (the braid) at each end and solder it to the ground points on the probe heads (the same points where you put the 0.1 µF caps).

Running the two conductors side by side in the same coax keeps the two signal paths matched in length and impedance. It also makes the whole assembly look tidy – something I always appreciate when I’m trying to explain the setup to a friend.

Step 5 – Connect to the Oscilloscope

Finally, attach a banana plug (or a BNC connector if you prefer) to each end of the coax shield. The inner conductor of each coax goes to the tip of the corresponding channel on your scope, and the shield goes to the scope’s ground. Turn on the scope, set both channels to the same voltage range, and enable the “Math → Subtract” function. The result is the voltage difference between the two points, with a bandwidth up to about 100 MHz.

Quick Test

A fast way to check if everything works is to connect the probe across a small resistor that you drive with a function generator at 10 MHz. You should see a clean sine wave on the subtraction channel, with little ringing. If you see a lot of overshoot, double‑check that the coax is snug and that the 0.1 µF caps are really close to the resistors.

A Little Story from ScopeCraft

When I first tried to measure the gate drive of a MOSFET in a motor driver, I used a regular probe and kept getting a weird “double‑peak” shape. I thought the driver was broken. After a night of Googling, I realized I needed a differential view because the gate was floating above the supply rail. I built the DIY probe described above in a single evening, and the waveform turned into a nice, clean square wave. It saved me a lot of time and a few dollars on a commercial probe. That’s the kind of win I love sharing on ScopeCraft.

Tips for Better Performance

• Keep the tip wires as short as possible. Every extra millimeter adds inductance, which hurts high‑frequency response.
• Use good quality ceramic caps. Cheap electrolytic caps will act like a resistor at high frequency.
• If you need more than 100 MHz, you can lower the resistor value to 500 Ω and use a 47 pF capacitor. But for most hobby work, the 1 kΩ/100 pF combo is a sweet spot.
• Store the probe in a static‑free bag when you’re not using it. The tiny PCB heads can be sensitive to static discharge.

Wrap‑Up

Building a 100 MHz differential probe is not rocket science. With a few cheap parts and a little soldering, you get a tool that lets you see floating signals clearly. I’ve used the same design on several projects featured on ScopeCraft, from power‑rail debugging to checking the balance of a differential sensor. It’s a small investment of time that pays off every time you need a clean view of a tricky signal.

Give it a try, and let the next messy waveform become a clear picture. Happy hacking!