This post will describe the DIY build of a new 1MHz, a 10MHz and a 100MHz differential probe with several power alternatives.
In 2017, my buddy in crime Bud and I (well, mostly Bud) build a DIY X10 100MHz differential probe that has been popular because so many other makers have build it, or even changed it to their liking. We know of a 1x and a 100x design modification. Our earlier design was described here:
Unfortunately, that 2017 design has some hard to get and rather expensive parts, especially the Opamps, the voltage regulator and also some hard to get capacitors.
In early 2025, Bud has started a new project to revisit the design and look into making the probe easier to build and make some desired and some needed improvements. He investigated the whole concept again and came-up with a main probe PCB that can be used to build a 10x 100MHz, a 10Mhz and a 1MHz version. The 10 and 1 MHz probes produce less noise. The other constraint on the earlier design was the external power supply. Now that we have USB-C PD capabilities available to us, Bud added a number of power options that can be added to power the probe to your liking.
When testing my diff probe, we found that only GaN technology, low power (<35W) good quality USB-C PD adapters are quiet enough to be used with the diff probe. Be aware! Below we explain why.
Another main issue that he wanted to address was a 3D printable enclosure and also making the probe slimmer so it's easier to hold while probing around with it.
There are some more improvements and refinements and if you're interested, you can follow along his very detailed design and test efforts here:
Bud has built several probes and verified and tested everything. Now that he is done, I will also build two probes and can verify the BOM's and describe the building procedures here and also add some more information that will hopefully help others building this very useful instrument.
When I'm done (and only then), the details like schematics, pictures, the BOM's and Gerber information will be available in a new Github project. I will also enter the information in the Shared Project list of my sponsor PCBWway so PCB's can be easily obtained there too.
If you're looking to build a diff probe, all I can say at the moment is to stay tuned for a little while longer. I will most likely be finished in October/November of 2025, so if you can hold-off a little longer, I recommend you do so.
What I'm going to build
I already have the earlier 100MHz probe, but wanted to build another one so I can test it with my gear that is a little bit different from what Bud has or uses. More specifically, my DSO is a 350MHz version, and while he has a modern and much more improved and new DSO (12-bit, touch screen, etc, etc. I'm jealous!) it is a 100MHz version. He has a function generator, but that only goes to 60MHz. I have a fast edge (<1ns) pulse generator, and also a nanoVNA.
So I completed two purchase orders that is mixed between LCSC and DigiKey to get all the parts for the 100MHz version, most of the different parts for the 10MHz version, and then the USB-C based power supply items.
Building the 100MHz probe
The schematic is largely based on the previous design, but with a number of refinements and changes. The input attenuator is changed so we have equal resistors, and that allows us to have equal capacitors that are now parallel to every resistor.
As with the old probe, during the verification and calibration, you will have to add capacitors to the not placed (NP) C11 and C12 so the trimmers C15 and C16 can properly adjust the AC compensation. There are extra values listed in the BOM that need to be ordered so you have the possible values at hand.
The next change is the first gain stage. The positive output now has an optional offset adjustment that can be installed when you need it. When you don't need it, don't install it because it degrades the output a little bit. Both outputs from the gain stage go to a summing amplifier. Unfortunately, Bud was unable to find suitable alternatives for the two Opamps, which may be a little difficult to get.
The rail splitter Opamp is replaced by a device that is easier to get, and there are several alternative devices that can be used. Some refinements were necessary because we no longer use the difficult to get small Tantalum capacitors. The ones we use now need a tiny resistance (act as a snubber circuit) to make everything stable.
Next additional and optional circuit deals with a power on LED. Depending on the way you power the probe, or what power supply you use, you can populate these two parts to get a visual power indication.
Last major change is the power input circuit. Note that there is no "real" connector on the PCB, but only two pins. The idea behind it is that you can add a selection of tiny power modules (called daughter boards) to the bottom of the probe, based on your particular taste or need. More about that later in the power section.
Lastly, as you can see from the picture at the beginning of the post, there is now a 3D printable enclosure available.
Here is the schematic for the 100MHz probe:
The PCB looks like this in the 3D viewer:
You will note that it's a much slimmer design, that will be easier to hold and maneuver when you're probing around in a circuit.
Note also that the middle pin of the front-end will have to be removed to create the creeping space. The 3D model for the header is not modified, so it shows all three pins.
The bottom of the PCB houses the rail splitter circuit, and is also the place where the power supply PCB will be added. That's where those four square holes are for. Bud calls these little boards Daughter Boards.
The calibration went without a glitch and I added two 47pF trimmer caps to get the AC gain in the range.
The new Power Supply Options
There are two components to the power supply. One is the input, and one is the regulator. The three probe models require different input voltages, 5.3V for the 100MHz and 15V for the 10 and 1MHz probes.
The USB-C input board
The input voltage for the probe regulator can come from a variety of supplies. The most optimum is a USB-C PD Trigger Board board that fits inside the probe enclosure. These Trigger Boards are very inexpensive, are widely available, but you have to select a certain type and size.
These Trigger Boards communicate with the USB-C Wall-Wart supply, and request the desired voltage. There is a CPU on the Trigger Board that takes care of the rather complicated communication protocol.
These tiny boards are available in different voltage configurations, like 9V, 12V, 15V and 20V. Although the voltage you order is fixed for the board, you can still change it by closing or opening a bridge (a 0402 0 Ohm resistor or a solder blob). So 9V can be configured to 12V by adding a bridge, and 20V can configured to 15V by removing a bridge.
The board in the picture is a 20V version, and if you remove the resistor in the top right, it will be configured for 15V.
The 100MHz probe requires the 9V Trigger Board version, and the required voltage regulator will bring that down to 5.3V. The 10Mhz and 1MHz probes require the 20V Trigger Board version and the required voltage regulator will drop that to 15V. I ordered a number of each of them, they typically come in lots of 5.
These Trigger Boards need to be securely fixed (glued) on to the probe regulator board. The voltage regulator board is fixed to the probe by soldering 4 power/support pins. The Trigger Board with the USB-C connector needs to have a firm connection to the Trigger Board, because of the force that will be applied when you push the USB-C cable into the connector.
The output of the Trigger Board needs to be connected to the probe regulator board by two wires that are soldered on the pads.
Below is a picture of Bud's version.
This construction will fit snugly into the 3D printed enclosure.
Obviously, these USB-C Trigger Boards need to be fed by a USB-C cable going to a Power Delivery (PD) supply.
The diff probe does not draw a lot of power (about 50mA) such that a 20..30W supply for mobile phone chargers will do.
WARNING
I was planning on using a 100W supply myself, because I wanted to use it for many other applications. The one I ordered turned out to be a mistake! Don't get a high power one, it may not work well with the probes. Read on to find out why.
The probe voltage regulator options
Bud create a number of voltage regulator boards to satisfy your particular need or preference. There is a switched power regulator, an LDO linear regulator and a discrete voltage regulator.
As mentioned earlier, there are two output voltages required for the probes from the regulator, 5.3V for the 100MHz probe, and 15V for the 10MHz and the 1MHz probes. The 100MHz probe requires an input voltage of 5.3V (+/- 5%), to create accurate +/-2.62V positive and negative rails for the Opamps.
The 10MHz and 1MHz probes require an input voltage of 15V to create the +/-7.5V (+/- 3%) positive and negative rails.
I decided to build a linear regulator (the VREG SOIC-8) and also try a VREG discrete regulator for the 100MHz version with a 9V output, and another VREG discrete regulator for the 10MHz version with 15V output.
The VREG SOIC-8 daughter board
The VREG SOIC-8 Daughter Board (DB) is intended to be used with the 100MHz probe, because it outputs 5.3V and requires an input voltage of 9V.
It can be configured with a number of LDO voltage regulators. I have decided to use the LP2951 LDO voltage regulator. It comes in an SOIC8 package, and Bud designed a Daughter Board specifically for these devices.
Here is the schematic:
As you can see, there are a number of voltage regulators that can be used on this PCB.
Here is the 3D picture of the PCB for it:
The parts are located on the bottom of the PCB, here is the top:
And here is a slanted 3D picture to make it a little bit more clear:
The USB-C Trigger Board and the Voltage Regulator combo
I populated the SOIC-8 low drop regulator board, and connected it to a 9V Trigger Board. That all worked well, but I ran into a gotcha that took quit some time to investigate.
These two boards need to be firmly glued together, but I first wanted to see what the power-up sequence was for the whole chain (USB-C PD supply, 9V Trigger Board and the 5.3V SOIC-8 regulator. It turns out that it performs really well with a nice and gradual power-up. The output voltage was a perfect 5.3V
Detecting a nasty surprise...
After I installed the board combo to the probe and verified the calibration and while further testing the probe, I noticed nasty spikes on the probe output signal. Up until now, I had used my Lab Supply to power the probe and running the calibration and tests. The output was noisy, as expected, but otherwise clean.
Eventually, comparing notes and measurements with Bud, we found out that the root cause of my problem (he does not have these switching glitches) is the 100W USB-C wall-wart I purchased. When I looked for one, I wanted to make sure that it produced all the voltages and I could not find many that listed them. I ended-up buying one that was advertised as a laptop charger and that may have been a mistake.
I purchased mine from Amazon that is called a Basicvolt 100W laptop charger. This is what it shows on the label:
Note the second caution. Is that the give-away?
It turns out that this supply is very noisy. So much so that it is unusable with our probe. The hf switching transition stuff comes through everything. Below is the wall wart output at 20V. The ripple is a non issue, the regulator will take care of that without even blushing, but the hf stuff at several 100's of mV is another story. This screenshot is from the supply, the probe is not connected.
The output of the connected 9V Trigger Board looks like this. Note the 2V p-p hf noise:
The Trigger Board itself does not contribute of create the hf noise. It only tells the USB-C supply what to do.
Here is what the probe output looks like on my DSO:
It's a little hard to see, but the hf is very present on the GND output signal of the probe. The circuits on the regulator and the probe itself filter the hf noise a lot, but it is not sufficient.
The net-net is that I need to change my USB-C supply and see if I can get a better one. The trouble is that almost all of these power adapters are specified for laptops or phones, and they don't care.
Bud is using 20W and 30W wall-warts and they are clean, so I ordered a 20W one as well.
This is one of the ones he is using:
Here it is with a 9V Trigger Board:
Mine is in the mail and will arrive this evening, so I can report more about this issue.
In the meantime, I tested a genuine Apple PD charger that supports 5 and 9V outputs.
This is the result.
Just for kicks, I then tested my Dell laptop supply, also a USB-C PD version.
Unusable!
By now I was getting a bit worried...
The one I ordered is this small power supply, hoping that would be the answer:
This is a simple USB-C PD supply.
However, below is the zoomed-in switching noise riding on the output:
However, below is the zoomed-in switching noise riding on the output:
After more consulting with Bud, it transpired that he is using two GaN technology wall-warts. Could this be the answer? With the GaN technology, the switching frequency can be up to 10MHz. The switching noise is then much easier to tame.
So, I found a seemingly suitable one and ordered it:
When it arrived, I found that even the packaging was very well done. It felt solid with a surprisingly "heavy" weight of 138 grams. My hopes were going up. It produced this:
Notice the much higher switching frequency, and the V/Div scale at 200mV/Div.
However, I was still disappointed, so with some more consulting from Bud, I tried to tame this tiger by adding a snubber circuit of a 4.7 Ohm resistor and a 4.7uF capacitor, mounted right on the 9V Trigger Board. I then also used two 1K at 100MHz ferrites to connect to a 510 Ohm 1W resistor that acted as the 60mA load. All four added parts are 0805 size.
With this addition, the switching noise is completey gone. All that is left is the residual noise of the adapter, my environment and that of the DSO.
Time to take a step back, and realize what we have here. Bud already put a 10uF/1 Ohm snubber on the probe in addition to two ferrites. Is that already sufficient?
So the next step was to use a clean 9V Trigger Board, connect it to the diff probe and measure the noise again.
Below is the setup I used.
The new GaN USB-C PD supply is connected through a good quality USB-C cable of 1 mtr to the USB-C monitor connected to the 9V Trigger Board. I did not want to modify the battery supply connection to the probe, so from there it connects to a 9V battery adapter that is plugged into the 9V battery holder connector. The battery holder is connected to the discrete VREG Daughter Board mounted on the probe through at least 35cm of plain but flexible 26AWG wire. The diff probe is outside the shielded enclosure. The input of the probe has no signal, but is terminated by a 50 Ohm feed-through. This must be a realistic but worse case condition setup.
And this is the output of the diff probe itself on my DSO.
Apart from the inherent noise generated by the diff probe amplification, there is no switching noise from the adapter visible. So the on-board measures are already sufficient to remove the switching noise!
The net-net is that we now highly recommend that you use a good quality GaN technology low power PD (<35W) adapter powering the diff probe. The designed-in filtering on the DB regulators and diff probe should be sufficient to eliminate the adapter noise.
However, if you end-up with one that is a bit noisier, you can still add the snubber circuit right on the Trigger board as I tested earlier.
Problem identified and solved.
A battery + VREG power option
Because of the noise mishap, and while waiting for the new wall-warts to arrive, I switched to using another power option for the probe. Obviously, the most optimum low noise power for the probe is by using a battery. I decided to keep it simple and selected a 9V rechargeable battery to power the VREG board.
Because I will also build an 18V supply by putting two 9V cells in series for the 10MHz probe, I wanted to add the high voltage protection that Bud designed for the discrete VREG option. When I tried it, it nicely cuts out at about 13V and drops the output to about 3V, so no harm is done when I accidentally use the 18V battery supply with my precious 100MHz probe.
Below is the construction. I use a 9V battery holder that has a power switch. They are widely available. I then soldered two flexible 26AWG wires to it and they were soldered to the VREG board +VIN and -VIN solder pads. The VREG board lowers the 9V to the required 5.3V for the probe with more than enough head-room for a decaying cell voltage.
Because there is no strain relief for the wires, I decided to solder the wires coming from the other end of the VREG board, so the folding of the wires is over the board and I hope that will create a form of strain relief.
Shielding the probe
When the power supply worked well, I continued putting the probe together with the required shielding. The probe functions without it, but your fingers and near equipment will have a large effect on the signal fidelity due to the close proximity to the very sensitive attenuation parts at the input of the probe. Any additional disturbence gets amplified and will show itself on the DSO. Unfortunately, Bud found out that shielding at the inside of the enclosure ruined the calibration and functioning, so he elected to add the shield the outside.
If you're experienced with this, you could try a conducting paint spray to cover the insides of the enclosure and figure out a way to connect that layer to the earth ground. I have no experience with that, so I skipped it.
When Bud put together his probe, he used a copper foil trace to go from the SMA connector to the shielding in the front, and wrapped the shielding around the closed probe. He then used shrink-wrap tubing to make it looks nice and prevent accidental grounding of the shield (earth ground!) to the DUT.
pictures are from his Hackaday post with details:
I wanted to try another approach, that would allow me to open up the probe without damaging the shielding.
Here is the result of what I did:
I added the copper foil shielding to both halves of the enclosure in two sections. A smaller one for the front part just past the bend to the thicker part, and one larger section just overlapping the front part by about 3-4 mm. As you can see, I folded the shielding around the edges to the inside of the enclosure. I then used a sharp knife to cut it nicely.
The top half copper foil has an extra tab and I soldered the ground wire to it before pressing it to the plastic, to avoid warping the plastic enclosure. The other end of the ground wire was soldered to one of the SMA connector ground pins.
When you close the probe, the foil gets pressed together at the seams and makes surprisingly good contact.
The covered trimmer holes need to be opened on the one side, and the screw holes on the other. Note that you need to cut away the shielding around the two holes for the capacitor trimmers, because you can create a short to ground if you use a metal screw driver. I use the plastic trimmer tool in the picture that has a metal insert the size of an 0603 part as the tip to reduce the metal influences.
The resistance from the SMA connector to the very beginning of the probe tip was about 0.5 Ohm to both halves, so a perfect connection. My "finger" effects on the output signal are now completely gone. It works better than I expected and hoped for.
When I calibrated the probe again while in this shielded enclosure, I was able to get a true flat line for the CMRR and a 0mV offset adjustment. Unfortunately, the capacitance trimmers are now at the edge of their range so I need to change the 47pF caps to 39pF, which I don't have in the 0603 size yet. They are on their way.
I will not yet add the shrink-wrap tubing yet because I will need to open the probe a few more times. At least one time to change the trimmer capacitors. I also want to experiment more with the USB-C PD power option.
The calibration procedure and some measurements with pictures will follow soon.
Building the 10MHz probe
The basic schematic is the same as the 100MHz probe, so it can be built on the same PCB, but there are several part and part value differences.
One of the major reasons to build a 10MHz and a 1MHz probe is to reduce the output noise of the probe, which is quite substantial for the 100MHz version. For these versions of the probe, Bud was able to find two alternative Opamps, the OPA2810 and the OPA810, that are 1/3 the cost of the LTC6268/9 pair of the 100MHz probe.
In order to finish building this version, I needed to order a few more parts. In that process I noticed a few items that Bud and I will use to update the BOM, like we did with the 100MHz version. Unfortunately, the main Opamp is on back-order at LCSC and the C trimmers are no longer available but there is an alternative so we're looking into that. It will take a few more weeks before I get the parts and can finish this probe and have all the documentation verified.
Building the 1MHz probe
As with the 10MHz probe, this version can also be built on the same PCB, and again there are part value differences to make it a 1MHz probe with the least amount of output noise. It uses the same Opamps because Bud could not find less expensive ones that had the right specifications.
At the moment, I have no plans at this moment to build this version myself.
The Calibration Procedure
When I have the 39pF caps for the 100MHz probe, I will describe the calibration procedure and add pictures of the calibration and testing of the probe.
The DIY pulse generator
In order to create a pulse that can be used to calibrate the probe, Bud and one of his friends designed a little circuit that can be put together easily on a protoboard. It uses parts that should be in your stock already, or are commonly available. More information can be found on his Hackaday postings.
Here is the circuit:
I'm powering the contraption with my Lab Supply at 30V to create a 30Vp-p pulse. The leading edge has about a 330nS rise time. I'm using one of my DSO probes to trigger on the waveform. The input signal comes from the 1KHz probe calibration output of my DSO. If you use the scope probe, you should not connect the scope calibrator GND connection, because it will create a ground loop. Use only one of them.
I used my grabber lead contraption from my earlier 100MHz diff probe to connect the calibration signal to the diff probe. There are 510 Ohm resistors in series of both leads to reduce reflections due to the inductance of the leads. I may have to select other resistor values for the new probe.
Look at the older Blog for details.
Stay tuned for more...
