This post will describe the DIY build of a 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 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 been looking 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 other constraint on the earlier design was the external power supply. Now that we have USB-C PD capabilities, Bud added a number of power options that can be added to the probe to your liking. Another main issue that he wanted to address was a 3D printable housing and 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 testing everything. Now that he is done, I will also build two probes and can verify the BOM's and 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 probe, all I can say at the moment is to stay tuned for a little while longer. It 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 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) generator, and most importantly a VNA.
So I just completed two purchase orders that is mixed between LCSC and DigiKey to get all the parts for the 100MHz version, the different parts for the 10MHz version, and then the USB-C based power supply.
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 as well, and parallel to every resistor.
As with the old probe, during the verification and calibration, you may 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 three 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, also a new device.
The rail splitter Opamp is also replaced by a device that is easier to get, and there are some refinements necessary because we no longer use difficult to get Tantalum capacitors. The ones we use now need a tiny resistance to make everything stable.
Next additional 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.
Last major change is the power input circuit. Note that there is no "real" connector, but only two pins. The idea behind it is that you can add a selection of tiny power modules 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 a 3D printable enclosure.
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 need 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 will be added. That's where those four square holes are for. Bud calls these little boards Daughter Boards.
Building the Power Supply
There are two components to the power supply. One is the input, and one is the regulator.
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 board that fits into the probe. These boards are very inexpensive, are widely available, but you have to select a certain type and size.
These 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 version, and the 10Mhz and 1MHz probes require the 20V version. I ordered a number of each of them, they typically come in lots of 5.
These boards need to be fixed (glued) on to the probe regulator board and the output of the board needs to be connected to the probe regulator board by two wires that are soldered on the pads.
Below is Bud's version.
This construction will fit snugly into the 3D printed enclosure.
Obviously, these USB-C input boards need to be fed by a USB-C Power Delivery (PD) supply.
The diff probe does not draw a lot of power (about 50mA) such that a low wattage supply will do. I'm using a 100W supply myself, because I want to use it for many other applications.
The probe voltage regulator
Bud create a number of voltage regulator boards to satisfy your particular need or preference. There is a switched power supply for two different kind of chips, an LDO linear regulator and a discrete voltage regulator.
There are two output voltages required required for the regulator, one for the 100MHz probe, and a different one for the 10MHz and the 1MHz probes. The reason is that 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.
The VREG SOIC-8 daughter board (100MHz probe)
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 VREG 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 PCB for it:
The parts are located on the bottom of the PCB, here is the top:
And here a 3D picture to make it a little bit more clear:
In the meantime, I have finished building my version of the probe, and tested and calibrated it. I ended up selecting 47pF values for the trimming capacitors that allowed me to nicely adjust the leading edge compensation of the probe. The other calibrations went well, and I will show a procedure and screen shots soon.
The USB-C and Trigger Board option
Initially, 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 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
Because of the gotcha, it took a while before I was ready to post the results and pictures. The main reason was that I ran into terrible hf noise issues with my USB-C supply. Eventually, comparing measurements with Bud, we found out that the root cause of my problem (he does not have it) is the 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 a laptop charger and that may have been a mistake.
I purchased mine from Amazon that calls it 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 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.
The output of the 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.
It's a little hard to see, but the hf is very present on the output signal of the probe.
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.
The battery + VREG power option
In the meantime, 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 VREG option. When I tried it, it nicely cuts out at about 13V and drops it 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 battery holder that has a switch for the 9V cell. I then soldered 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.
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 is over the board and would create a form of strain relief.
I tested the VREG board with my Lab Supply and a 60mA load to see how well the protection worked and what voltage it produced. That latter was an almost perfect 5.30V.
Shielding the probe
When that worked well, I continued putting the probe together with the required shielding. The probe functions without it, but your fingers will have a large effect on the signal due to the close proximity to the very sensitive attenuation parts at the input of the probe. Bud found out that shielding at the inside ruined the calibration and functioning, so he elected to shield the outside.
He used a copper tape trace to go from the SMA connector to the shielding in the front, and wrapped the shielding around the probe. He then used shrink-wrap tubing to make it looks nice and prevent accidental grounding to the DUT.
all from his Hackaday post :
I wanted to try another approach, that would allow me to open up the probe without damaging the shielding.
Here is what I did:
I added the 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. 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 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 ground pins.
When you close the probe, the foil gets pressed together at the seams and makes very good contact.
The 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 crew driver. I use the plastic trimmer tool that has a metal insert the size of an 0603 part to reduce the metal influences.
I measured the resistance from the SMA connector to the very beginning of the probe tip and it was about 0.5 Ohm to both halves, so a perfect connection. The "finger" effects on the signal are now completely gone.
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.
I will not yet add the shrink-wrap tubing because I 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.
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. Initially, 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 previous model.
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 one part is no longer available but there is an alternative so we're looking into that. It will take a few more weeks before I 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 to build this version myself.
Stay tuned for more...
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For those that already did, thank you!
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