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 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 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 or 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 design and test efforts here:
Bud is now in the final stages of verifying and testing everything, and when he's done, I will add the information here also and provide the BOM and Gerber information in a new Github project.
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 September of 2025, so if you can hold-off, 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 300MHz 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, and 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.
These boards need to be 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.
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 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:
The VREG DB
This voltage regulator is a discrete design that can be configured for different voltages.
It can be use for all three probes.
This board has an optional maximum voltage protection in case you directly use a USB_C PD board that outputs a voltage that is too high for the probe. We've found that the output voltage of some of these PD boards are not reliably outputting the correct voltage.
By changing a few components and not populating a few, this regulator can be configured to output 7.5V for the 100MHz probe, and 15V for the 10MHz and 1MHz probes.
The Buck Convertor DB
There is also a Buck convertor design available.
Although this board has many possibilities, Bud is still in the process of investigating and verifying the devices. As an example, the TPP parts looked functional, but Bud discovered that the ripple output in the Pulse Skipping Mode is unacceptable for our application, so they are no longer recommended.
From the list, the NEX40400B is still our first choice, although I will built one with the RY8310 to give that a try.
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 several 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.
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.
Stay tuned for more...
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For those that already did, thank you!
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