It's 2025-07-08. I received the core probe PCB, the generic micro USB DB and the buck converter DB. They all work, but I will need to make changes on the daughter boards to prevent problems when stacking onto the core probe.

The Probe Evaluation:

I did not take rigorous data this time. The probe performed as expected, with a few problems that were easily resolved. I had a couple of issues during assembly. I did not have the 412Ω or 3.92k resistors in my inventory. Luckily, I measured two 3.9k 1% resistors to be 3918Ω and stacked two 820Ω resistors on top of each other to get 411Ω. I did not have the 620mΩ snubber resistors either, so 1Ω would have to do.

When I powered it up it drew 25ma or 60mA randomly as I fiddled with the board. Clearly there was an intermittent connection somewhere -- it turned out to be the power pin of the LTC6269-10. A bit of work with the hot air rework station fixed that. I now draws 60mA and I get a signal out when a signal is applied to the input.

Then I noticed that there was a difference of a few hundred mV between the positive and negative supply rails. The rail splitter opamp was oscillating (a sawtooth waveform) at 10kHz. I disconnected the 10uF cap at the output of U4 to no effect. Then I replaced the FB3 ferrite bead with a 3 Ohm resistor and the oscillation went away. Lesson learned. The small resistor is 10X the resistance of the bead, but there is not much current flowing through it -- even at full scale output voltage swing the effect on the GND voltage will be just a few hundred uV.

After a quick check to see if the gain was correct I calibrated the DC common mode and the two AC gain paths without any issues. It did not require any change in the large attenuator capacitors, or any additional capacitor in the open 0603 capacitor location. I had calculated those values based upon the -7.5% change on the 10pF input capacitors and it proved correct this time as well. It also helped to have a 15.5pF trim range.

I was able to trim the output offset to less than 1mV, using a DVM. This offset moves several mV as the supply voltage changes a few hundred mV. I would not recommend swapping DC-DC adapters, that will have different voltages, if you are going to implement the offset adjust feature. But using a daughter board with a regulator should be fine.

Overall DC gain was a smidge high -- about 1%, which I attributed to my stacked 820Ω parallel kluge. I can't measure AC gain much past 10MHz, but it was good to that point. I applied 20Vpp (the max of my function generator) and did not see any clipping at the output.

The noisy output is still there. I can see about 100mVpp of hash at the output when the scope is set to 10X mode.

In short, this probe looks nearly identical to the old one. That's good!

The Buck Converter Daughter Board:

l built two versions of the buck converter -- one with an RT8259 chip and another with an RY8310 chip. They are nearly identical performers. Digikey sells the RT8259 for more than $1.5, while LCSC sells the RY8310 for around $0.10. There is a side benefit to the RY8310 in that it doesn't require a freewheeling Schottky diode at the switch node. 

Both boards worked well. They both were almost exactly 5.3V at the output. There was a 20mVpp 1.4/1.5MHz square wave ripple at the output, as expected. The load and line regulation was excellent -- about 2-mV change for 50mA change in load current, and about the same 2mV change as the input voltage changed from 9-12.5V. The overhead was also very good as both converters would not drop the output voltage until input voltage was below 6V.

The RY8310 was more efficient -- 80% @12V, 85% @9V. The RT8259 was about 5% lower. Still not bad. At this low current (60mA) both boards remained cool, as they should because they only dissipated around 150mW.

Pairing the buck converter DB with the probe:

I tried to mount the RT8259 daughter board under the probe and immediately found a problem. The two input voltage pins on the DB are through-hole and are located at the back of the DB where the SMA connector is located on the probe. The wires running from the DB to the trigger board must be soldered to the underside of the DB and might short to GND if they contact the housing of the SMA connector or its rather large pins that are soldered to the back side of the probe PCB. This issue will require a change to any daughter board that has components mounted on the bottom side, and the input voltage pins will need to be relocated to the front of the DB and made surface mount only to the bottom side of the DB.

By carefully soldering the power leads to the DB and mounting the DB above where it would interfere with the SMA connector I was able to attach the DB to the probe. When I powered it up I could not detect that it was being powered by a switch mode converter. It helps that there is a ton of noise at the probe's output. It might show up with a spectrum analyzer, but I don't think so. The back side of the probe has a solid ground plane to act as a shield to any switching noise produced by the converter.

I think that this combination of switching converter and USB-C trigger board will be my goto solution to powering the probe. It's small and cheap and cool compared to the LDO solutions.

Putting the Probe into the Case:

The bottom housing of the case had to be modified in order to get the probe with DB attached mounted properly. Eventually, I got to the point where everything snapped together (without the trigger board attached yet.) Here is the result:

The case is yellow to match the color of the first channel of the scope. I cut out a section near the LED indicator so it would allow a brighter glow from the LED. It is not as bright as the photo would suggest. I'm pleased with the result.