DIY rebuild of the Tek SG505 V3

This Blog post will detail the third version of my project to rebuild the Tektronix SG505 instrument.

Here is the link to the post describing the second version of the design: 

https://www.paulvdiyblogs.net/2025/08/diy-rebuild-of-tek-sg505-instrument.html

Here is the first post about this project:

https://www.paulvdiyblogs.net/2025/03/diy-build-of-tek-sg505.html

The reason for the third version

After the investigations of the second version, I wanted to separate the Power Supply from the Generator in the same enclosure. I also wanted to put the Generator circuits into a full metal enclosure to make the output of the generator as clean as possible and as a minimum, remove the mains hum.

After a very long search, this is the only enclosure I could find that has the required height and width for the front panel layout. It is a ProMa 130 0044 and is also available on Amazon. The outside dimensions are 165x110x80. I would have liked a black enclosure, but alas I couldn't find one. There is one available from ProMa though, with part number 130 0045. I can always spray paint it myself if I develop the urge. The current Front Panel design will need some modifications and will replace the aluminum panel. The generator PCB will slide in a slot close to the bottom. Unfortunately, the dimensions are a little different so I can't use the current PCB in this enclosure, not even to try it out. 

This all means a new PCB for the Generator, for the Power Supply and for the Front Panel.

Splitting the Power Supply

There will be a separation of the noisy mains related parts of the circuit, that need to go outside of the enclosure for the generator. In essence, it means that the transformer, the bridge and the main capacitor reservoirs and the 40V regulator need to be on a separate PCB that will be housed outside of the generator.

The shunt supplies for the +/-16V rails can move to the main generator, and also the 12V supply can move to the main board. They are quiet and will have no negative effect on the generator. I hope. It also makes the interface from the Power Supply enclosure simple, because I will only need to use two wires for the 40V supply that feeds the other three rails.

The circuit after the transformer and the bridge will get some more filtering to avoid mains related noise getting into the generator.

The Mains Power Supply

This is the circuit that is inherently noisy and will no longer be in the same enclosure as the generator. This is the schematic I'm now working from. There are not too many changes, but I added ferrite coils in between the larger reservoir caps, and I used a common mode coil at the output. I also added a few more capacitors to quiet the thing down as much as possible.

The Power Rails on the Generator PCB

These three rails are very quiet by themselves and can now move to the generator PCB.

No major changes from the previous design, I just added a few extra capacitors and ferrite beads to the power input lines. This may still change a little based on the new layout.

The Generator circuits

The other circuits stay the same, will just get a revised layout and incorporate the three power rails and needs to fit in the new enclosure.

I finished the generator PCB, now with the power rails on it.

This is as close as I have it now.

The Front Panel

This is the new front panel fitting the new enclosure.

Stay tuned for more information while I go about making all these changes...

A Github repository is available here it will be updated with information during the project when I have verified the correct operation. I'm still working on it, so there is very little information there at the moment so please be patient.

DIY rebuild of the Tek SG505 instrument

This post will describe the building of a complete instrument based on the earlier investigation of the Tektronix SG505 Signal Generator described here: 

https://www.paulvdiyblogs.net/2025/03/diy-build-of-tek-sg505.html

Enclosure and Front Panel

I'm going to use the same enclosure as I used for the SG502, which is a TEKLA KL-22. This is a black plastic enclosure with a metal front panel and back panel. I would have liked to use a full metal enclosure but the ones I found where either too small, too low, or too large and mostly all of them had the wrong dimensions to use a front panel.

To get a feel for the proper handling of the knobs and the overall layout, I typically create a mock-up using a paper front panel with a card board backing functioning as the PCB.

I made a number of changes to the initial layout to make sure I can turn and switch everything without cumbersome finger acrobatics. I also want to group controls together as much as possible, and make the positions logical. Also the position of the BNC output and the main frequency control contraption needs to be positioned correctly.

Here is that mock-up in the final version, after spending a few days relocating and printing and mounting the controls again. (this is not the final version (see the PCB below), some text got moved a bit here and there)

Here is the view from the back:

And here the detail for the mounting of the main potmeter contraption.

This is an earlier version, the mounting holes for the bracket line up in the final one. (;-))

After finishing this, I could finelize the design of the Front panel PCB for The KL-22 , so it has a nice looking feel for the instrument. Here is the 3D viewer result of the final version:

The rectangle in the top is the cut-out for the OLED display that will show the frequency. There are no mounting holes on the OLED display board itself, you have to find a way to somehow glue it to the back of the front panel.

All the holes are isolated from the front and back ground poor to add some more strength and EMI blocking. I will order the black solder mask with white silkscreen lettering as I usually do for all of my front panels.

New PCB Layout

I have also started to create a new layout for the main board, and added the relay's for the range switching, as well as the output amplifier. 

Here are the circuits for the frequency range selection, using a simple rotary switch located on the front panel.

This is the frequency setting for the phase shift amps.

Here are the dampening circuits for the AGC and the Peak Detector.

And here is the circuit for the frequency multiplier rotary switch:

A new Power Supply?

I'm very happy with the Jung SuperReg, but it is more than a bit of an overkill for this instrument. I'm now looking into testing a dual shunt supply that I also very recently used for my Twin-T notch filter and the Victor Mickevics oscillator. I'm using Victor's design to start with. I've used LTSpice to learn a bit more about the principle and I think I now know enough to be dangerous and feel brave enough to give it a try. I also need to add the 12V rail for the relay's and feed the Arduino and LCD display.

This will decide whether I'm going to add the components to the main PCB, or create a separate one, and then add the Arduino counter circuit to it.

Well, after some fooling around with prototypes, I finally got it going on two protoboards.

Here is the schematic of the supply:

The prototype below does not have the transformer snubber and the bridge capacitors implemented.

The power LED is optional. If you're going to use the OLED display, it will show that there is power. During testing, I used an LED to tell me that power is on, otherwise I can't see it visually, and before I added it, to my dismay, I left the unit powered on for a whole night. Not good.

The Shunt Supply principle is quite simple, almost literally taken from my Twin-T notch filter design and I only added trimmers for the key voltages and tweaked the current distribution. I'm now also using a much smaller transformer that I can mount on the PCB.

Here is the first result:

For some reason that I do not understand yet, there are more harmonics of the principle visible and you can also see a bit more of the mains frequency. I'm hoping that a real PCB and complete mains and transformer filtering will reduce that.

The top protoboard on the left is the raw supply circuit fed with the ac from the 2x 28VAC transformer. It has the rectification and filtering plus the LM317HVT that feeds the 44V to the dual shunt supply, located on the lower board. The two boards are connected together using the two 91 Ohm resistors that provide the shunt supply current headroom.

I'll do some more long term testing and look at the temperatures, but it looks like this supply is adequate for this application.

I finished the layout of the new power supply. It will be mounted up-side down from the top of the enclosure. There is a normal transformer located on the bottom for the +/- 17V rails, and the top rectangular in black is the 12V DC power module. That circuit is completely isolated from the +/-17V rails.

And finally, here is a 3D picture of the main board:

The 12V supply for the relays, the Arduino and the OLED display is completely separate from the generator circuits itself.

A reconsideration for the front panel

While working on the final layout for the main board and the front panel, I became more and more agitated with the two screws that mount the potmeter reduction unit that are visible on the font panel.

After some hemming and hawing, I bit the bullet and redesigned the hardware construction such that there are no more screws visible on the front panel. It meant that I had to move some parts on the main board out of the way, and add another bracket and holes to the main board. I also used this moment to slightly change some positions and changed some of the silkscreen text.

This is how the front panel looks now:

And here is the new construct for the potmeter and the reduction unit:

The reduction unit is now no longer mounted on the front panel, but has it's own bracket mounted on the main PCB. The two brackets are made of an L shape aluminum 1mm thick and is 20x30mm. Commonly available in DIY stores.

Here is the main PCB:

On 22-aug-2025 I uploaded the three PCB fabrication files to PCBWay and asked them to produce them under the sponsoring agreement. They already passed their review successfully. I hope to get their approval for the sponsoring soon in which case they can be here in 7-10 days.

Construction Details

The three boards have arrived from my sponsor PCBWay and as usual, look great. Especially the front panel came out very well with the black color of the solder mask and the crisp white silkscreen. Compared to other fab houses, PCBWay uses a special matt black color that is especially interesting for front panels because does not shine or mirror and does not leave finger prints. It is more difficult to clean though, so don't spill any flux on it. I use it on all my front panels and I quickly counted 5 of them on different instruments.

This time I forgot to ask them to put their manufacturing number on the back side of the board. By default, they put it on the front, as you can see here in the bottom left. It's not hindering though.

The Power Supply

I already populated the power supply board (top left above) the same afternoon. The next day I tested the vitals and all the voltages where there. 

One important notice that I now also added to the schematic is that with this supply, you always need to connect a nominal load to the output of the shunt supplies, otherwise the TL431's will get too hot. They will try to reduce the voltage by sinking all the current. I used two 680R 1/4 W resistors as a load, connected to the board as you can see in the picture below.

I could easily adjust the voltages, the trimmers have a good range. The LED that I mounted on the board, to have a visual indication of the applied power, was too bright. I changed the series resistor from 10K to 47K to reduce the brightness.  Now that I know that the supply is working, I can give it a good soak & scrub. 

The Main Board

After knowing that the power supply works, I can start on the main board. I normally keep the prototype in a functioning state, but there are so many special parts on it that I can't do that. I'll simply transfer the parts from the prototype to the new board.

The challenge will be that I used the Tek part identifiers on the prototype, but started anew for the new board. I will have to use the iBOM plugin for KiCad on both PCB's to figure out which is which and where.

Here's a very valuable tip.
If you don't have this iBOM plugin installed yet, you're missing out on a fantastic addition. Highly recommended! My buddy Bud looked at it when I told him, and initially was not impressed. However it took only a little explanation of the features to make him a true believer as well. Now he swears by it.

Here is how that looks like:

The top screen shows the iBOM windows for the prototype with the parts list on the left, and the layout on the right. Everything is linked, so when you select a part in the layout, iBOM parts list or iBOM layout, the cursor will go there and highlight the part(s).

The bottom screen shows the Version 2 board, using a second browser. So I select a certain part value on the prototype, and also select the same part value on the V2 and then know where it is located on the prototype and where it needs to go on the V2 board. I first apply solder paste on the V2 board, and then use my heat-gun to remove the part from the prototype and place it on the V2 board. When everything is done, I can use the heat-gun or reflow plate to solder all the parts on the V2. Easy-peasy in theory, not so easy in practice, but after discovering and correcting a few errors, I could also add the other parts. 

In the parts list, there are checkmarks you can fill in so you can keep track of where you are.

The main board is now ready for a reflow soldering process on my new hot plate. This is the largest board with a lot of parts, so yet another good test of the hot plate design.

Wave soldering went well, although I had to nudge a few tiny (0402 diodes - I ran out of the 0805 types) components in place. I'm now soaking the boards for several hours so I can clean them properly in order to inspect them better with my microscope. With the amount of flux and very tiny solder ball bearings in the way, that's not really possible.

Aftermath of reflow soldering

This is a little deviation, but I wanted to share it with you nonetheless. I will probably move this information to a new post, but at least you know why I did not present any progress for about a week.

After a reflow process, you end-up with a lot of flux that was part of the solder paste. Unfortunately, you need to remove that completely because it will have influences on hf circuits, and is hygroscopic so will attract water that will result in corrosion. Another side-effect of using solder paste is that you end-up with a number of so called ball-bearings. They are very tiny solder balls that are the result of the part sucking down to the PCB and that will push solder from beneath the part to the sides of the component.

Below is a good example of this issue. Mind you, this is after cleaning and scrubbing the board twice already!

The whitish goo is the solder flux and you can clearly see several ball bearings. 

The remedy is to soak the board in a solvent for a while, and then vigorously scrub the PCB with a brush to remove the unwanted flux and the ball bearings. 

Sounds easy, but is not so. The selection of the most optimum solvent is a science by itself, and is highly depending on the solder paste you are using. Some have water base paste, most have not. When you soak the board with a solvent, and use a brush, you typically end up with a very sticky board. To remove that, you can use IPA 99% to rub the PCB some more, and then generously poor fresh IPA on the board to let is drop of the board, and hopefully take the sticky goo with it. After that process, you can use clean (distilled to avoid calcium deposits) water to wash the remaining IPA off, and then dry the board. You can use a hairdryer, or use a solder heat gun, as I do, because that will allow you to blow the remaining liquid out of small pockets.

That's the process, and I have been using that for a long time.

This time however, it did not go well at all. First of all, the PCB was the largest I ever used on my reflow hotplate and that meant that the solder in the middle was overly done, and on the edges just about. There was an abundance of flux and I have never seen so many ball bearings. Cleaning as I describe above did not give a good result. Even after soaking for several hours, the goo stayed and the ball bearings with it.

Rather frustrated, I finally bit the bullet on purchasing an ultrasonic bath. I had been eyeing them for a while, but thought I could do without yet another box and also save the money. So I bought one for 2 liters that is just large enough for this PCB (160mmx115mm). It fits, but not in the wire-bucket.

After trying it with my solder paste solvent in the pure state, I first set the heating of the solvent to 50C and tried it for 5 minutes. That was much, much better, but still not good enough so I tried it for another 10 min. after adding some liquid dish washing soap to further breakdown the flux. That resulted in about a 90% successful cleaning. The edges of the board were not as clean as the middle, but that's because the single transducer is in the middle, and I'm pushing the size limit. 

In the process, I noticed that one of the tiny 0402 diodes that I soldered by hand after the reflow soldering got loose (note the circle below). It must have been a cold solder joint, but I'm actually glad that happened because finding that error would have been difficult and I'm warned for the next time.

You can also clearly see the cloud of the solder paste solving into the liquid.

In any case, I'm happy with the results of the cleaning session now.

In the meantime, I may have discovered the reason for my reflow issues. It turns out that my solder paste is probably expired. I used SN4258 (Sn42/Bi58/Ag1), 138C melting point with a mesh (size of balls) 20-38um, or size #4. Unfortunately, there is no expiration date.

The effects that I see are exactly what is described when using expired solder paste. I always keep mine in the fridge, but it explains a lot. Unfortunately, I can't find the purchase date anymore but must be less than a year. I ordered it when I was working on my reflow hotplate. Lesson learned...

First Power Up

I connected the 12V rail to my Lab Power Supply to keep an eye on the current and used a jumper to select the frequency ranges, and listened for the relays clicking, which they did.

I then added the +/- 17V rails again using my Lab Supply and set the current to the expected value of 35mA. That didn't work, the supply went into current limiting, so something was wrong. Upon inspection, it turned out that I goofed with the location of two of the NE5534 Opamps, they were rotated 180 degrees. The marking is very difficult to see on them.. Unfortunately, trying the power again did not result in a lower current. I probably fried them, because after replacing them, I got the expected current result.

By then I could use the current shunt power supply for the SG505 and use that from now on. While checking the voltages, I noticed that the +17 could no longer be adjusted higher than 16.4V, so I lowered the -17V to also 16.4V to have symmetry. I need to investigate that in more detail later on, but I now had the 12V rail and the +/- 16.4V rails.

I then added the three potmeter connections and used my DSO to look at the generator output. Nothing, just some weird very low frequency flopping of the trace. That smelled of a bad connection in the overall loop so I hunted for poorly soldered parts. It took a while adding some flux (here we go again) and using either the heat gun or my solder iron to go over all the possible parts. Eventually I found the culprit and was presented by a beautiful sine wave. The frequency setting worked, the vernier worked and the frequency ranges worked as well.

With that result, I can now investigate and test the output amplifier and that's next on the list.

Wired-up all the required parts to make the amplifier functional, and eventually got it all working. 

Oops!

I found one serious issue that I didn't see when using the prototype. I never tested the counter and then also looked at the sinewave distortion. In addition, I also used a slightly different pick-up point to tap the output frequency for the counter. That's a mistake, the 1K series resistor loads the feed-back loop of the output amplifier way too much due to the clamping of the diodes on the other side. I even tried 10K and that's still not good enough. Higher values result in no signal at the diodes. 

The fix is rather easy and only involved a single trace cut from RV2 to the 1K of the squarer circuit and connect it now via a jumper wire to the Sine Out test point. I will fix this in the schematic and layout of course.

Tuning the highest range

While it is still lying on my desk and easily accessible, I decided to tune the X10K range because the frequency was a little too high compared to the lower ranges. I replaced the 100pF capacitors for C16 and C26 to 140pF. I didn't have that value, so I used a 150pF probably an X-version. That resulted in a frequency of 105KHz instead of 100KHz. The 140pF COG should get it closer. 

Looking into the rail situation

I also investigated the fact that I could not get the positive rail up to +17V, while the -17V worked. The raw supply is also a little lower than I hoped for with the transformer I'm using, so I adjusted that output to 40.0V. With that, I adjusted the rails now to +/- 16.4V so they are equal. I don't think it will matter much. The original SG505 used +/- 15V rails, I decided to use the SG505 Option 2 rails that are +/- 17V which will give a bit more head-room for the Opamps.

I have not made any distortion measurements yet, I'll wait until everything is inside the enclosure and I have shortened all the leads that go to the front panel to the appropriate length.

Using the front panel

The next step is to mount the controls on the front panel and the PCB in the enclosure. I also need to make a few mechanical adjustments to the front panel because the holes do not match they way I designed it, and I made a mistake with the size for the main potmeter. These turned out to be a KiCad footprint issues and the result is that the holes are too small. Easy to fix but I also need to move the hole for the BNC connector a bit. Obviously, all the changes will be updated in schematics and the the Gerber files that I will publish.

Putting it in the enclosure

I finished the assembly of the front panel and put the main PCB inside the enclosure. It all works as advertised, but I had to make several changes to the front panel design.

Here is a view from the back that shows a few more details.

The power supply is still outside the enclosure as you can see, and I connected it with flying leads that still look untidy.

Now that I have the main potmeter reduction unit installed, the adjustment of the frequency is very precise and feels great. If you're in doubt of getting one, do it, you won't regret it.

Adding the frequency counter

I also fixed the counter loading problem that I mentioned earlier so I can now start to add the Arduino Nano, the opto-coupler and the OLED display.

The installation went pretty easy, also because I used tape to secure the display to the front panel.

However, making it all work was more involved than I anticipated, even though I built a prototype that (seemed) to work very well. After a few hours of struggling, I ended-up making several hardware and software changes to make it work reliably throughout the frequency range. 

I eliminated the squarer circuit and replaced the BJT with a MOSFET to reduce the load on the frequency generator tapping point. I could now also use a much larger series resistor, and placed that very close to the generator output again, instead of using the output amplifier because that signal can be attenuated.

The trace that goes from the generator output to the MOSFET for the counter is very long, so I wanted to reduce the current and hence the possibility for cross-talk.

I also had to power the opto-coupler open collector output with the Arduino 3V3 instead of the 5V rail to reduce the 5V p-p hammering on the port. That made a significant improvement to the precision of the counter.

Lastly, I tried to get a bit more fancy with the frequency count display, and wanted to add decimals for frequencies below 1KHz. After trying for some time, I realized that the library I'm using is optimal for the higher frequencies, but not so much below 1KHz. For those frequencies, to get a decimal digit, I needed to select a gate time of 10s which makes the counter unresponsive for this application. Selecting a library that fixes that will not work well with the higher frequencies, and I have not figured out how to use both libraries and select them dynamically. It is what it is. The solace is that the counter is a lot better (more precise) than the frequency dial of the original SG505. If you need a higher precision, use a real counter!

The DMM shows the frequency from the SG505 output and can be compared with the counter display. Not a perfect match, but good enough!

Next step is to mount the power supply in the top half of the enclosure and create the correct length for the power interconnects. By then, it's time for some distortion measurements, some tuning/calibration and see if I need to add shielding to the inside. I think I will...

Oh boy, oh boy...

I finished with putting everything inside the enclosure, calibrated the 0dBm output level and found out that I need to make changes to the -10dBm switch circuit.. I have a new set of values that I tried and worked for the Pi attenuator but will need to take the board out to replace the resistors. That can wait. 

I wanted to make a distortion measurement to see where we are:

The 50Hz mains is everywhere, I kind of expected some of it because there are two transformers in the box but was hoping some serious shielding would help, but this is out of the question. The good news is that the principle has pretty good harmonic distortion results. THD h2-12 is 0.00063%, but's it's overwhelmed by the hum. 

It's abundantly clear that the power supply has to move out. There is no place for both of them in this house. Bummer. 

That means going back to an earlier idea and that was to use a separate enclosure to house the power supply and keep it away from the generator. That's pretty easy to try with some longer connection leads for the power rails.

Unfortunately, even with the generator in a metal box, the hum and the AC-DC supply generated noise is still there.

Here is the result with the +/-17V connected, but without the 12V supply connected. There is no sine wave because the relays don't work. This is kind off the noise floor.

It seems that the shunt supplies are working OK, it's the 12V supply and circuits. 

This is with my lab supply feeding the 12V rail, activating the relays.

And below is the same measurement but with the Arduino Nano out of it's socket. I also did a quick trimming of the H2 reduction, and that worked very well. Went from -110dB to -126dB.

The silver lining

Without the 12V AC-DC convertor, and without the Nano/LCD display, the generator works fine. 

THD-N is 0.0033%, THD H2-12 is 0.00020%, with H2 at 0.000050% and H3 at 0.00014%
That's good to know and very good news! Now I need to find some solutions.

Finding the root cause

We already established that it is mainly the AC-DC 12V supply, but is there more to it? I tried it also with the Nano powered, but pulling the LCD connector. This dramatically reduces the noise so it's not the Nano itself, adding little peaks at 15KHz and 19KHz, but more the i2c signals and the power devices on the OLED board itself:

Using a 1uF blocking capacitor and using the E-MU to look at the AC-DC 12V rails, it sees this:

And without the OLED display this:

So the dominant culprit is the OLED display, and there is also seems to be some back-coupling from the generator into the 12V rail. On a hunch, I disconnected the generator output signal to the counter input circuit and presto, we have proof that the harmonics are from the Nano and counter input circuits.

When I pulled the opto-coupler, the harmonics went away. Putting it back in and removing the 3V3 for the open collector pull-up did not make a difference. Disabling the Arduino loop reading the counter did also nothing. It sure looks I need to cut the power to the Arduino and the display to make that circuit quiet.

The two part solution:

Powering the 12V section of the Arduino counter with a SPST switch, I can now eliminate the generated noise from that circuit. It works great, when I power the 12V with my lab supply. Have a look:

With the Arduino Nano circuit turned on, it looks like this:

Mind you, the noise is still below 120dB, but the root of the problem is the AC-DC 12V supply. That will need some serious filtering, or another solution.

The second part of the solution

I need to filter the AC-DC supply much better, or create a different solution. Seriously reducing that noise is going to be a challenge to force it below 120dB. I first need to profile that supply better.

The 12V supply draws 46-48mA when everything is powered.

We're looking mostly at 50Hz mains related hum, plus many harmonics in the higher frequencies.

I can try to filter the hum and noise, but the whole reason for the AC-DC convertor feeding the separate 12V supply and to avoid coupling to the generator fell flat on its face. That's not the solution.

So, when the coupling is there anyway, I might as well do away with the 12V circuit and tap the 12V rail from the main supply. I quickly added a few components to the 5V section of a bare Jung supply PCB and modified the output for 12V. It's actually 11.5V but that has no consequence. When powering the 12V rails with the new supply, we're making real progress. This is when the Nano supply is switched off:

I still want see if I can get rid of the mains related hum on the left, but the part on the right is now clean.

To give myself some peace of mind, I also tried the Jung SuperReg again for the +/-17V rails. Surprisingly, with the current configuration, it produced more harmonics. Go figure.

So, all in all, I decided to redesign the power supply by removing the AC-DC converter, and add a third rail fed by the 40V Vreg supply, to provide the 12V supply. The 12V rail does not connect to the common ground, so it stays floating from the rest. 

I tried many things to reduce the mains hum and was partially successful by using more capacitance on the main supply.

The next step is to work on the grounding/shielding of the generator. Right now, the front panel is isolated from GND, and I need to change that now, and also ground the switches and potmeters on the front panel. The whole generator will continue to be isolated from earth ground.

Major changes and a departure from this version

After considerations off the issues described above, I have now decided to make a significant change to the generator, now that the power supply is no longer going to be in the same enclosure as the generator. I'm now going to use a full metal enclosure for the generator and put the noisy parts of the power supply in another enclosure.

This means that with the other changes I already had to make/want to make, I will go through a redesign of the main PCB, the Power Supply and the Front Panel.

The sponsoring agreement with PCBWay requires a new Blog for every set of PCB's that they sponsor, so I will start a new Blog for the new Version 3 of the DIY SG505 rebuild. This one was already too long, so it's a good reason to start a new one anyway.

The V3 information can be found here:  

https://www.paulvdiyblogs.net/2025/09/diy-rebuild-of-tek-sg505-v3.html

A Github repository is available here it will be updated with information during the project when I have verified the correct operation. I'm still working on it, so there is very little information there at the moment so please be patient.

Building a new 100MHz Differential Probe

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:

https://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-make-buy.html

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:

https://hackaday.io/project/203296-a-slim-10x-100mhz-10mhz-1mhz-differential-probe

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...