Requesting feedback on 150MHz differential probe - Page 1
Forum section: electronics / projects

author: craigMWS
author_rank: Contributor
timestamp: January 10, 2023, 07:05:55 pm
content: Hello,I’ve been working on a compact 150MHz differential scope probe which I’m planning to make into a product. I’m interested in any feedback you might have. Is it something that you would find useful? Are there any additional features, information, or testing you’d like to see? Thank you so much for taking a look!I started this project because I had trouble finding a differential probe for general purpose and high frequency testing that costs less than $1000USD. Most lower cost ones are high voltage type probes with larger probe bodies and bulky leads. While great for high voltage use, I have found them to be cumbersome for probing around small components, and hard to use for reliable high frequency measurement.The probe is also designed to allow easy user tuning of common mode rejection and zero bias. These adjustments are automated and do not require additional test equipment to perform.The probe is in an advanced prototype stage. I’ve included a brief description below. For more details, seehttps://milkywaysys.com/and for more information on testing, seehttps://milkywaysys.com/pd150a-r5-testing/.Selected Specs & Features:-Pricing: targeting <$300USD-150MHz 3dB bandwidth-20:1 Attenuation-Differential range of +/-20V with up to +/-30V common mode-Input damage threshold of at least 100V-CMRR ≥70dB at 60Hz and≥60dB40dB(TBR) at 10MHz, returned to input-50Ω output impedance-Probe size: 2.4 x 1.5 x 10.0 cm-Input sockets with 0.1in (2.54 mm) spacing-Automated DC bias and common mode rejection ratio tuning-LED oscilloscope channel indicator-Powered through USB Type-C connectorProbe with Enclosure:The probe inputs are at the tip of the probe on the right, below the “+” and “-” labels. The signal output coaxial cable and USB power cable are on the left. The two buttons and RGB LED provide a user interface for several functions – changing the LED color to act as a channel indicator, zeroing out DC bias, and tuning CMRR.Probe PCB:The input attenuator under an EMI shield at the left (lid removed in photo). The two tuning capacitors under the EMI shield are for frequency compensation. All other tuning is electronic, without any need to adjust potentiometers.

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 10, 2023, 08:35:38 pm
content: Having only 10dB loss in CMRR between 60Hz and 10Mhz is going to be tough, also CMRR in differential probes is something to be tested in each probe, ie a routine test, since it usually can’t be designed in and has to be tested (and corrected) for each probe

author: bson
author_rank: Unknown
timestamp: January 10, 2023, 09:27:38 pm
content: I understand if it's an interesting project, but is it really economical compared to buying a used probe, say a LeCroy AP034? These are directly supported by Siglent scopes with a probe bus style interface (using a Siglent adapter that adds to the cost), but can also use a LeCroy powered probe adapter (forget what they call it off-hand) with just about any instrument; it connects to the probe and provides a BNC output. I use the powered adapter mainly to use the AP034 with a bench counter. But it could also be used to say trigger a logic analyzer capture. I think I paid $350 for the probe, plus if I recall $45 for a 10:1 attenuator and also got an assortment of random used HP and Tek probe tips off eBay. (It was cheap because it had no accessories, but those pop up once in a while if you keep looking.) I see a naked AP034 on eBay for $400 right now - it will need an attenuator and some basic tips, but otherwise they claim it's "calibrated" which I would take to mean it checks out okay. But it's certainly an interesting project, and the enclosure design looks great! (Some of the current active single-ended probes are awfully tiny, like the LeCroy ZD series, somewhat to ergonomic detriment IMO but good when clipped on or wedged between pins with a blade tip.)

author: craigMWS
author_rank: Contributor
timestamp: January 11, 2023, 12:16:38 am
content: Thank you for pointing that out PartialDischarge! That spec is indeed suspect. The latest prototype measures 43dB at 10MHz. I probably was looking at data which was not scaled by the probe’s 20x attenuation when working on the specification. 40dB may be a more reasonable value at 10MHz. I may leave it as TBR until I compensate and measure a large number of probes.The probe’s operation at low frequencies is more unique. Its built in tuning feature can automatically balance the resistive part of the input attenuator. Its performance is limited by the discrete step size of the adjustment. My worst-case analysis shows 70dB. The prototype measures about 75dB at 60Hz after running automatic adjustment.

author: craigMWS
author_rank: Contributor
timestamp: January 11, 2023, 01:07:30 am
content: Quote from: bson on January 10, 2023, 09:27:38 pmThank you bson, that is a good point! It’s very hard to beat a used probe or other equipment from the big names on performance and quality. I often go that route too when I have the time. I wasn’t aware of Siglent’s adapter for LeCroy interfaces or their powered adapter – thank you for pointing those out. It reminds me of an interesting project for adapting to Keysight's autoprobe:https://electronicprojectsforfun.wordpress.com/using-the-keysight-autoprobe-interface-in-your-own-projects/. Working on the enclosure was quite an experience for me! Being an electronic guy, it was slow going learning FreeCAD, which I used for the 3D modeling. It was amazed by how useful that software is after getting the hang of it.

author: jonpaul
author_rank: Super Contributor
timestamp: January 11, 2023, 04:36:19 am
content: The 0.1" input and clip choice is limiting, a delicate and awkward connector.I doubt the performance to 150 MHzThe probe needs completely metal Shielded caseInput protection is critical, 40 V way low.With persistent searching on epay, ham fleas, surplus, you can find classics tektronix P6046, Yokogawa, Tektronix and Lecroy HV differential probes.We have several of each Tek, Yokogawa, but in half century of experience we seldom used any of them.Designing and debugging SMPS, Electronic ballast, EMI, transient protection, we employed Mains Isolations transformers, SCOPE CH1-CH2, Tektronix 7A22, AM501.Huge bravo for your initiatives!Have an absolutely fantastic day!Jon

author: craigMWS
author_rank: Contributor
timestamp: January 11, 2023, 02:21:46 pm
content: Quote from: jonpaul on January 11, 2023, 04:36:19 amThanks so much for your feedback Jon, and hope you have a wonderful day too!Yes, this probe is limited to low to moderate voltage work, though it will survive at least +/-100V (40V is its linear range). It would be a lower cost alternatives to a Keysight N2792A or LeCroy ZD200, but wouldn’t replace a HV diff probe for high voltage applications.The input sockets are a bit fragile, though I haven’t managed to damage them yet. I’m hesitant to make them too large are far separated since I have found the closer spaced sockets to be helpful for high frequency work and for probing near smaller components. I had thought of adding some sort of “connector saver” or other solution to make it easier to replace the input sockets.Regarding bandwidth and shielding, I have had success with selectively shielding the signal path as opposed to a full metal enclosure. For example, the high impedance signal path is within an EMI shield and groundplane. There is a longer 50Ω trace outside the shield, which is as stripline and sandwiched between upper and lower groundplanes. The probe is quite insensitive to EMI and has a pretty flat response. I have for instance tried placing the probe body right up against small ~1MHz buck converter and did see interference on the probe’s output.I’ll drop in a test plot or two showing the prototype’s frequency response. Keep in mind the 150MHz figure is a 3dB bandwidth, so you could expect accurate amplitudes up to about ~30MHz, with more noticeable amplitude roll-off afterwards.Craig

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 11, 2023, 03:45:27 pm
content: Quote from: craigMWS on January 11, 2023, 02:21:46 pmIf you have a cheap 144MHz walkie, transmit with it with 5W at 30 to 60cm from the probe, this will generate fields similar to those used in IEC radiated test

author: craigMWS
author_rank: Contributor
timestamp: January 11, 2023, 04:03:52 pm
content: I've been testing the probe's frequency response in a few ways and dropped in some test data belowFrequency response testing with a VNAHere, I measured the probe's frequency response in a 50system using a VNA (NanoVHA-H). The test setup is pretty simple. It used a VNA, two SMA cables for the VNA, and small fixture for the probe input which contains a 50termination. After calibrating the VNA at the cable outputs, I connected the fixture to the VNA cable on the source port, and the probe output cable to the SMA cable on the VNA's 2nd port using an adapter. Thus the measurement includes the probe and its 3ft output cable in the response. The VNA was then swept from 100kHz to 200MHz and |S21| recorded. I added 26dB to the results, which is equivalent to the probe's 20:1 attenuation. This test is convenient since it covers all but the very low end of the probe's bandwidth. It's also good for comparison since some commercial probes offer similar S21 data. It has some drawbacks as well - it is a single ended measurement, and the input amplitude is limited by the VNA's output power (unless adding an amplifier). The NanoVNA used has a lot of harmonic content and lacks output level control, though wouldn't be an issue with a higher end VNA.Here is the full VNA sweep:And zooming into .1-30MHz for a better view of gain flatness:Pulse ResponseI've also looked at the probe's pulse response. It's an indirect way to verify 3dB bandwidth, as BW=0.35/(rise time), and is also convenient for detecting things like overshoot or undershoot. For this test I've used simple homebrew pulser to generate a differential pulse with a fast rise time. The probe's response is shown below. The scope's measurement feature gives a 10% to 90% rise time of 2.287ns which matches up pretty well with the spec. 0.35/2.879ns = 153MHz.Future Testing-Signal generator and scope:The simplest test is probably using a signal generator and scope. It's how I started and I've certainly done a lot of testing this way. For sinusoidal signals, I'm currently limited by my function generator's maximum frequency of 100MHz. Hoping to get some filters to use with my SynthUSB, or a new signal generator in the future.-Signal generator + RF amp + Balun + scopeI've tested the probe with higher voltages at DC and lower frequencies. At higher frequencies, I've been limited by signal generator output. The amp and balun shown below can blast the probe input with high frequency and high amplitude voltage. I haven't done much testing with it yet, but happy to run some tests if people are interested.

author: mawyatt
author_rank: Super Contributor
timestamp: January 11, 2023, 04:35:40 pm
content: Interesting project and nicely doneQuestion, is the silver ring around the front end as shown in the image, for a shield? If so, does the shield have holes for accessing the two trimmer capacitors?Best,

author: craigMWS
author_rank: Contributor
timestamp: January 11, 2023, 07:42:31 pm
content: Quote from: mawyatt on January 11, 2023, 04:35:40 pmThanks for the kind works mawyatt! The silver ring is for a shield as you suspected. That part is a shield frame from Masach Tech. The frame is soldered around its circumference to the surface of the PCB. It connects to a ground plane on the bottom of the PCB through several vias along the perimeter. The frame has a corresponding snap on cover. It has been a convenient design to work with. The cover snaps on tightly, but is durable enough to be pried off without damage.So far I have been removing the shield cover to adjust the trimmer capacitors and then replacing it when testing the probe. But the cover is actually supposed to have two small holes directly above the trimmer capacitors. The holes would align with the two unmarked openings on the top of the enclosure to allow adjustment without taking the probe apart. I haven’t tried creating the holes in a cover yet. I plan to try using a metal punch or small drill press, and may need to work on a simple fixture to keep the holes aligned.Here are the shield parts in case anyone is interested:https://www.digikey.com/en/products/detail/masach-tech-ltd/MS233-20F/12084521https://www.digikey.com/en/products/detail/masach-tech-ltd/MS233-20C/12084530

author: jonpaul
author_rank: Super Contributor
timestamp: January 11, 2023, 07:53:09 pm
content: Rebonjour a Craig MWS (Magnet Wire Supply?): Very fine work, you are ham? Retired EE? Work in broadcast/industry?0/ The use of the USB is interesting but limiting. How much power is used? USB C? The rtn of your probe is to ref GRD to the PC or USB PSU. Some diff probes have battery power or use an isolation PSU internally.(see Yokogawa below)1/ The pulse tests are interesting, what is the BW and speed rise of the scope used? Did you account for the sum of squares Tr = T1 exp 2 + T2 exp 2 SQRT error in your Trise measurement?can you use a very fast rise gen 50 Ohm like the great Leo Bodnar 40 pS pulser?2/ TEK and others use a Z shaped swivel tip to accommodate different test point spacing, and the spacing at the tip is larger, perhaps 10 mm3/ One can order plastic injection molded cases with a foil shield lining.4/ Pix my accumulation of diff amps back to 1980s: Ham radio fleas, epay, silent key estate...TEK TM500 and 7000 Mainframe: AM502, 7A22 super sensitive to 10 uV /div, with selectable LF/HF BW limitP6046 50 MHz classic 1970sYokogawa 701921 high voltage diff 700 VCAT III 100 MHz x10/x100Your thoughts appreciated!Just the ramblings of an old retied EEJon

author: moffy
author_rank: Super Contributor
timestamp: January 11, 2023, 09:54:32 pm
content: Congrats CraigMWS, that is a flat input amplifier frequency response and a beautifully damped step response, that is not easy.

author: craigMWS
author_rank: Contributor
timestamp: January 12, 2023, 04:57:39 pm
content: Quote from: PartialDischarge on January 11, 2023, 03:45:27 pmThanks for the tip PartialDischarge! That seems like a quick way to check the probe's EMI sensitivity.

author: craigMWS
author_rank: Contributor
timestamp: January 12, 2023, 09:36:54 pm
content: Rebonjour a Craig MWS (Magnet Wire Supply?): Very fine work, you are ham? Retired EE? Work in broadcast/industry?Rebonjour a Jon! MWS=Milky Way Systems – a local business for all denizens of Earth (and beyond). I’ve enjoyed working at a space industry company on research and low volume products for ~10yrs. I’m a generalist with emphasis on analog, electromagnetics, and some RF. I recently embarked on a new path that’s now a mix of consulting, parenting, and working on projects like this probe. Are you working on projects lately as well?0/ The use of the USB is interesting but limiting. How much power is used? USB C? The rtn of your probe is to ref GRD to the PC or USB PSU. Some diff probes have battery power or use an isolation PSU internally.(see Yokogawa below)The probe’s maximum power is a little above one watt. I have to be pretty careful with thermal design even at these small power levels given how tiny the board is!The probe uses a USB Type-C connector. The next gen power supply is compliant with USB-C and USB Type-A ports with the right cable. It can be powered from a USB charger, charge port, and other ports that the probe can successfully communicate with and request ~300mA.This prototype has an internal isolated supply to guarantee no DC & low frequency current flow between the scope and power source ground. However, I have experimented with shorting the isolation barier and have not been able to produce an issue such as DC bias error when powering through various non-isolated USB ports. I’m undecided currently on whether isolation is worthwhile, since it does add to the product cost. Any thoughts appreciated.1/ The pulse tests are interesting, what is the BW and speed rise of the scope used? Did you account for the sum of squares Tr = T1 exp 2 + T2 exp 2 SQRT error in your Trise measurement?The scope used is an Agilent 54832B with a BW of 1GHz. Its rise time spec is 350ps, though it’s just a calculated rather than measured value (i.e. 0.35/BW).I had not accounted for the sum of squares error – thank you for pointing that out. I will try calculating. I’m curious how the result will compare with the VNA 3dB bandwidth measurement.can you use a very fast rise gen 50 Ohm like the great Leo Bodnar 40 pS pulser?Thanks for the suggestion. That is a fast pulser - I will look into getting one. So far I have been using a simple homemade pulser based on LVC CMOS logic gates. Its rise time measures 863ps on the scope without accounting for the scope’s limited rise time.2/ TEK and others use a Z shaped swivel tip to accommodate different test point spacing, and the spacing at the tip is larger, perhaps 10 mmThat is a nice feature. I’ve been looking into accessory options to see what I could offer. Hoping to improve on the probe tip and accessories as much as my limited mechanical design skills allow.3/ One can order plastic injection molded cases with a foil shield lining.That is a good point, I will keep it in mind. I do remember coming across a case like that at one point.4/ Pix my accumulation of diff amps back to 1980s: Ham radio fleas, epay, silent key estate...That is a nice collection, thanks! It’s amazing what the test equipment engineers were able to achieve with discrete components and very limited ICs in the 70s. The 7000 series plugins brought back memories. I used to have a 7603 mainframe with a few basic plugins. It was an Ebay find in pretty poor condition. I recall trying to fix some issues with it, with limited success, but did at least learn from the great manuals.

author: craigMWS
author_rank: Contributor
timestamp: January 12, 2023, 09:41:03 pm
content: Quote from: moffy on January 11, 2023, 09:54:32 pmThank you moffy! It took a lot of iteration and occasionalto get there.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 13, 2023, 02:34:01 pm
content: nice project! cheers.

author: craigMWS
author_rank: Contributor
timestamp: January 14, 2023, 03:04:57 pm
content: Thanks Mechatrommer! Cheers to you too.