Scope Current Probe Deskew Fixture PCB Project - Page 1
Forum section: electronics / testgear

author: steve1515
author_rank: Regular Contributor
timestamp: August 01, 2020, 11:16:36 pm
content: After some discussion with the members over on the Siglent SDS2000X Plus (https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/) thread, I decided to try and make my own deskew fixture PCB with adjustable frequency and rise time. The adjustable rise time is to account for differing inductance since the current loop would be a wire added to terminal blocks and not part of the PCB. This would allow for varying numbers of turns. I've drawn up the following schematic. It works well on the bread board, but I wanted to post here for others in case they were interested.

author: Martin72
author_rank: Super Contributor
timestamp: August 01, 2020, 11:57:13 pm
content: Hi,First question, why adjustable frequency ?Because of the limited bandwith of some probes ?

author: steve1515
author_rank: Regular Contributor
timestamp: August 02, 2020, 12:01:35 am
content: Quote from: Martin72 on August 01, 2020, 11:57:13 pmYes, that was my thinking. I wanted it to work with more probes... even cheaper ones that don't have high bandwidth.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 12:13:04 am
content: Quote from: steve1515 on August 01, 2020, 11:16:36 pmInterested to see some current probe captures with it.BTW, your thread subject would benefit from an edit and the insertion of the word Current.

author: steve1515
author_rank: Regular Contributor
timestamp: August 02, 2020, 12:28:50 am
content: I've updated the thread title. Thanks for the tip.I've also taken some screen shots. My probe isn't the best. It's a Fluke i80-110s set to 100mV/A. I have 1k on the gate of the MOSFET and 4 turns of wire on the current loop. If I put about 500R on the gate, there is a slight overshoot but the rise time is about 200nS. Are these on par with other deskew fixtures?

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 02:04:16 am
content: Quote from: steve1515 on August 02, 2020, 12:28:50 amVoltage/current rise times need to be fast enough to measure the propagation delay differences between a voltage and current probe and if 200ns is indeed the measured rise IMHO it's too slow.I think faster is better as it allows us to confirm the risetime of a current probe meets spec.So sure you might achieve 5ns but few current probes are that fast anyways.IIRC my 60 MHz Tek P6021 has ~7ns spec.Later I'll check RT of the Siglent deskew fixture.

author: steve1515
author_rank: Regular Contributor
timestamp: August 02, 2020, 03:37:23 am
content: I tried it with a shorter wire (1-turn) and a 150R resistor at the gate and I get about 85ns rise time. I eagerly await to find out what the Siglent fixture has for rise times in both of its loops.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 05:04:38 am
content: 2 data points for you.Voltage probe on the test point of the current loop.1 turn 147mA loop8 turn 125mA loopYour 85ns RT looks to be in the right ballpark.

author: steve1515
author_rank: Regular Contributor
timestamp: August 20, 2020, 02:12:10 am
content: I've updated my schematic to simplify it a little bit and also have a 3D render of my board. What do you guys think?

author: Martin72
author_rank: Super Contributor
timestamp: August 20, 2020, 06:48:52 pm
content: Looks good so far.

author: tautech
author_rank: Super Contributor
timestamp: August 20, 2020, 08:07:21 pm
content: Quote from: steve1515 on August 20, 2020, 02:12:10 amI'd suggest additional USB footprints for the 5V supply.Plain old USB-B as most of us have plain A-B cables lying around that came with our instruments.

author: Kean
author_rank: Unknown
timestamp: August 21, 2020, 04:33:42 am
content: An alternate power connection seems like a good idea. Micro USB is convenient I guess, but I also like having a convenient way to power things off a lab supply.I see you changed from 2 x 10R 2W resistors in series, to a single 20R 2W resistor. What footprint is that? It looks huge compared to the DPAK, so guessing 4527? A bit harder to source?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 21, 2020, 06:45:51 am
content: Hi,Assuming 50% duty-cycle.Dissipation in R40.5 x 2.5 x 2.5 / 20 = 0.156W (no need for a 2W resistor)Dissipation in R3 = 0.5 x (2.52+ 52) /20= 0.78WI would use 5 x 1001206 in parallel.+1 on alternative to the USB for power (too fragile) add alternative in parallel.Regards,Jay_Diddy_B

author: tautech
author_rank: Super Contributor
timestamp: August 21, 2020, 07:47:59 am
content: The resistors in what we are copying this from, the Siglent deskew fixture do get warm but not very warm.This is the unit in this pic however as it's nearly 5MBI'll only link the post here:https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg3142106/#msg3142106Later a few posts down is Martin's RE diagrams.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 21, 2020, 01:15:15 pm
content: Hi,I have looked carefully at the high resolution picture that was posted by forum member Martin72. This is what I see from the photo:I have not drawn the cmos 555 circuits.I have indicated the power dissipations in the resistors.I have shown correctly the relationship between the coil, the resistors and the voltage test points.By connecting the voltage test point to the bottom resistor, this guarantees that the voltage and current are in phase.Regards,Jay_Diddy_B

author: 2N3055
author_rank: Super Contributor
timestamp: August 21, 2020, 06:02:16 pm
content: Quote from: Jay_Diddy_B on August 21, 2020, 01:15:15 pmJay,those are very much correct. Pulse generating circuit is not important. It can even be driven with pulse generator..Regards,Sinisa

author: Martin72
author_rank: Super Contributor
timestamp: August 21, 2020, 08:04:44 pm
content: Quote from: tautech on August 21, 2020, 07:47:59 amRemember, I´ve promised 2N3055 a hires pic..

author: 2N3055
author_rank: Super Contributor
timestamp: August 21, 2020, 10:51:48 pm
content: Quote from: Martin72 on August 21, 2020, 08:04:44 pmAnd as a man of honour, you kept it!!

author: _Wim_
author_rank: Super Contributor
timestamp: August 22, 2020, 01:56:20 pm
content: Quote from: steve1515 on August 20, 2020, 02:12:10 amI would connect your voltage header just above R4, as this is more representative for the current actually going through the coil during the turn-on and turn-off of the FET.

author: _Wim_
author_rank: Super Contributor
timestamp: August 23, 2020, 08:51:09 am
content: Green trace: deskew voltage out (short coax)Blue trace: deskew voltage out (scope probe 10x) => 8ns slower!Red trace: Pearson 411 passive current probe (same short coax length as green trace) => no measurable delayYellow trace: Fluke I50A current probe => 17ns slowerTest Fixture: 12ns rise time, 18ns fall timeRemark: Pearson has slower rise time as bandwidth is only 20Mhz vs 50Mhz for the fluke probe.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2020, 06:55:29 pm
content: Hi,Given the results from _Wim_ it looks like a rise and fall time in the 10 -20ns are is required to deskew the probes.I would suggest selecting the MOSFET carefully. Do not oversize the MOSFET this is will slow it down.You are looking for low gate charge.The RDS(on) could be 1.Regards,Jay_Diddy_B

author: Ribster
author_rank: Frequent Contributor
timestamp: August 23, 2020, 07:28:29 pm
content: So i've made a design based on wim's prototype.I holds USB C input of 5 volts, also a connector for wired 5V0.The mosfet is a GaN mosfet with high switching parameters (EPC2014C)I also put in a selector switch for the internal oscillator with the NE555, or selection for an external clock.The clock input is fed via an inverter to a mosfet driver to switch the mosfet as hard as we can.Will order today and post with the results later

author: steve1515
author_rank: Regular Contributor
timestamp: August 23, 2020, 10:10:24 pm
content: Thanks for all the great suggestions!I wanted to update everyone on my project. For this revision, I'm going to leave it with just a USB micro input. I waffled on the power input for a long time. In my case, I usually have a USB micro cable around vs other cables and I wanted to use this board as a learning experience for trying to place some parts I haven't used before.I did modify the voltage reading to be after the coil as Jay_Diddy_B and _Wim_ mentioned. Good catch!I'm a little embarased that I missed that.The large resistors are WSC4527's from Vishay. I ended up with those by reviewing the Agilent U1880A fixture. The were pretty easy to source for me.If I do another revision, I'll likely change it to the 1206 resistors in parallel and add an alternate power input as that does make sense. I also like the idea of a pulse input!_Wim_, what FET did you use to get such fast rise times?_Wim_ and Ribster, would you be able to post schematics of your projects?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 23, 2020, 10:19:55 pm
content: Hi,I would suggest looking at the BSS316N MOSFET. This is in a more friendly SOT-23 package.If I compare gate charge:BSS316NEPC2014CThe BSS316N should be faster with the same gate drive.Hot loopAlso watch the layout. The 'hot loop', where the high speed currents are is from the 5V supply to the ground connection at the bottom resistor. The decoupling should be optimized for this loop.Regards,Jay_Diddy_B

author: _Wim_
author_rank: Super Contributor
timestamp: August 24, 2020, 05:59:34 am
content: Quote from: steve1515 on August 23, 2020, 10:10:24 pmHi, I used just some parts I had in my parts bin. The FET is an FDS4559 (N-channel used, complementary P-channel unused) driven by an abused 74LCT2524 clock driver (is a 3.3V part, but I am running it at 5V...)Attached is a schematic (made in Microcap 12), but clock driver is not shown as that part was not available in Micro-cap and I was too lazy to add it. A very crude drawing of the prototype layout is also shown, as is the Micro-cap simulation file.

author: Ribster
author_rank: Frequent Contributor
timestamp: August 24, 2020, 08:04:15 am
content: Quote from: steve1515 on August 23, 2020, 10:10:24 pmSteve,I've added the schematic to the design.It's not all that complicated. I've reused your NE555 circuit with some fixed resistors. I'm just opting for a fixed frequency and fastest rise time possible.The project is done in Altium Designer. If you need the project, PM me.Quote from: Jay_Diddy_B on August 23, 2020, 10:19:55 pmI don't know if you can see it or not, but i via stiched the whole ground plane. Look at the attached schematic.If you look at page 3 of my pdf, you see the top copper in red, and all those green dots. Those are stitching via's.So i think my ground inductance will be okay, since the bottom copper layer is almost uninterrupted.I could add some more capacitance on the other side if required. I could experiment with that once i get them built up.Quote from: Jay_Diddy_B on August 23, 2020, 10:19:55 pmWell that's a nice find.I've added the part to my parts library. I'll order some samples of them together with my parts order for this.It should be straight forward to replace the GaN mosfet with this one, and compare!@All:What do you think about the solution proposed with the molex connectors ?Those are plugin connectors, so you can add a wire or a coil, but its unpluggable, so it's handy to handle.I'm just a bit worried about the distance between the connectors (net distance), but you can have the loop coming forward.I don't see the use of making the pcb bigger for that reason. And i quite like the odd shape.This is the male PCB sidehttps://www.digikey.be/product-detail/en/0395026002/WM25530-ND/2735307?utm_campaign=buynow&WT.z_cid=ref_octopart_dkc_buynow&utm_medium=aggregator&curr=eur&site=us&utm_source=octopartThis is the female plug in sidehttps://www.digikey.be/product-detail/en/molex/0395005002/WM23176-ND/2735117

author: _Wim_
author_rank: Super Contributor
timestamp: August 24, 2020, 05:52:12 pm
content: Quote from: Jay_Diddy_B on August 23, 2020, 06:55:29 pmI personally would avoid an RDS(on) of 1 ohm. This means about 20 mA keeps flowing through the coil during the "off" period (with the resistances I used), and as some current probes have it difficult to nicely return to zero after a steep peak, I find it important that the off period is as close to zero current as possible. This then also tells you something about your probe. I see this more as a "current probe test fixture" than as a "deskew-only fixture".So I would advocate for a low RDS on also, but I agree also to take a small FET with low input capacitance, although a little higher input capacitance can be covered by putting the clock driver really close.BTW, I started first without the clock driver, but got rise time similar to the results from steve1515.

author: _Wim_
author_rank: Super Contributor
timestamp: August 24, 2020, 06:06:12 pm
content: Quote from: Ribster on August 24, 2020, 08:04:15 amWhat is the remaining net distance? This we could compare to the width of some of the "typical" probes. I will start with mine:- Fluke i50A: 17mm- Pearson 411: 26 mm- Singer CP315: 32 mm- ILA SMZ802: 27 mm(yes, I had a TEA-current-probe-problem a while ago)But anyway, your current pcb footprint allows for the use screw terminals also, so I think the design is flexible.

author: Elasia
author_rank: Frequent Contributor
timestamp: August 24, 2020, 09:00:16 pm
content: Quote from: Ribster on August 24, 2020, 08:04:15 amThat looks pretty good, i'll have to remember to hit you up once its done being hashed on here less you eventually attach the ad files

author: Ribster
author_rank: Frequent Contributor
timestamp: September 10, 2020, 02:43:00 pm
content: So i've gotten around to assembling them.I need to still solder on the mosfets. Then i'll test the BSS316N and the GaN mosfet.Will report back when testing is done

author: steve1515
author_rank: Regular Contributor
timestamp: September 16, 2020, 12:53:36 am
content: Here's my board all finished up.What do you guys think?Here are some of the measured rise times. The first image is with 1 loop and the second image is 8 loops.I noticed if I measure at the input to the loop, I get a voltage spike. This is most noticeable with the 8 turn loop. Can someone check the Siglent board to see if the same thing happens?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 16, 2020, 02:17:27 am
content: Hi,In the LTspice simulation that I showed a few messages ago, you get the voltage spike if you connect the voltage probe to the top end of the coil.Try connecting the voltage probe to the other end of the coil.Regards,Jay_Diddy_B

author: theatrus
author_rank: Frequent Contributor
timestamp: January 08, 2021, 12:08:56 am
content: I was just looking for a de-skew board since I'm not paying $700 for a PCB with a 555 timer - this design looks nice.

author: Thebrakshow
author_rank: Newbie
timestamp: September 04, 2021, 02:55:12 pm
content: @RibsterDid you ever get a chance to test out your fixture? I've been really interested in making a fixture for my Tek TCP312 and lecroy CP030 probes. Your design looks really nice. If you haven't had a chance to test it and are willing to open up the design, I for sure will be willing to give it a go!Thanks!Brak

author: Ribster
author_rank: Frequent Contributor
timestamp: September 04, 2021, 05:03:22 pm
content: Hey there!My design works and i’ve made the prototypes.A forum member here contacted me and i sold him my prototype. If you’re interested i’ll produce a batch of 20 more and sell them on a tindie store.Is there interest in buying a unit?You can always hit me up at shop (at) ashlabs.beI’ll send some scope plots or the unit with the GaN mosfet. Which i will produce 20 of.Kind regards

author: Thebrakshow
author_rank: Newbie
timestamp: September 04, 2021, 07:10:30 pm
content: Thanks for the offer, however, I was planning on making my own with some tweaks (to accommodate parts I already have in my lab). I was more so curious about the performance. Mainly curious if you ultimately went with the GaN (EPC2014C) or mosfet (BSS316N) or were both acceptable? What kind of rise times were you getting? When you use the fixture are you happy with the 555 timer as the clock source at ~7.6KHz? Or do you find yourself using an external clock source?Thanks for your input!As always you guys (entire EEVBLOG community ) rock 🤟!-Brak

author: blurpy
author_rank: Regular Contributor
timestamp: September 05, 2021, 12:02:04 pm
content: @RibsterWhere do you hook up the regular oscilloscope probe on your design? Or is yours not made for the same purpose as steve1515s design?

author: Thebrakshow
author_rank: Newbie
timestamp: March 11, 2022, 02:11:22 am
content: I finally got around to building one of these fixtures. I just recreated Ribsters design / layout. The main modification I made was that I added footprints for both BSS316N and EPC2014C FETS. I've only tested out the BSS316N so far with pretty good results.@blurpy I know this is an old post but if you look at Ribster's design it, has test point locations which you can use to probe the voltage with the tip and barrel method. I've attached a photo that should clear it up.

author: blurpy
author_rank: Regular Contributor
timestamp: March 12, 2022, 11:11:02 am
content: Quote from: Thebrakshow on March 11, 2022, 02:11:22 amThank you, that explains it

author: Ribster
author_rank: Frequent Contributor
timestamp: September 15, 2023, 11:20:11 am
content: Quote from: blurpy on September 05, 2021, 12:02:04 pmI've added 2 points that can be soldered on.So you can attach it there.I've finished the product including 3d printed case.Only 1 is left at the shop, but ordered more PCB's for a new batch.So if you want one, head over there

author: ZGoode
author_rank: Frequent Contributor
timestamp: September 11, 2024, 06:31:51 pm
content: @Ribster Any plans on releasing the design files for this? Definitely looks like something I could use in my lab.

author: Njk
author_rank: Frequent Contributor
timestamp: September 12, 2024, 09:20:14 am
content: It seems there is nothing that prevents it from being built on a regular prototype board (I don't mean a bread bord) using a through-hole components. The goal is to synchronize the I and U curves, not to generate perfect edges with no ringing. Alternatively, the Rigol fixture (RPA246) is available for about 50 USD/Eur

author: ZGoode
author_rank: Frequent Contributor
timestamp: September 12, 2024, 05:33:15 pm
content: Oh dang, didn't realize Rigol had one. It's cheap enough that I'll just go with that one instead of spinning my own.