Active probes - Page 1
Forum section: electronics / testgear

author: Safar
author_rank: Regular Contributor
timestamp: May 12, 2017, 10:35:27 pm
content: A question on active probes. I have a Pico oscilloscope and look for active probe for HF "non-invasive" measurements. Now I'm looking for used probes from Tektronix, Agilent etc.Do I understand correctly that question is only in power supply and 50 Om termination? And the digital interface in these probes is used only to store the calibration values.Is there a model (not new) with BNC connector that is not capable with other oscilloscopes?Thanks

author: rx8pilot
author_rank: Super Contributor
timestamp: May 12, 2017, 10:40:56 pm
content: Tektronix has remote power adapters for some of their active probes - can't remember the models exactly. What bandwidth are you needing?Single ended, differential, max voltage?

author: swingbyte1
author_rank: Contributor
timestamp: May 12, 2017, 11:05:03 pm
content: Quote from: Safar on May 12, 2017, 10:35:27 pmLook at the tek1103 series probe power supply for information on the way to connect with other Scopes. There is a diy project around hereSent from my Nexus 5 using Tapatalk

author: Safar
author_rank: Regular Contributor
timestamp: May 13, 2017, 10:27:12 pm
content: OK, thanks all. I think that Tek P6243 or P6245 will be an good option. They need -5v and +5v power (and compensation in P6245). Plan to add cable from internal board and connect it to a separate unit via mini-Din connector

author: MrW0lf
author_rank: Frequent Contributor
timestamp: May 13, 2017, 10:45:12 pm
content: Something for DIY fans also:http://www.ebay.co.uk/itm/RF-Active-Probe-0-1-1500-MHz-1-5-GHz-HF-VHF-UHF-SHF-analyzer-oscilloscope-/291998278881$15 for new1.5100kHz...1GHz probe

author: exe
author_rank: Unknown
timestamp: May 14, 2017, 08:30:07 am
content: Quote from: MrW0lf on May 13, 2017, 10:45:12 pmLooks cool, just I'm not sure about input impedance. How is it calculated? I see the gate capacitance is > 1pF which is already bad-enough at 1GHz. Did I miss something?

author: MrW0lf
author_rank: Frequent Contributor
timestamp: May 14, 2017, 11:20:31 am
content: Quote from: exe on May 14, 2017, 08:30:07 amNo idea, just noticed, bought just in case along with some other stuff - but so far been too lazy to solder cable to it. But can find quite a lot info on the net, seems "accepted design", for example:https://elektrotanya.com/files/forum/2009/10/e04a036.pdf

author: abraxa
author_rank: Frequent Contributor
timestamp: May 14, 2017, 06:31:59 pm
content: It's a great find for sure, however personally I'd be a bit reluctant to purchase the one you linked to on ebay. Three reasons: first, he sells them as 1.5GHz probes when the BF998 is only rated for 1GHz (I doubt they incorporate a 50% BW margin). Second, he changed the design by using different passives - e.g. 5x2M instead of 10M (R1), 51R instead of 47R (R4), 5k1 for both R2 and R3 instead of 4k7 and 6k8, respectively. Capacitors are unknown, so who knows what he uses there, but he also replaced the input capacitor C1 by three capacitors in series. Third, C1 is supposed to be a PCB cap, not a discrete component. Hence, he turned a non-existant component into three existing ones.I'm not against changing the values as such, they could of course yield better performance. However, the 3x?nF and 5x2M changes make me wonder if he had to "make do" because of component availability/cost in the Ukraine. Not only do they both add significant trace length on the input side (and thus increase stub length and impedance mismatch when probing 50R transmission lines) but also increase the non-linear impedance seen by the signal.Also, what about the ground return path? The original Elektor design has a direct return where this has... none?Overall, I'd try out the original design for sure but stay away from the design sold by the ebay auction seen above.

author: voltsandjolts
author_rank: Unknown
timestamp: May 14, 2017, 06:40:59 pm
content: Cross ref, just in case this is of interest:https://www.eevblog.com/forum/circuit-studio/example-project-tektronix-tekprobe-adapterbreakout/

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2017, 06:43:04 pm
content: RP wrote an article on a DC coupled probe as well that was dirt cheap to make. I made a few but they are long gone. The BW may have been 50 to 100MHz? All basic parts. If interested, I can see if I can find the article.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: May 14, 2017, 07:11:25 pm
content: Quote from: abraxa on May 14, 2017, 06:31:59 pmDamn now might have to actually solder cable to it for finding outPaid no attention to details, was cheap and was ordering SWR bridge anyway from same dude (because it was also cheap). If turns out crap... what you gonna do - cant always win with these ultra cheap momentary mood purchases...BTW, about return path, he says on one of pics:"Do not forget to solder the ground wire somewhere on the ground surface of the probe..."Edit: Fired it up with temporary connections - electrically works. Found some info in russian space:Long Word doc, mostly in russian, with lots of schematicsand this:Rough translation of important stuff:- input capacitance approx 0.7pF formed by C1-C3- input resistance 10Mohm- gate shift voltage is created by resistive divider R2-R3 and supplied thru R1=10Mohm- input capacitance of BF998 is approx 2.1pF, because of that probe voltage will be 3x lower than actual voltage on DUT- R4 must be matched to used cable impedance- probe works from100kHz to 1GHz, with max error of 2.5dB- on>1GHz frequencies error significantly rises- transistor gate and C1-C3 must not be placed on grounded plane or in shielded enclosure to avoid forming capacitance- input wires max 10cm in length- C1 feed wire must be at distance from PCB(ground plane?)and enclosure (shield)

author: serggio
author_rank: Regular Contributor
timestamp: May 14, 2017, 11:07:27 pm
content: Interesting, how much goods him need to sell for purchase right soldering tools for SMD componentsProbably in this design need focus to 0603 or 0402 capacitors at least for eliminate components/PCB capacitance .. Isoprophil alcohol can help remove colophony too.... Disaster

author: Safar
author_rank: Regular Contributor
timestamp: May 15, 2017, 12:56:00 am
content: Quote from: serggio on May 14, 2017, 11:07:27 pmGas oven is enough... And too much tin that also add capacitanceI seen this project early but think that Tek do much better. But bit pricey of course.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: May 15, 2017, 08:38:15 am
content: Naturally this is crude contraption and of course should go for Tek if money no issue. But I hear some even use Rigol scopes with amplitudes being thrown around up to 5dB between different ack modes with same input... So it not that critical when little money involved. Think I like this little soldering disaster and it was good purchase. T34 tank was also extremely crude but blasted many engineering perfections away. Crude slavic contraptions rock

author: james_s
author_rank: Super Contributor
timestamp: May 17, 2017, 05:54:32 am
content: I would be absolutely shocked if that "1.5GHz" probe is really good for even 500MHz. Anyone want to buy one and evaluate it?

author: Dubbie
author_rank: Unknown
timestamp: May 17, 2017, 07:30:56 am
content: I bought one to have a play with. If it's any good I'll make a little enclosure for it. I'd be happy if it does 500Mhz

author: MrW0lf
author_rank: Frequent Contributor
timestamp: May 17, 2017, 07:43:17 am
content: Naturally I have unit also, but absolutely no time right now to assemble & test properly, totally preoccupied with designing Arduino-PLC based home electrical system. Just ordered 0.5km of cablingSorry in advance if it was s*it find on my partHowever I do find also that even if its 500MHz - ok for money already. So fingers crossed and good luck!

author: abraxa
author_rank: Frequent Contributor
timestamp: May 17, 2017, 10:31:04 am
content: Quote from: MrW0lf on May 17, 2017, 07:43:17 amNothing to be sorry for! It pointed me to the Elektor design, which I didn't know about before.

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2017, 06:59:17 pm
content: The probe interface also usually includes an offset signal from the oscilloscope to the probe to set the input offset. The digital interface if present identifies the probe type and any calibration data.If you are just dealing with HF frequencies up to 30 MHz, then something like Bob Pease's active probe is easy enough to build and has even lower input capacitance.

author: exe
author_rank: Unknown
timestamp: May 17, 2017, 09:24:53 pm
content: Quote from: David Hess on May 17, 2017, 06:59:17 pmUnprotected direct JFET input? How durable is this? I thought they a very susceptible for ESD...

author: james_s
author_rank: Super Contributor
timestamp: May 17, 2017, 10:04:19 pm
content: I've never owned an active probe myself, but I've been under the impression that high performance active probes are notoriously fragile. Any sort of protection ends up compromising performance.

author: G0HZU
author_rank: Super Contributor
timestamp: May 17, 2017, 10:35:58 pm
content: QuoteThat looks like a neat circuit but I'd expect that it won't prove to be unconditionally stable. I think it could self oscillate up in the MHz region when probing certain (passive) circuits. I think it needs refining if you want to make something that should prove fully stable.

author: David Hess
author_rank: Super Contributor
timestamp: May 18, 2017, 02:20:20 am
content: Quote from: exe on May 17, 2017, 09:24:53 pmAn ESD protection diode (it only takes one to protect a JFET) would at least double the input capacitance.Quote from: G0HZU on May 17, 2017, 10:35:58 pmThe 51 ohm source resistance improves stability.

author: G0HZU
author_rank: Super Contributor
timestamp: May 18, 2017, 07:55:03 am
content: I think that it's still going to be unstable with certain circuits and needs some refinement. I simulated this circuit and it isn't fully stable.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 18, 2017, 05:18:43 pm
content: Quote from: G0HZU on May 18, 2017, 07:55:03 amWishing I had your skills and software....

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2017, 06:26:04 pm
content: The skills required to simulate circuits are not unattainable. There are some reasonably easy to use tools these days, you don't need a phd to figure it out, just some patience. If you already understand electronics then the hard part is done already.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 18, 2017, 08:05:31 pm
content: Quote from: james_s on May 18, 2017, 06:26:04 pmI was thinking about RF simulation - but the stability of this circuit is more like a job for the various flavors of SPICE which is much less of a mountain to climb. Still, I really need to practice and study simulation. I feel like I waste a lot of time as a result of modest SPICE skills.G0HZU had some great contributions to one of my threads dealing with higher frequencies and physical structures - the software was beyond me and beyond SPICE. Not sure it helps on this project.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 20, 2018, 01:46:50 pm
content: Hi,I have completed a hombrew FET probe that is good for at least 1.2GHz.Details here (scroll down to see the latest version):https://electronicprojectsforfun.wordpress.com/rf-measurement-techniques/high-frequency-probes/

author: Dubbie
author_rank: Unknown
timestamp: November 25, 2018, 11:02:26 pm
content: Excellent writeup Wolfgang.Thanks for taking the time to document so thoroughly.Will add it to my list of projects to build!

author: Wolfgang
author_rank: Super Contributor
timestamp: November 25, 2018, 11:21:10 pm
content: Thanks ! Feel free to ask questions whatever they may be.