DIY Magnetic field Probes - Page 1
Forum section: electronics / projects

author: yanir
author_rank: Regular Contributor
timestamp: March 29, 2013, 07:58:43 pm
content: I recently ordered a spectrum analyzer for work (Rigol DSA815-TG, like Dave reviewed) for checking ground noise, and basic EMC testing. I want to construct some Magnetic/electric Field probes for finding noise sources on my boards. I found an excellent series of articles on the Rigol site that show how simple they are to build but they don't go into specifics. Does anyone have experience building these or know of a good how-to guide?In the picture attached are some images showing construction.Thanks

author: Unknown
author_rank: Unknown
timestamp: March 29, 2013, 08:26:20 pm
content: I remember reading some information on it onemcesd.com, Doug Smith's website.

author: Rufus
author_rank: Super Contributor
timestamp: March 29, 2013, 08:33:36 pm
content: https://www.eevblog.com/forum/reviews/near-field-probes/Has a photo of one I made, middle left style in your attachment.

author: yanir
author_rank: Regular Contributor
timestamp: March 29, 2013, 09:51:05 pm
content: Quote from: Rufus on March 29, 2013, 08:33:36 pmNice, that's what I need to do. Do you have a recommendation for wire gauge?Quote from: alm on March 29, 2013, 08:26:20 pmThis a great resource, thanks

author: Fraser
author_rank: Super Contributor
timestamp: March 29, 2013, 10:51:40 pm
content: I have a professional set of these probes that came with an SA that I purchased. They are very useful indeed and I have been surprised at the frequencies at which they will provide a useful signal. I believe mine are specified at 1 or 2 GHz but I have used them at 4GHz without issues.The EMC kit contains the following 'heads':Monopole E field probe'Ball' E field probeSmall Loop (approx. 1cm diam)Medium Loop (approx. 2.5cm diam)Large Loop (approx. 4cm diam)The loops are specified in terms of output for a given field density, with the large loop being the most sensitive. The loops appear to be constructed as in the second type shown in the OP diagram. They have a opening in the screen at the loop mid point. My colleagues have replicated such loops using mine as a pattern and all have worked very well indeed.I will photograph the probes over the weekend and provide exact dimensions for you. Everyday rigid coax is the material to use . My coleagues used ~3mm diameter, but the manufactured EMC probes use more like a 2mm diameter attached to a 6mm coaxial shaft terminated in a BNC connector.It is possible to use an in-line amplifier to improve the signal level from the loops and an MMIC type is very easy to build these days.UPDATE: I have just found teh PDF version of teh manual and data sheet for my probe set. I have attached them. I think the paper copy of the maual has more frequency response graphs. I will have to see if I can find it. At least the manual provides the loop and probe sizes.My EM-6992 set was provided with an Advantest R3261A. I believe Advantest charged quite a lot of money for them, but they do not look anything particularly special in terms of build. The parts have been plastic coated and you could use a product called plastic dip to do the same if you want to prevent dirt incress or corrosion issues.Fraser

author: Rufus
author_rank: Super Contributor
timestamp: March 30, 2013, 12:17:09 am
content: Quote from: yanir on March 29, 2013, 09:51:05 pmI don't see it matters much electrically. It is a loop picking up magnetic fields with as much electrical field shielding as you can get without creating a shorted turn. Coax happens to be a wire with ready made shielding and semi-rigid coax holds its shape. If I were to make another I would probably thread a ferrite bead on the cable before forming the loop as suggested in your attachment.

author: w2aew
author_rank: Super Contributor
timestamp: March 30, 2013, 02:02:01 am
content: The figure in the original post is pretty clear using semi-rigid coax. If you don't want to build your own, there are some relatively low cost commercial alternatives. I use these for work, and they are very nice:http://www.beehive-electronics.com/probes.htmlI used one of them when I made this video on troubleshooting a transient EMI problem on a circuit board.

author: JackOfVA
author_rank: Unknown
timestamp: March 30, 2013, 09:02:34 pm
content: I will second the Beehive probes - I have a 100C model I use to measure the uniformity and magnitude of the magnetic field inside a 2 meter diameter Helmholtz coil. I wish it had an SMA connector, not the SMB however.One thing I discovered today about the Beehive probe ... I cleaned it with a small squirt of IPA and it dissolved the printed lettering.I'm reasonably sure the probe is a piece of double sided PCB with the two winding coil being top and bottom traces. It's then dunked in liquid vinyl or the like after the connector is attached.

author: yanir
author_rank: Regular Contributor
timestamp: March 30, 2013, 10:15:47 pm
content: Quote from: JackOfVA on March 30, 2013, 09:02:34 pmHmm, that sounds fun and easy to make.

author: Rufus
author_rank: Super Contributor
timestamp: March 31, 2013, 12:23:12 am
content: Quote from: JackOfVA on March 30, 2013, 09:02:34 pmThe coils would have no electric field screening. If it is PCB the coil would have to be on inner layer(s) with outer layers and probably edge plating forming the screen.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: March 31, 2013, 03:13:46 am
content: Hello,For many years I mostly built and repaired home audio and studio recording equipment, but lately I too found that I needed to find a way to "see" the EMI/RFI that was causing me so much grief with my audio equipment. This had to be done economically (luckily, I'm mainly interested in relative measurements. I just need to see if my added shielding, capacitors, ferrite beads, etc were helping squash the unwelcome high frequency interferance).The URLs listed below are where I found most of the info I used to build some "sniffer probes" and also two high frequency current probes to use mainly with my low cost RF Explorer-3G Combo handheld spectrum analyzer (15MHz - 2.7GHz BW). The included photo is of my "poor man's EMC troubleshooting kit". It containing an assortment of home-brewed E-Field and H-Field probes, and the two high frequency current probes. To my surprise, I put together this complete kit of probes for a grand total of about $100 USD (inexpensive, interesting, but also fairly tedious). I later included a couple of attenuators and sma to BNC adapters in the kit too for good measure.The materials needed to build all of these devices was pretty reasonably priced. I bought a few used semi-rigid coax cables from ebay. These already had the male sma connectors needed for my handheld analyzer installed. I just cut each coax in half and made two different diameter sniffers from them. This saved me the trouble of soldering the sma connectors on.The black square shaped probe on the left side is not made from semi-rigid coax. This one is "The Paperclip Probe" from Doug Smith's site, where he states that "a paper clip (any stiff wire will do) is bent into a square shape and covered with insulation, forming an unshielded magnetic loop probe".The high frequency current probes are made with the components specified in Ken Wyatt's multi-part article listed below. One current probe is made from a toroid core and the other one (directly below it in my kit) is made from a clamp-on type RF choke. That one allows me to conveniently snap it onto the wire or coax that I'm investigating for noise issues._____________________________________________________________________________With all the info, so generously provided at Mr Smith's and Mr Wyatt's sites, I was able to make what I needed for my home testbench.Douglas C. Smith's site (www.emcesd.com) is loaded with terrific EMC related info. He also has some interesting videos on youtube.Also see the multi-part article by Kenneth Wyatt at the following URL...www.tmworld.com/electronics-blogs/emc-emi-rfi-esd/4378152/An-EMC-Troubleshooting-Kit--Part-1a-Emissions-Mr Wyatt's own site (http://design-4-emc.com) has lots of good EMC info too.Mr Henry Ott's site is also very much worth visiting (www.hottconsultants.com/tips.html)._____________________________________________________________________________The late, great Jim Williams has an app-note that shows a very slender tipped diy sniffer probe which has a stated spacial resolution of about 1 millimeter.http://cds.linear.com/docs/en/application-note/an118fa.pdfSee Appendix E of this application-note. That section shows how to build the probe and also shows some situations where the probe can be used to great advantage.I made one of these probes (the one with the "plump" body and BNC connector in the photo) and I'm impressed with its ability to pinpoint a high frequency noise source on a circuit board, etc. I use this probe with my scope and my RF Explorer-3G (I really like this recently purchased little analyzer)._____________________________________________________________________________I finally have a fairly low cost way to "see", and hopefully tame, some of the EMI/RFI that is affecting my audio gear. I hope the above URLs help you out too.Regards,FrankEDIT 4/1/13: If anyone is interested in making the two High Frequency Current Probes shown in my DIY Probe kit photo below, see the following pdf.www.interferencetechnology.com/wp-content/uploads/2012/04/Wyatt_NA_DDG12.pdfThis pdf had all the info that I needed to construct mine (see figures 4 and 5 on page 3 of the pdf), plus Mr Wyatt has a section at the end of the document on how you can easily characterize the performance of the probes after you build them.These are the various DIY probes in my "EMC Kit" that I fabricated for use with my scope and RF Explorer-3G spectrum analyzer (my scroll saw came in hand to cut out the enclosure's foam inserts to form the pockets which hold the probes neatly in place).

author: fpliuzzi
author_rank: Regular Contributor
timestamp: March 31, 2013, 11:43:56 pm
content: Quote from: Aurora on March 31, 2013, 10:46:17 amThanks for the thumbs up Fraser. The only preamp that I have available on my bench is an HP461A (gain 20 or 40dB, 1KHz - 150MHz). I use that with my scope sometimes, but luckily the RF Explorer-3G Combo is sensitive enough on it own when I use it with my DIY probes. I hope that others will post some of the probes that they have fabricated for use with their RF spectrum analyzers.Speaking of active probes... I have the components needed for the "Poor Man's 1GHz Active Probe" project by Elektor magazine.www.elektor.com/magazines/2004/october/poor-man-s-1-ghz-active-probe.57219.lynkxI'll build this probe when I get time to make the needed PCB. It should make another nice add-on for my RF Explorer-3G.Regards,Frank

author: yanir
author_rank: Regular Contributor
timestamp: April 01, 2013, 07:09:28 pm
content: FrankWow, thanks, that's a lot of great info. I should be able to make some suitable probes. The Beehive probes are also great option at $295.I'll be sure to post any creations or interesting results.

author: fpliuzzi
author_rank: Regular Contributor
timestamp: April 01, 2013, 07:52:56 pm
content: Quote from: yanir on April 01, 2013, 07:09:28 pmHi yanir,Glad that you found my post helpful. Actually, I stumbled across the illustration that you had attached to your original post (emcprobes.png) about a year ago. Four of the probes that I show in my DIY Probe Kit photo at the bottom of my lengthy post above were constructed using the info shown in that illustration. It helped me a great deal.Regards,Frank

author: saelig
author_rank: Contributor
timestamp: October 07, 2014, 07:29:03 pm
content: We have some really cheap probes here in case they're helpful:http://www.saelig.com/MFR00149/M00149002.htm