Near-Field Antenna for sniffing differential bus - Page 1
Forum section: electronics / rf-microwave

author: Rerouter
author_rank: Super Contributor
timestamp: April 29, 2023, 12:34:59 pm
content: Hello,I'm working on a way to clip over some RS485 wires and read off a copy of whats being sent over the bus, by lying spaced wires over antennas, (Its for diagnostics)The better performing options I've found for other applications (CANBUS), use a meander antenna, but I cant find very much information on it for this kind of application (wire sitting directly on top of the antenna) and could use some pointers on what I should focus on to optimise how much signal I can extractThe example antenna I found was for edge rates around 30-40V/us, gets fed into a differential amplifier that then gets fed to some schmidt triggers, I also expect I need to manage some kind of filtering to get a rough bandpass between 9.6KHz and under 10Mhz,Dimensions of the existing example I can find is 0.2mm trace, 0.5mm pitch, 5mm wide, 22mm long per wire, with a 1mm exclusion of the ground plane,Thank you for any input or suggestions,

author: RoGeorge
author_rank: Super Contributor
timestamp: April 29, 2023, 03:52:41 pm
content: Don't know about near field antennas, or which one would work best.I would try magnetic coupling, because RS485 is a current loop. For example, a miniature clamp-meter that encircles one wire (to make a current transformer, e.g. from coiling a few turns on a ferrite gland, the ones to reduce EMI, so that the gland encloses one wire).I didn't try, but I guess magnetic heads (from former cassette tape recorders, or floppy disks, etc.) put in the proximity of the wire (without heaving to enclose the wire) should work, too, would make a magnetic probe to sense the current in a RS485 wire.Non contact magnetic probes, the ones sold for oscilloscopes (easy to DIY), might work too.

author: nctnico
author_rank: Super Contributor
timestamp: April 30, 2023, 05:39:32 pm
content: No. RS485 is not current loop. That is another RS4xx electrical specification. RS485 is differential voltage signalling.Sniffing a differential signal is likely strongly influenced by how (and if) the wires are twisted.

author: Rerouter
author_rank: Super Contributor
timestamp: April 30, 2023, 08:44:52 pm
content: Yeah, differential voltage bus, not much current,The wires would be untwisted over a small distance and each wire laid upon an antenna

author: RoGeorge
author_rank: Super Contributor
timestamp: April 30, 2023, 09:45:02 pm
content: RS485 always needs a terminator. Without the terminator it doesn't work reliable. The Tx sees about 120ohms, which implies plenty of current.The transmission line (the wires) together with the terminator makes a current loop for the Tx driver. That is why RS485 is so immune to interference in industrial environment, over huge distance and speed (when compared with voltage transmission e.g. RS232). Being differential helps, too (in comparison with RS232), but that helps mostly against common mode noise, not for speed. Other examples are the LAN and SCSI interfaces, also RS485 type.Rx sensing is made in voltage, indeed, but it can work so well in noisy environments only because there is a current loop through the Tx driver + the transmission line + the terminator.

author: nali
author_rank: Frequent Contributor
timestamp: May 01, 2023, 07:56:07 am
content: There are commercial products available that do just this for CANbus so I don't see why not for RS485.E.g.https://www.canm8.com/cannect-contactless-reader/canm8-cannect-contactless-reader.html

author: tszaboo
author_rank: Super Contributor
timestamp: May 01, 2023, 08:36:30 am
content: Just get a pair of sharp probes and pierce the isolation of the wire. RS485 can handle a lot of nodes, some drivers will do KMs of cable with hundreds of nodes on it. Connect the probe to a 485 to USB receiver and done. If you use sharp probes the isolation closes back and you can never tell that you were connected.But it always comes down to the missing termination resistors, or cable not connected. If the software is written well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2023, 09:04:14 am
content: I mean, current depends on where you're measuring it. And also if you need directionality, which could be helpful to measure.In the middle of a double-terminated bus, there will be significant current flow: signal voltage over characteristic impedance.Right up at a receiver, not much.But at an active transmitter, that (or ~double the current, because both terminations act in parallel, if applicable).The meander doesn't do anything for E-field sensing; just slap down some wide-enough traces be done. More helpful will be integrating JFET followers -- minimize stray capacitance to the sensors, maximize signal gain. Maybe combine with a diff amp to drive 50 ohm BNC for direct scope usage?Tim

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 01, 2023, 01:21:46 pm
content: If you wish to pick up the signal without cutting into the wires or connectors, perhaps this would work?Clamp-on RF Current Meter (ifwtech.co.uk)Use the concept of the clamp on sensor and see if you get enough signal to analyze. All you do in wrap a wire around a clamp on ferrite and use that wire as a signal source. I do not think you need the other circuitry.Member A. Z. mentioned this in another post.