Directional Antenna for Detecting Radiated EM Emissions
Directional Antenna for Detecting Radiated EM Emissions
(OP)
I am testing a design for a device with a built in Ethernet switch for CE certification. We failed the EM Emissions portion of the test due to noise spikes at 125, 250, and 375 MHz. I immediately suspected the Ethernet communication because the actual clock rate of 100Mbit/s Ethernet is 125Mhz. When I forced the connection to 10Mbit/s the noise spikes went away (this reinforced my suspicion of the Ethernet communication).
I have some ideas for routing modifications that might solve the problem, but I would like to test my modifications in-house before going back to the external lab for certification. We have a spectrum analyzer so I thought I might be able to connect some sort of directional antenna to it and probe the circuit board for noise at the problem frequencies before and after the modifications.
I am having problems trying to buy/make the antenna. When I look online for directional antennas in this frequency range, all I find are roof-top TV antennas (VHF band). I also tried making a small loop from coax cable, but I wasn't able to pick up spikes at the frequencies of interest.
Any thoughts on what I could try next?
I have some ideas for routing modifications that might solve the problem, but I would like to test my modifications in-house before going back to the external lab for certification. We have a spectrum analyzer so I thought I might be able to connect some sort of directional antenna to it and probe the circuit board for noise at the problem frequencies before and after the modifications.
I am having problems trying to buy/make the antenna. When I look online for directional antennas in this frequency range, all I find are roof-top TV antennas (VHF band). I also tried making a small loop from coax cable, but I wasn't able to pick up spikes at the frequencies of interest.
Any thoughts on what I could try next?





RE: Directional Antenna for Detecting Radiated EM Emissions
If you want me to look at it, email me the PCB for just that section and I'll give it a once over (though I will say most transceiver chip datasheets do a good job at giving you a well-shielded layout).
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Directional Antenna for Detecting Radiated EM Emissions
If you have the room I've had good luck with both of the solutions you mentioned.
I have an old rooftop TV antenna (you can get them fairly inexpensively from the big box stores) that I connect directly via coax to a 50 ohm spectrum analyzer; the 75 ohm/50 ohm mismatch isn't a problem for seeing signals that are strong enough to fail EMC tests.
I often come back into the lab and 'sniff' for emissions with a loop on the end of coax; 0.5" diameter has been good for the frequency range you mention. The loop will sense near-field emissions, so if you aren't picking anything up your source may be in a location other than where you are looking. Check the power supply leads for starters.
John D
RE: Directional Antenna for Detecting Radiated EM Emissions
If you're seeing Ethernet frequencies being leaked, then maybe they're coming out on the (commerical, unshielded) Ethernet wiring. Cat5/6 is twisted pairs, but that only gets you so far.
RE: Directional Antenna for Detecting Radiated EM Emissions
Another place to check is if you have a switching voltage regulator. These give off a lot of RF noise. Sometimes just adding a capacitor can greatly reduce this.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Directional Antenna for Detecting Radiated EM Emissions
RE: Directional Antenna for Detecting Radiated EM Emissions
The device is for an industrial environment and we use shielded twisted pair for the Ethernet media. We started the test with the shield un-terminated on the laptop side and ac terminated to earth via a 1nf capacitor on the EUT side. We also tried terminating the shield to earth on the laptop side by stripping the insulation and connecting a piece of copper tape around the shield and onto the ground plate. Next, in the same manner we terminated the shield directly to ground on the EUT side. The emissions did not change much (1 or 2 db) with the different termination techniques.
One concern with the termination on the EUT side is due to the test constraints the EUT must be 1m above the ground plane. We try to reduce the impedance in the ground connection of the device by making our ground connection through a 2cm wide copper braid, but I think the impedance of the braid is still pretty high at 375Mhz.
For my in-house testing I made a new loop antenna following zappedagain's specifications. For the loop I used a coil from an old RFID tag that was laying around the lab. The new antenna works great. When wave the loop over parts of the PCB I get noise spikes at the frequencies of interest that are 20dB over the noise floor.
Good suggestion to check the switching supply. However, the switching supply we use on this device is a MC33063. It switches pretty slowly. In our past experience, if there is an emissions problem with this switcher it will be at frequencies below 100MHz.
My plan is to use the loop antenna to find what parts of the PCB are noisy then try to quiet them down. So far it looks like the noise comes from the vicinity of the Ethernet switch.
RE: Directional Antenna for Detecting Radiated EM Emissions
That length at that frequency is close to a complete tour around the edge of the Smith chart.
RE: Directional Antenna for Detecting Radiated EM Emissions
Whenever anybody comes on with EMC issues I always give a plug for where I learned how to solve EMC/EMI issues:
http://www.emiguru.com/books.htm#EDN
Remember, cables make great antennas! Any common-mode currents will radiate. Most Ethernet circuits have a choke right before they leave the board; yours didn't get cost-reduced out, did it?
John D
RE: Directional Antenna for Detecting Radiated EM Emissions
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Directional Antenna for Detecting Radiated EM Emissions
RE: Directional Antenna for Detecting Radiated EM Emissions
RE: Directional Antenna for Detecting Radiated EM Emissions
You can not dither the clock frequency, as you are probably trying to meet some ethernet clock speed spec, but spread spectruming the clock would make the heights of the frequency spikes go down a lot.
There are shielding and absorbing materials that you could use. Tell us how exactly this unit is to be used in the final product.
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: Directional Antenna for Detecting Radiated EM Emissions
The idea of adding a capacitor between the Ground Plane and Earth Ground came from an app-note. Before I start the re-layout I would like to have a better understanding of what is happening. I think noise is coupling from the electronics side of the transformer to the Cat5 side. The capacitor is giving this noise a return path so it does not need to travel down the the ground braid or Ethernet shield. In addition to the capacitor I also plan on increasing our isolation zone between the 2 sides of the transformer from 1mm to 1.5mm.
I am just looking for a sanity check here. Does it sound like I am on the right path? Does anyone have any other suggestions?
Thanks!
P.S. I also linked the referenced app-note.
RE: Directional Antenna for Detecting Radiated EM Emissions
The transformer in the Ethernet interface (layer) is supposed to block the common mode signals; maybe at 125 MHz they still leak through? Or there might be some distortion that is getting 'rectified' by non-linear effects and radiating? I'm thinking out loud now because I have a similar problem that I need to fix eventually when I get back in for emissions testing. With an unshielded Ethernet cable I fail immunity testing because lots of noise is introduced into my system; a shielded cable passes. The Ethernet keeps communicating just fine with either cable during the immunity testing. So not to sidetrack your thread, you also might see if things are different with a shielded vs unshielded Ethernet cable.
I'm glad to hear you passed.
John D
RE: Directional Antenna for Detecting Radiated EM Emissions
For the first question, if the return path involves the analog ground plane, then I assume the CM noise source is either the Ethernet Phy or something coupling on the traces between the Phy and Transformer. For fix this, I will try to increase the separation between the diff pairs and other traces. Also, I read an app-note that suggested adding a small (22pf) capacitor in parallel with the termination resistors on these traces. The capacitors will remove the fast edges on the Ethernet traces, not necessarily the CM noise. I did try this modification during our FCC test and it did help some in my situation.
For the second question, the small Ethernet transformers have some non-negligible capacitive coupling between the primary and secondary windings (15pf in my case). This capacitive coupling could be a path for CM noise. Maybe a different transformer would help?? For my design, a CM choke is integrated in the transformer, I would think the choke would remove most of this noise. The other path I can imagine is capacitive coupling across the isolation zone between the primary and secondary of the transformer. This is the reason I plan to increase this zone from 1mm to 1.5mm.
These are just my thoughts from my investigation so far. Hopefully they will help with your issue.