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Build your own EMC Probes

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Jay_Diddy_B:
Hi,

EMC probes are useful for locating sources of EMC on PCBs. They are available at a moderate cost from

Beehive Electronics

https://beehive-electronics.com/probes.html

and

Tekbox

https://www.tekbox.com/product/tekbox-tbps01-emc-near-field-probes/

Dave made a series of videos on using these probes and making your own H-field probe:

In EEVBlog 1178 Dave constructs an H-Field probe using semi-rigid coax and coating in Plasti Dip.



H-Field Probes

These are simply single turn coils that used to detect varying magnetic fields. They are shielded from the E-Field using coaxial construction.

E-Field Probes

These are simply a short conductor that is capacitively coupled to the noise source.

3-D Printed construction

I decided to use a 3-D printed construction to provide the mechanical support and insulation.
The probes are constructed from three parts:

The body of the probe, two pieces, is made from PLA.
The strain relief for the coax is made from TPU.

All the electrical connections where made with RG316 coax.

Photograph




Regards,
Jay_Diddy_B



Jay_Diddy_B:
Hi,

Here are some of the details

Fusion 360 Design

H-Probe







E-Probe



Construction






This shows how the coil is constructed.

SMA Strain Relief



A matching strain relief was made for the SMA connector using TPU filament.

The E-Field probe is constructed by removing the outer jacket and braid shield from the 12mm at the end of the cable and gluing the cable in the probe body.



Regards,
Jay_Diddy_B

David Hess:
Appendix J of Linear Technology application note 70 shows the design of a magnetic EMI sniffer probe which can be used for much more precise applications:

https://www.analog.com/media/en/technical-documentation/application-notes/an70.pdf

T3sl4co1l:
Here's a lower profile one, though lacking shielding:



Two turns #24 hookup wire, soldered to a length of RG-174.  I forget if I put a termination resistor on it or not...  (Huh, just checking, it measure 26 ohms at the connector end.  Now I really don't know what I did... :palm: )

The effect of no shielding, is some E-field sensitivity.  This can be somewhat inconvenient in practice, but when understood, can also be kind of useful.

It could also be made with a balun, turning the balanced loop into coax.  This would perform about as well as the coax loop type above.

I've also tried the slotted cylinder shield; it has lower sensitivity, and an off-center axis (because the slot acts like an angled end).  The coil inside is still a dipole, you literally just can't put anything on the other end of it, mechanically or electrically.  I don't think this is all that helpful.  I prefer the open loop style.

Tim

floobydust:
Bozo engineer broke a Rohde & Schwarz near-field EMI probe they are excellent, compact, flat response and work quite well. But did not purchase probe "extended warranty"  :P and they wanted over $800 for a replacement... so I took it apart and was surprised to see the wand is actually a ball point pen, nicely dip-coated and the connector just fits in the end.
The Beehive ones I find inexpensive and big and clunky. With compact PCB designs you can't get a fat probe near things.

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