Bruce Carsten's 'EMI Sniffer Probe'

On Sunday, August 16, 2020 at 7:24:09 PM UTC-4, jla...@highlandsniptechnology.com wrote:
> On Sun, 16 Aug 2020 14:25:44 -0700 (PDT), dagmargoodboat@yahoo.com
> wrote:
> 
> >On Sunday, August 16, 2020 at 5:09:06 PM UTC-4, jla...@highlandsniptechnology.com wrote:
> >> On Sun, 16 Aug 2020 13:46:56 -0700 (PDT), dagmargoodboat@yahoo.com
> >> wrote:
> >> 
> >> >I've many times made ad hoc sniffer coils for hunting out EMI &
> >> >found them invaluable. This more-refined implementation came
> >> >across my radar this morning, and seemed worthy of revisiting
> >> >for the group.
> >> >
> >> >The EMI Sniffer Probe is a small pick-up coil for probing
> >> >high-frequency magnetic fields with an oscilloscope, featured
> >> >in Jim Williams' classic AN118, Appendix E.
> >> >
> >> >https://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf
> >> >
> >> >The probe is a solenoid of 10 turns of #34 wire, 0.060" diameter,
> >> >encased in a brass shielding tube that has been slit to avoid the
> >> >shorted turn. The coil is terminated into 50 ohms, and the whole
> >> >works are mounted to a BNC for the 'scope connection.
> >> >
> >> >AN118 gives construction and use details vis a vis SMPS.
> >> >
> >> >Insulated, this sort of thing would be handy for non-contact
> >> >tracing fast high-voltage currents.
> >> >
> >> >I may build one into some semi-rigid coax, possibly with a ferrite
> >> >rod for tighter resolution.
> >> >
> >> >Cheers,
> >> >James Arthur
> >> 
> >> If you're looking for EMI emitters, why shield the coil?
> >
> >To block the electrical field--you're hunting currents, not
> >capacitively-coupled voltages.
> >
> >> I sometimes hang a short piece of wire on the end of a coax, into a
> >> spectrum analyzer. For mag fields, a cheap unshielded drum core
> >> inductor works great.
> >> 
> >> Friday, trying to find my jitter problem, just waving a scope probe
> >> around was all I needed. I have two dc/dc converters on my board, and
> >> therorized that one or both were getting into my nearby timing ramps.
> >> Parking the probe above each converter showed its switching frequency.
> >
> >I do that too.  Also, for mixed hash, trigger on the magic wand
> >probe, while viewing the hash on another channel.  That freezes
> >the various hash components selectively, one-by-one.
> >
> >> https://www.dropbox.com/s/g3ohf1538qth6wl/J270_E02.jpg?raw=1
> >> 
> >> Once I figured out the frequency of each converter, I varied the
> >> trigger rate into the box. At some cardinal rates, I could see the
> >> trigger rate heterodyning the jitter pattern. Neither dc/dc switch
> >> rate corresponded. The thing that did correspond was the oscillation
> >> frequency of an LDO.
> >> 
> >> Sometimes a good fet probe doesn't even need to touch a signal. Just
> >> get close.
> >> 
> >> Electronics presents us with riddles to solve.
> >
> >Yep. I'm intrigued with air-core power converters at the moment,
> >hence the appeal of B-field probes.
> 
> Fun. Maybe make a transformer from really thin kapton flex.

These guys put 35W through a 10mm coreless PCB-printed transformer:
https://ieeexplore.ieee.org/document/6099785

That's cool.
 
Cheers,
James