Author Topic: DIY RF EMC Current Probe Set Design  (Read 13441 times)

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Offline doktor pyta

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Offline jonpaul

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Re: DIY RF EMC Current Probe Set Design
« Reply #26 on: July 01, 2022, 06:11:57 am »
Rebonjour just returned to this topic...

a few notes

1/ study the Pearson shield, your shield configuration is not good.

2/ in the frequency ranges mentioned and dimensions there's no " cavity" resonance. Check the wavelength vs shields dimensions

3/ termination should be at the winding and distributed

4/  try various types of core material, suggesting high permiability 10,000 u ferrite or thin tapewound cores.

5/ wire, insulation, core coatings or wrap, and winding design have significant effects on frequency response, transients response and flatness.

6/ I have never seen filling with beads around the toroids, it should have little effect.

Just my thoughts

Jon



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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #27 on: July 01, 2022, 09:31:41 am »
Rebonjour just returned to this topic...

a few notes

1/ study the Pearson shield, your shield configuration is not good.
The shielding you see (Al foil over 3d printed plastic) is a quick, cheap and fast method of trying different shielding geometries. 
The final version will be machined from billet aluminium.

I have never seen/touched a Pearson sensor so not really a practical option.
Looking at their products on-line indicates shielding that is a close fit to the coil.   There could be a resin supporting the coil/core inside the shield.  Can someone confirm?
Quote
2/ in the frequency ranges mentioned and dimensions there's no " cavity" resonance. Check the wavelength vs shields dimensions
Agreed.  Bad explanation on my part.  I think the space between the coil and the shielding is acting as a capacitor to the coil forming an LC resonator.
Quote
3/ termination should be at the winding and distributed
If you mean that the coil winding termination should be very close to the core, agreed.  I have proved that by experiment.  There is resonance at 23MHz due to the length of bare wire between the core and the coax cable.
The eccentric shape of the prototype allows just enough spacing for a panel mounted SMA connector.  The need to keep the coil terminations as short as, lead to the choice of SMA rather than BNC. 
BNC is easier to use and entirely adequate for the job, but the lead lengths would be longer. 
Quote
4/  try various types of core material, suggesting high permiability 10,000 u ferrite or thin tapewound cores.
The bare open coil/unknown junkbox core has proven to provide adequate bandwidth, good  flatness and everything else I am looking for. 
The enclosure has proven to be far more important than the core and windings. 

Exotic ferrite materials are only available through companies like RS, Element14, Digikey etc.  They all have minimum purchase values and they are all expensive.
I do have some known cores purchased specifically for what I am doing right now.  The junkbox cores and wire wrap are already working beyond expectations. 
Quote
5/ wire, insulation, core coatings or wrap, and winding design have significant effects on frequency response, transients response and flatness.
Experimental results do show that the choice of wire does make a difference.   I am using 40 year old mil-spec wire wrap made with real silver plating and real copper on the inside.    The advantage of wire wrap is that it minimises the gaps between the core and the conductor, minimizing stray magnetic fields. I would have to use something very exotic and expensive to make any further observable improvement.
Quote
6/ I have never seen filling with beads around the toroids, it should have little effect.
It definitely works.   They soak up energy that would otherwise be used to resonate.

Given that I have LC resonance that is reducing the upper bandwidth, I can either reduce LC to increase the resonance frequency out of the desired measurement band
or
dampen the resonance by loading the LC resonator with ferrites acting as resistors.

Both have advantages and disadvantages.  Adding ferrite beads is the option of last resort.

Quote
Just my thoughts
There is a thing called the principle of 1+1=3.  When two minds work together, the outcome is more than the sum of the two.  It is three.
Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #28 on: July 01, 2022, 10:02:18 am »
Hi
I have just tested a new enclosure.  The enclosure has the same basic eccentric donut shape.  I just made the cavity that the coil sits in smaller.  This shape will be easier to machine from Al.

The frequency response stills has about the same bandwidth but the resonance is less severe.

The plots below are all with the coil grounded to the shield.  This configuration still significantly reduces the bandwidth compared to the open coil.

Reducing the volume of the cavity has improved the frequency response. 

The next thing I will try is to make the cavity a tight fit for the coil.    The new 3D printed enclosure is now in 3 parts, so I can now replace just the middle part to reduce the size of the cavity.

Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #29 on: July 01, 2022, 10:36:55 am »
just one thought:
somewhere i've seen black conductive esd foam used as an RF absorber to get rid of cavity resonances.
https://www.conrad.pl/p/pianka-rozpraszajaca-pe-553-esd-dl-x-szer-x-wys-150-x-75-x-5-mm-394611?&vat=true&gclid=CjwKCAjwk_WVBhBZEiwAUHQCmW8EHtHLcBsMiKBzSlJ94GDAVcBp5Lq_C6-qb884SAUjkQQ0Pj--wxoCOU4QAvD_BwE&gclsrc=aw.ds

Hi
Yes there are various products. They all do the same thing, absorb surplus energy RF to dampen resonance.     A quick search on Digikey shows a range of different products.  https://www.digikey.co.nz/en/products/filter/rfi-and-emi-shielding-and-absorbing-materials/869 I find that any product with "EMC" in the title costs at least 10x more.   

The combination of shielding and absorption is rarely a bad thing to do. 
Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #30 on: July 01, 2022, 10:47:48 am »
Hi
Looking at Pearson's website, I found a test fixture.  https://www.pearsonelectronics.com/applications/emi-testing-applications

It weighs a lot more than my simple version, and I am sure it would cost a lot more, but the fundamental principle is the same.

I have found that the fixture is essential for testing these current sensing toroids.    It takes the uncertainty out of the measurements, meaning I trust what I see on the spectrum analyser. 
Dazz

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Offline jonpaul

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Re: DIY RF EMC Current Probe Set Design
« Reply #31 on: July 01, 2022, 11:22:50 am »
Have several Pearsons, 410, 411, also had contact to him in 1980s.

Paul Pearson's old  patents precisely explain the techniques.

https://patents.google.com/patent/US3146417A/en

Several other patents were granted over the years.

All are long expired


The "exotic" cores are easily available in USA, I will check a few sources,

Bon Journee,

Jon
« Last Edit: July 01, 2022, 11:41:00 am by jonpaul »
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Offline mag_therm

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Re: DIY RF EMC Current Probe Set Design
« Reply #32 on: July 01, 2022, 12:56:57 pm »
I remember the use of a high voltage rated square Pearson along with a scope, I think it was a small Philips with battery power
and insulated front panel controls.
That was used to set the grid current and  neutralization on 40 kW Class C triodes with 10 kV anode.
( But no measurements were made on anode side)

That measurement problem might come around again
I see the newly emerging "Statcom" inverters are being located in HV switching stations and up the power transmission poles under the cables.
Or maybe those inverter modules will be better designed, to be racked out and serviced off-line.
 

Offline jonpaul

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Re: DIY RF EMC Current Probe Set Design
« Reply #33 on: July 01, 2022, 01:13:02 pm »
Indeed Paul Pearson was a pioneer in the field of wideband current transformers.

Several ex employees and transformer firms made inferior knockoffs.

The idea of distributed termination was brilliant.

The shield is cut in two axis so circulation currents and losses in axial and radial are minimized.

I believe the potting was high solids silica filled epoxy and not ferrite beads.

All of my Pearsons have BNC term not SMA.

Bon Weekend

Jon
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Offline tautech

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Re: DIY RF EMC Current Probe Set Design
« Reply #34 on: July 01, 2022, 08:59:20 pm »
just one thought:
somewhere i've seen black conductive esd foam used as an RF absorber to get rid of cavity resonances.
https://www.conrad.pl/p/pianka-rozpraszajaca-pe-553-esd-dl-x-szer-x-wys-150-x-75-x-5-mm-394611?&vat=true&gclid=CjwKCAjwk_WVBhBZEiwAUHQCmW8EHtHLcBsMiKBzSlJ94GDAVcBp5Lq_C6-qb884SAUjkQQ0Pj--wxoCOU4QAvD_BwE&gclsrc=aw.ds

Hi
Yes there are various products. They all do the same thing, absorb surplus energy RF to dampen resonance.     A quick search on Digikey shows a range of different products.  https://www.digikey.co.nz/en/products/filter/rfi-and-emi-shielding-and-absorbing-materials/869 I find that any product with "EMC" in the title costs at least 10x more.   

The combination of shielding and absorption is rarely a bad thing to do.
Would conductive foam be a cost effective solution ?
https://www.farnell.com/datasheets/1485187.pdf
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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #35 on: July 01, 2022, 10:50:53 pm »

Would conductive foam be a cost effective solution ?
https://www.farnell.com/datasheets/1485187.pdf

I am sure that would work.
Way back at university, sheets of carbon copy paper embedded in polystyrene foam were used to make a non-reflective load at the end of a wave guide. 

I have also considered using carbon heater in flexible sheet form.  It is cheap and readily available.   I just haven't had the need to try it.   


Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #36 on: July 02, 2022, 12:19:30 am »
Hi
Hi
I made up another 3D printed shield, this time sized to be a close fit to the coil.    This further smoothed out the frequency response but the 3dB band still ends at 83MHz.  Much less that the coil/ferrite is capable of unshielded.    The frequency roll off is much like an LRC 1 stage filter, which it is.    I think I have gone about as far as I can with this line of experimentation

I also slightly reshaped the coil termination area to create a better transition from coil to coax.  This slight tweak has eliminated the resonance spur at 23MHz, even with the still relatively long (bad) coil terminations.   So this result indicates that a BNC connector (as used on Pearson coils, noted in the post above) would be OK.

Below are photos of the 3D printed prototypes showing the evolution from open to tight fitting shielding.
Also shown are the screen shots of the frequency response.   

For the record, I think that ferrite beads would be unlikely to succeed in this application.  I think they would most likely react unfavourably to the magnetic field produced by the current being sensed.  I think they would reduce sensitivity of the coil.  That is why I stated back in Reply 27, ferrite beads would be the option of last resort.   I have successfully used ferrite beads to suppress resonance in RF circuits, but in this application, the resonance was suppressed by altering the geometry of the enclosure.     I think that is probably a result of bringing the ferrite of the core closer to the surface of the shield.  Similar to the effect of laying ferrite onto the surface of a shield to damp the current flow. 

The experimental results show that the enclosure (e-field shield), and not the coil/core, is the dominant limit to bandwidth.    The next logical step is to address the LC relationship of the core and shield.  I have already considered using radial cuts to divide the e-field shield to distribute the LC around the core, documented way back in reply 24.  I am not trying to take anything away from Pearson's creative invention of distributed termination.   The only point I am trying to make is that anyone who works at this problem for long enough is going to end up at a stage where they identify the need to move from lumped LC to distributed LC to achieve wideband performance.    Pearson found the solution and all credit to him.  Also credit to those here who pointed to his work.  I would not have found it myself.    Having read Pearson's patent, I will just jump to his very good solution.    It is clear that Pearson did a lot of R&D before producing the Patent.   I had a look at some of the specs for the coils they now sell.  Very impressive, especially at the lower end of the frequency range.  I don't see any risk of me getting anywhere near their bandwidth. 

So the next step is for me to figure out a DIY method of applying Pearson's distributed termination.   That might take me a while.  I need to build and test prototypes, most likely printed in plastic.   When I purchased the 3D printer, I never expected to be making RF parts.   I need to figure out a construction method for the final version that uses the tooling I already have in my workshop.    Those include a couple of lathes and milling machines, but no CNC. 

 
Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #37 on: July 02, 2022, 03:22:41 am »

The "exotic" cores are easily available in USA, I will check a few sources,

Bon Journee,

I'd be interested in what you find.
Dazz

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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #38 on: July 02, 2022, 10:50:09 am »
Hi
Having read Pearson's patent, I did a bit of Googling and found Anderson's later patent 1972 here:  https://patents.google.com/patent/US3701003A/en

The eureka moment occurred when I saw the Fig 1 diagram attached below.   It shows the distributed resistor network as a ring (20) around the core (11).  The ring is basically a large flat washer to act as a low impedance floating ground (not connected to any actual ground).  The critical detail is that the ring and discrete resistors are floating.  The ring is not directly connected to the coil outputs.  The distributed resistors suppress the natural LC oscillations of the coil.

I almost have this arrangement already.    Rather than using discrete resistors to suppress oscillations, I am already using the ferrite in the core.   The shield (Al foil) is the equivalent of the large flat washer.  The shield is a low impedance floating ground.  The wirewrap is 0.5mm diameter, so the shield is in close proximity to the ferrite core.  At high enough frequencies, ferrite looks like a resistor.  The ferrite acts as a true distributed resistance load.  The stray flux from the coil,  sandwiched snugly between the shield and the ferrite core, makes the connection between the resistor (ferrite) and the floating ground (shield).    The critical difference is that I have (had) one end of the coil connected directly to the shield and coax outer conductor.    That unbalanced connection messes up everything. 

So I have modified the latest prototype version 5 and disconnected the shield from the coax.  This immediately raised the bandwidth to about 187MHz (I wouldn't trust measurements above that frequency).  Then I wrapped a second split shield around the exterior of the assembly.  I connected one end of the coil output to the shield and to the coax outer conductor.  The other coil output was connected to the coax centre conductor.    The result was that the bandwidth remained at about 187MHz.  The frequency response has some bumps in it, but when you see how bad the outer shield is fabricated and fitted, this will not be a surprise. 

So, I have no plans to replicate Pearson's and later Anderson's discrete resistor based oscillation suppression.  I could, but it would be far too much work. 
The double shield metal enclosure will be easy to manufacture.  The inner shield just needs to be a donut cup and lid.  The outer shield needs to be an eccentric donut cup and lid.  The eccentricity will create space for the BNC connector without the need for any CNC or complex milling operations.  Eccentricity can be done on a standard lathe.   The gap between the two shields can be filled with a 3D printed spacer/insulator. 

The quick and dirty prototype adaption is good enough to justify putting some effort into making a decent 3D printed prototype.
The resistive material properties of the ferrite core will affect the optimal gap between the inner shield and the ferrite core.  The gap will determine the effective resistance.  If the gap is too large, the coil will resonate, as seen in the initial enclosed prototypes.  If the gap is too narrow, the coil will be over-damped and the coil sensitivity (bandwidth) will be degraded.  Obviously the gap cannot be less than the thickness of the wire wrap (0.5mm).   I have no idea of the make/model of the ferrite core I am using.  I know it works.

It just so happens that the flat wound wire wrap DIY  Litz wire is exactly right for this application.
 
I think I will try a coil with a few more turns to improve the lower frequency 3dB limit.  I will not be able to achieve the really low bandwidth achieved by Pearson coils, but I don't need to.  I am making coils for EMC pre-testing work.  In saying that, there is no such thing as too much bandwidth.

I was expecting to need at least two coils to cover the desired frequency range, but I think I will be able to do it with one. 

It is a lucky thing that I didn't see Pearson's/Anderson's patents earlier.  If I had, I would have followed a different R&D path.
Dazz

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Offline Hydron

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Re: DIY RF EMC Current Probe Set Design
« Reply #39 on: July 02, 2022, 12:20:28 pm »
Sadly I think you're right about the chances of finding a Pearson on trademe - it's difficult enough in the UK with a max of one or two at non-crazy prices seeming to come up per year, and part of my Pearson collection has come from getting US ebay finds sent to friends living there (then collecting them in person the next time I crossed paths in NZ or the UK).

I have also spent some time making and testing CTs for tesla coil use - one key trick is to "cascade" them, with the secondary of one CT feeding the primary of another, with the final equivalent turns ratio being the product of the two CT ratios. This gives a much higher upper -3dB point than would normally be possible with a CT of equivalent ratio (greatly reduced total turns count means much more benign parasitics), but the downside is that you lose some BW at the low end, and it works best when you want quite a high turns ratio (e.g. 1024:1 can be done with a total of 64 turns, 32 on each CT). May not be an option with your sensitivity requirements.

It may also be worth looking at "nanocrystaline" cores - these will be very high Al compared to ferrite and will help the lower -3dB point. A bit pricey, but people have used them on the forum to do things like build wideband injection transformers for PSU testing. Pearson definitely uses finely laminated steel cores (I assume for the same reason of low frequency performance) - I can see them in the two split-core ones that I own.

Finally for the peak in laziness, try and terminate a Murata 56050C into 50 ohms to give 1V/A - my measurements of this combo suggested a ~20MHz -3dB point - sure it's miles off the 200MHz that a similarly sized Pearson 2877 will have, but the cost of a couple of dollars can't be beaten!
 

Offline mag_therm

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Re: DIY RF EMC Current Probe Set Design
« Reply #40 on: July 02, 2022, 06:31:44 pm »
Hydron,
I have used that cascading method on the 1 ~ 10 kHz C/T . Higher than about 3000A, the winding temperature becomes a problem even with water cooling.
So, a cascade is made eg 5000:200 & 200:5. They  have better linearity and fidelity too.
( One time there was poorly made terminal on the litz cable to the second C/T. The main went up in an arc and ball of flame, scaring everybody. )

On the larger toroidal high freq power transformers, the LV side might consist of a number of windings ,
 all connected in parallel to two concentric rings  of 4 inch busbar so the leadouts are as short as possible. That improves the balance around the core, and reduces leakage and stray inductance.
I never thought of doing that on a measuring C/T, but it might have similar improvements.
 

Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #41 on: July 02, 2022, 11:09:42 pm »


It may also be worth looking at "nanocrystaline" cores - these will be very high Al compared to ferrite and will help the lower -3dB point. A bit pricey, but people have used them on the forum to do things like build wideband injection transformers for PSU testing. Pearson definitely uses finely laminated steel cores (I assume for the same reason of low frequency performance) - I can see them in the two split-core ones that I own.

Finally for the peak in laziness, try and terminate a Murata 56050C into 50 ohms to give 1V/A - my measurements of this combo suggested a ~20MHz -3dB point - sure it's miles off the 200MHz that a similarly sized Pearson 2877 will have, but the cost of a couple of dollars can't be beaten!



I note that the Murata 56050C is cheap and the datasheet states a  bandwidth of 20kHz to 200kHz.    This is fine for the intended applications. "The 5600 series of current sense transformers are designed to monitor AC currents. They can be used for high frequency current sensing, for example in switched-mode power supplies, motor control and electronic lighting ballasts." 

I am sure you are correct in recommending nanocrystaline cores.   Lowering the -3dB point would be a good thing.    Right now the dominant effect degrading and limiting the performance of my current sensor is the geometry of the enclosure (shields).    That is where I should focus my efforts.    Unlike analog electronics, I can't just plug some numbers into a formula to get a working answer.    I used to have a job project managing the building of power stations so I am familiar with the HV CTs used in those applications.    My professional background includes radio and microwave  engineering but I am not a ham-radio guy.    One of the things I like about RF engineering is that I can make a physical object that works.  I can't make a microprocessor or any other electronics component.    In this case, I can make a functioning RF current sensor from stuff in my junk box.

I can't hope to replicate Pearson''s >50years R&D effort to match their coil performance.   So what am I trying to achieve?  One of the main reasons is to exercise my brain.   Early 2021 while commuting to work on my bicycle I had a major crash and I am still recovering from the injuries.   The doctors advised that I needed to exercise the brain so I tried assembling a plastic model kit of a racing car and couldn't do it, but I found I could diagnose and repair multiple faults on an ancient 1980's vintage Wavetek 2520 RF sig-gen.  You can read about that saga here: https://www.eevblog.com/forum/repair/wavetek-2025a-0-2-2-200mghz-rf-sig-gen-repair/] [url]https://www.eevblog.com/forum/repair/wavetek-2025a-0-2-2-200mghz-rf-sig-gen-repair/[/url].  I have finished that so now I am working on this RF current probe.    My brain is improving, but it isn't like it used to be.  If I look like I am writing nonsense, that's my excuse.  If I can make a RF current probe with a reasonably flat and wide bandwidth, that would be a bonus.


Dazz

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Offline Hydron

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Re: DIY RF EMC Current Probe Set Design
« Reply #42 on: July 02, 2022, 11:26:35 pm »
Sorry to hear about the accident - sounds worse in a lot of ways than a purely physical recovery process.

My comment about the Murata option came from using one in an application where shielding wasn't an issue, and I wanted a quick and dirty option for a measurement - just soldered a BNC straight to it. The upper limit being WAY above datasheet spec was a nice surprise - it was probably specced for the series as a whole, which goes up to much higher turns ratios.

As for a project, this certainly sounds like a fun one (and that I've toyed with having a go at myself, though given I have some Pearsons I think I'll try and DIY a high bandwidth rogowski coil instead)

@mag_therm:
Would love to see what the watercooled CT looked like!

In the tesla coil case another big reason for cascading is laziness - many fewer turns to wind! Application is a few 10s of kHz up to a few hundred, up to 1-2kA at quite a low duty cycle, don't want a big phase shift (so a good upper -3dB point is handy), so the technique works well.
 

Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #43 on: July 02, 2022, 11:30:27 pm »
Hi
Here is a photo of the dual shield prototype I made up.  The interior shield is fully floating and not connected to the coax at all.
The outer Al foil shield you can see is connected to the outer shield of the coax cable through the tweezers (being used as a conductive clamp).
If you are looking at this and thinking that the test setup is rubbish, you'd be absolutely correct, but look at the Spec-An frequency response.   It is flatter than it should be out to >185MHz. 

The important claim of Pearson and Anderson in their patents was the use of distributed resistors to dampen resonance.  When I first looked at Pearson's patent, I wondered how was I going to add the  distributed resistors to the coil.   I could see that was going to be difficult and time consuming to make.    You can see from the frequency response attached that my latest current sensor prototype has none of the really bad resonance seen in my earliest enclosure prototypes.  The resonance is completely eliminated without the use of a wired network of pseudo distributed resistors.   Every winding contributes to the damping without a physical connection.  The frequency roll-off is well behaved.    The Anderson patent provided the clue that steered me down this path.
Dazz

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Offline tautech

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Re: DIY RF EMC Current Probe Set Design
« Reply #44 on: July 03, 2022, 12:15:49 am »
You can read about that saga here: https://www.eevblog.com/forum/repair/wavetek-2025a-0-2-2-200mghz-rf-sig-gen-repair/
I have finished that so now I am working on this RF current probe.    My brain is improving, but it isn't like it used to be.  If I look like I am writing nonsense, that's my excuse.  If I can make a RF current probe with a reasonably flat and wide bandwidth, that would be a bonus.
It's amazing what you can accomplish with a clear bench !
For a good while that Wavetek was looking to be your nemesis and a damn fine thing it's behind you so that fog can clear and allow you to get on with this stuff that obviously has been tucked away for a bit.

Carry on Daz and still can't believe it's a year since I dropped in while down your ways.
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Offline nctnico

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Re: DIY RF EMC Current Probe Set Design
« Reply #45 on: July 03, 2022, 01:06:45 am »
@dazz1:
The problem with the test setup is that it is under the assumption that the current probe isn't affecting the current going through the terminator. For these kind of tests you better use a 2 channel network analyser which can show the ratio between the two inputs. Or make the measurements in two steps if your spectrum analyser allows to use math to show the ratio between two traces.

The bumps you are seeing could be caused by the discontinuity that is inherent to your test setup. Try to measure the signal through your test setup first. What is current is flowing through the terminator? A good test is to replace the terminator with a 20dB attenuator and measure what comes out of the attenuator.
« Last Edit: July 03, 2022, 01:18:23 am by nctnico »
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Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #46 on: July 03, 2022, 03:29:25 am »
@dazz1:
The problem with the test setup is that it is under the assumption that the current probe isn't affecting the current going through the terminator. For these kind of tests you better use a 2 channel network analyser which can show the ratio between the two inputs. Or make the measurements in two steps if your spectrum analyser allows to use math to show the ratio between two traces.
I don't disagree with what you are saying but I don't have a network analyser either.

Quote
The bumps you are seeing could be caused by the discontinuity that is inherent to your test setup. Try to measure the signal through your test setup first. What is current is flowing through the terminator? A good test is to replace the terminator with a 20dB attenuator and measure what comes out of the attenuator.
If you look at the earlier results with the open coil, there is no evidence of significant errors from the fixture, or the presence of the coil in the test fixture.  By significant, I mean enough error to lead to a false conclusion.  Not the same as zero error.    The test fixture and setup is a combination of using what I have and using something "good enough" to guide me down the right path.

I do have a RF bridge for the spec-an.   As I write this, I am planning to run some tests with that.  This includes the SWR of the fixture and of the coil.



Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #47 on: July 03, 2022, 03:31:44 am »
On the larger toroidal high freq power transformers, the LV side might consist of a number of windings ,
 all connected in parallel to two concentric rings  of 4 inch busbar so the leadouts are as short as possible. That improves the balance around the core, and reduces leakage and stray inductance.
I never thought of doing that on a measuring C/T, but it might have similar improvements.

I haven't tried cascading CTs but as I recall both Pearson and Anderson mention it in their patents.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Offline dazz1Topic starter

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Re: DIY RF EMC Current Probe Set Design
« Reply #48 on: July 03, 2022, 03:43:50 am »
As for a project, this certainly sounds like a fun one (and that I've toyed with having a go at myself, though given I have some Pearsons I think I'll try and DIY a high bandwidth rogowski coil instead)


I hadn't heard of a Rogowski coil before now.  An interesting concept. 
The power industry is very slowly moving away from copper CTs to fibre optic versions. 
Copper CTs have multiple issues.  If the burden resistor goes open circuit, the CT output reaches lethal voltages (even on small CTs).  If there is an arc-over or some other connection between the HV cable and the CT windings, there are a lot of flashes and bangs.  Fibre optic CTs don't share those sorts of problems, but it will be decades before they replace copper CTs.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

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Re: DIY RF EMC Current Probe Set Design
« Reply #49 on: July 03, 2022, 04:21:06 am »
Hi
I fitted an improved version of the external shield with a better (not good) connection to the coax outer shield.
The results are attached.

The improved outer shield did make an observable difference, but only above 83MHz.
Below 83MHz, the frequency response is stable and within 1dB of being flat.
Above 83MHz up to the end of the bandwidth at about 190MHz, the frequency response varies a lot depending on the flex in the termination between the coax and the coil.   This includes connection of the coax shield to the outer coil shield.

The connection between the coil, coax and shield was always bad, but now the error is too great to ignore.   The RF on the coil will be leaking out the hole in the shield.  I now need a proper connector with a proper connection with the outer shield.  When I was getting resonance 40dB down, it didn't matter that I had a few dB errors from things like poor connections.  Now that I am chasing sub 3dB improvements, it matters.

I am rapidly approaching the point where errors from 3D printed plastic and kitchen foil will be too great to ignore.  I think I can get away with one more plastic and foil iteration before I need to switch to machined metal.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 


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