DIY RF EMC Current Probe Set Design - Page 1
Forum section: electronics / projects

author: dazz1
author_rank: Frequent Contributor
timestamp: June 22, 2022, 09:33:32 am
content: HiI have read posts on EEVblog Forum, and papers elsewhere that describe how to DIY build a RF current probe (like these professional oneshttps://www.fischercc.com/type/current-monitor-probes/) for EMC work. Most of these posts and articles are quite old and describe one size.I would like to build a set of say 3 different sizes specifically form EMC work. I'd like to use the latest materials and techniques to make professional grade probes at DIY prices.The reason I want to build a set is because I don't think one size will fit all situations.Small sizes can be expected to have higher frequency range but lower current limit. A small size could be easier to use.Conversely, larger probes will have a lower frequency range but higher current limit. The increased physical hole will be useful.Is there anyone interested in participating in the design phase?

author: jonpaul
author_rank: Super Contributor
timestamp: June 23, 2022, 09:12:27 am
content: See Tektronix Circuits Concept book Oscilloscope Probe Circuits for current probe theory and design.You can buy a fine Tektronix P6022, perhaps $50-150.larger P6021 is similarWe used Pearson 411 toroids for widebandAs you can wire in current sense jumpers, probe sizes are not a problem.Jon

author: dazz1
author_rank: Frequent Contributor
timestamp: June 23, 2022, 09:39:16 am
content: HiI looked up the P6022 an new price is about $USD3800https://nz.element14.com/tektronix/p6022/probe-current-6a/dp/7986882. e-bay prices are around $USD1000.The Pearson 411 coil is about $USD900 including import tax and shipping.https://www.bcgroupstore.com/Biomedical-Pearson_Pearson_411.aspxWaaaay too many $$$ for me.

author: jonpaul
author_rank: Super Contributor
timestamp: June 23, 2022, 10:34:42 am
content: TEK probes: Epay ...used prices but be patient. Probes must have termination box.Best laces at ham Radio flea markets.DIY is hard for clamp on, easier for toroid.Jon

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 10:27:24 am
content: daz, why would you want a current probe for EMC testing as opposed to a near field probe ?Would you not be using your SSA for this work ?

author: 2N3055
author_rank: Super Contributor
timestamp: June 24, 2022, 01:41:55 pm
content: Quote from: tautech on June 24, 2022, 10:27:24 amCurrent probes are used for conducted interference.. They are basically very wideband current transformers..

author: artag
author_rank: Super Contributor
timestamp: June 24, 2022, 01:56:00 pm
content: They're not that wide. P6022 is good to 120MHz, 6021 to 60MHz.Conducted interference frequency limits are lower than radiated, but I'd regard that as a bit low for EMC work.

author: jonpaul
author_rank: Super Contributor
timestamp: June 24, 2022, 05:44:01 pm
content: Bonjour,the conduced EMI range in EU and UK, US is 100k..150 k >>>30 MHzSee EN 55032 for Class A and Class B conducted emission limitshttps://e2e.ti.com/blogs_/b/powerhouse/posts/a-review-of-emi-standards-part-1-conducted-emissionshttps://metlabs.com/emc/en55032-replacing-en55022-multimedia-equiptment-march-5-2017/in 1970s...1990s I used P6021, P6022 and TEK probe amplifier 134, also Pearson 411 toroidal CT in conducted EMI lab work.An appropriate LISN and calibrated spectrum analyzer are essentials for conducted work.The conducted EMI is reduced by design: soft switching, transformer shields, CM, DM filters.After testing, one could then mod PCB layout, add cable and PCB shields and ferrite beads.Above 30 MHz EMI is radiated and NOT not conducted. Radiated is NOT read with current probes! Use a sniffer for E or H field.Bon ChanceJon

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 08:54:56 pm
content: Quote from: jonpaul on June 24, 2022, 05:44:01 pmYes for conducted EMC P6022 could be an excellent choice and I'm a fan of these old designs....6022 and 6021 both with terminations in my selection of equipment. I also have a Type 134 and 110&230VAC supplies for it.Like you said, with patience you can find these on eBay for 2-300 if you wait.The OP has IIRC a SSA3021X that does have a EMI mode but not with the preloaded CISPR ranges like the later SSA3000X Plus models however you can configure the OP's SA for the appropriate levels for radiated EMI.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 25, 2022, 03:57:31 am
content: HiI want to build up my tool set for EMC work. I have probes and I have a couple of log period antennae by WA5VJB that cover the rang 400MHz to 6.5GHz.My Siglent SSA has all available options enabled, including EMC and 3.2GHz freq range.I built a directional coupler.I have already purchased Fair-rite type 61 toroids (plus a larger unknownium toroid) to experiment with.I have recently repaired an antique Wavetek 2520 RF sig-gen that gives me a quantified signal source.I have a bi-conic antenna on the drawing board.What I don't yet have is any Litz wire, but I would save that for the final versions. For prototypes, I plan to use multiple strands of wirewrap to approximate Litz wire.With what I have, I am aiming to produce at least two current sensors. One with a bandwidth up to 1MHz-5MHz, and the other up to 30MHz (if that is even achievable).The specs for commercial units indicate that you can get sensors with a good lower frequency response, and sensors with a good higher frequency response, but not in the same coil.One issue that I haven't seen in any documents I have found, is the effect of the coil on the circuit. You can't measure something without having some sort of effect on the something. I would anticipate running a cable through a toroid will tend to attenuate any signal on that cable. This would apply even if a LISM is used. The risk is getting false confidence that the conducted emissions are less than they really are.If the toroids work well, I might consider making clamp-on versions to give more flexibility.

author: tautech
author_rank: Super Contributor
timestamp: June 25, 2022, 04:31:08 am
content: Quote from: dazz1 on June 25, 2022, 03:57:31 amBetter tools do list Insertion Impedance.See P4 of the attached Tek P6022 current probe manual.FYI the P6021 manual is also attached.

author: jonpaul
author_rank: Super Contributor
timestamp: June 25, 2022, 06:23:42 am
content: Bonjour Dazz1: fine work, a few points....1. Wideband transformers and CT toroids are very difficult to design and build, Pearson 411 current transformers are optimized for :-BROKEvery low freq to 50 MHz and cost $500-800.2/ CT TEK probes LF end can be extended with the 134 CT amp.3/ DC hall effect probes with a special plugin or amp can achieve DC-50 MHz.4/ See other posts on this topic for more info on EMC probes.5/ Litz wire is not needed for the CT.Jon

author: dazz1
author_rank: Frequent Contributor
timestamp: June 25, 2022, 09:25:09 pm
content: Quote from: jonpaul on June 25, 2022, 06:23:42 amI have no intention of trying to make a Tektronics type probe.Quote from: jonpaul on June 25, 2022, 06:23:42 amI have already built a rf-preamp that is powered from the USB port on my spectrum analyser, or any other 5V supply.Quote from: jonpaul on June 25, 2022, 06:23:42 amI agree it isn't needed, but I can make some up with my existing stock of silver plated wire wrap.

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 25, 2022, 11:55:09 pm
content: Hi Dazz,Some tips for a 1 Turn solid toroidal C/T design.Select a turns ratio and burden resistor for easy conversion eg 1V/A, 2mV/mA etc.The burden resistor should be of suitable precision, non inductive, and has to be rated for low temp rise to help precision.For a Ferrite core, initially aim to run B_peak at 0.1 T peak at lowest sq wave, and down to about 0.02T at highest frequency.You may be able to do a better range with testing.As the voltage is measured into a diff amp across burden resistor, the winding resistance does not affect accuracy.However it does effect the B_peak and core temp rise, so you need to calc or measure winding resistance over the frequency range.The only times I used litz wire was in the C/T's measuring up in 5,000 ~10,000 Amp range which were water cooled and winding and core loss had to be minimized.Your cores will hopefully have very low temp rise. As ferrite cores are usually optimised for around 100C, you need to get from data sheet, the core characteristics at ambient temperatures which are usually not as good as when hot.Good Luck ! Post some details as I for one, and sure jonpaul too, will be following your development.Edit: And should mention the low level IMD and distortion at very low flux density. I never encountered it in power electronics, but did in some hobby 3~30 MHz antenna tuner inductors.Search: Peterson coefficient, Legg coefficient, and Rayleigh loop. Distortion is a reason not to run down at very low flux density.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 26, 2022, 09:02:20 am
content: HiI made some progress today. I decided I needed to make a test fixture to be able to get consistent (not the same as accurate) results. I have not seen a commercially made test fixture, so I designed my own.I used a U bent piece of aluminium sheet to provide an earth return and to physically hold things in place. The coax centre wires are connected with a piece of bronze brazing wire, soldered to the BNC pins, that is cut close to one end. To fit a coil, I remove the BNC connector with the longer brazing wire. Fit the core in-place and refit the BNC connector. The terminal block then connects the two ends of the brazing wire.I have one side of the fixture fitted with a 50ohm load. The other side is connected to the Tracking Generator output of my spectrum analyser.The coil ends are connected to coax that goes to the RF input of my spec-an. The centre of the core is stuffed with a piece of foam to keep the brazing wire centred through the core. The aim being to get consistent repeatable results.I have no idea if this is the "right" test setup for RF current sensors, but I have used what I have available.I made a coil with the unknown core and randomly chose to do 10turns with some crappy scrap hookup wire. I had really low expectations but I was wrong.If you take a look at the spectrum analyser screen shots, this coil has a flat response out to 21MHz. There is a resonance spike but this is sensitive to the positioning of the coil ends and the coax connections. With not much tweaking, this spike disappears.This coil has a useful response out to about 150MHz, far higher than I expected.On the low frequency end, the frequency response is also much better than expected.To test whether the results are actually real, I removed the 50ohm load. As expected, the frequency response went nuts and was full of spikes and resonance as the rf from the tracking generator reflected back. It means that the coil is responding to what is passing through the centre. So I am reasonably comfortable that what I am seeing is real.I will have to use a DC current source to figure out the saturation point. The equipment I plan to test draws less than 1A @ 220VAC so I don't expect any issues. I need to add some electric field shielding and do some more testing.These initial results are good enough to justify further time and effort to do more R&D.

author: jonpaul
author_rank: Super Contributor
timestamp: June 26, 2022, 04:32:49 pm
content: Daaz1, BRAVO for your fine efforts and progress.We used wideband CTs for HID and arc lamp ignitors and SMPS designs, decades ago 1980s ..1990sWe had contact to Mr. Pearson, a pioneer in the field.A few moments for a trip down memory lane...1/ Cores are high perm, besides 10k..15k ferrite, I believe thin tapewound cores and amorphous tapewound cores were used.2/ Application with high DC bias need a gapped core or powdered iron.3/ Commonly the problem is sidestepped with a DC bucking technique on a second primary.Thus the core sees only the AC component, the DC component is cancelled by the buck winding.A TEK high current clamp on extender CT-4 comes with an accessory DC buck coil.https://w140.com/tekwiki/wiki/CT-44/ Winding is sectionalized and a distributed termination is used. Each section has a term resistor.5/ Coil term is optimized for best BW or HF transient response, independent of required system Zo eg 50 Ohm.6/ The coil term is then adapted to the BNC or connector Zo with a matching network.7/ Calibration constant is adjusted in the term and matching network eg 100 mV = 1 A8/ Number of turns increase with required LF corner.9/ You can test at higher excitation with a multiturn primary eg 10 T multiplies the primary A*T by 10.10/ The finished toroid is shielded against EMI and capacitive noise by a metal shield which has breaks to avoid shorted turns.11/ occasionally can find a used Pearson 410, 411 at Ham fleas, surplus, or epay. Patience and persistence.We have several, paid $25.....$75 over the decades.12/ I think the design details are in old fine papers, app notes and patents from Pearson and others.Just the ramblings of an old retired EE.Jon

author: dazz1
author_rank: Frequent Contributor
timestamp: June 27, 2022, 09:41:17 am
content: HiI think I still have a lot of work to do.e-field shielding and resonanceI suspect putting this coil into a metal shield will add capacitance between the windings and ground. The capacitance plus the stray inductance from the coil windings is a recipe for a tuned cavity resonator. I expect that will really mess up the frequency response. The problem with typical shielding is that it doesn't absorb the stray energy. It is like yelling in an empty concrete room. The sound reverberates and echos. If the room is lined with absorbent material, the sound dissipates.You can buy specialist EMC RF absorbent material that sticks onto the inside of shielding to absorb stray RF. Naturally this is difficult and $$$$ to source. I glue a layer of very small ferrite beads onto the interior surface of shielding. Grade, size, shape etc is not important. I can get 100x for $1 from China if I am patient. I focus on corners and edges where currents are a max. This method has worked for me.MatchingI haven't even looked at matching the coil to the 50ohm source. With a 10:1 turns ratio, the 50ohm load on the through wire is going to be something less than 10ohms through the coil. I anticipate the coil is going to look like a mostly highly reactive load (it's an inductor), so matching will require reactance. Achieving wideband matching would be a challenge to say the least. This would be further complicated by the effects of the measured impedance reflected through the coil. At present the SA Tracking Generator and fixture load are 50ohms but if I replaced the 50ohm load with a capacitor, the coil impedance is going to look quite different. I don't have a network analyser to fully characterize the coil. I do have a RF bridge to do some limited measurement. This only gives me amplitude, but not phase information.I am going to try varying the turns to see what effect that will have on bandwidth, gain etc. At this stage, I won't be trying to match the coil to the spectrum analyser input.Litz WireThe frequency response shows a downward slope, most likely due to loss in the ferrite or the coil. I suspect the increased resistance due to skin effect in the cheap hook-up wire will be observable by comparison with the Litz wire.I am going to try some DIY Litz wire, using wire wrap, to see if that makes any difference.PearsonThe range of used electronics test equipment available here is pitiful and expensive. There is absolutely no danger of finding a cheap Pearson in my country. I look with green eyes of envy at the range of cheap test equipment on USA e-bay but the shipping costs rule out this option. Also, I hate dealing with e-bay. e-bay has no presence in New Zealand because it was wiped out by the local competitionwww.trademe.co.nzGoing towww.ebay.co.nzredirects to ebay.com and no local listings.There is a really nice HP Network Analyser for sale herehttps://www.trademe.co.nz/a/marketplace/electronics-photography/other-electronics/other/listing/3646247523?archive=1&bof=WJH0nzdgbut the asking price is eye watering.

author: jonpaul
author_rank: Super Contributor
timestamp: June 27, 2022, 12:19:13 pm
content: Dazz1:I Understand now your issues with epay, etc.Perhaps contact an NZ or AU Amateur Radio club, or ARRL branch for ham fleas....The Shielding of the Pearson's is simply a metal clamshell split in both axies, for E field electrostatic shield.Litz is very specific, many very fine strands with dia ~ skin depth, interwoven not just bunched or twisted.https://en.wikipedia.org/wiki/Litz_wireYou may find some on old RF chokes....just unwind.As the currents are so small in these CT toroid's, i cant imagine use of Litz makes any difference ....I think the Pearsons used regular solid magnet wore, not Litz.Bon Chance,JonPS: Had no idea you are in NZ, the EEVBlog user flag icons are soo small they are often unrecognizable....

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 27, 2022, 02:39:02 pm
content: That is a good looking test jigger you made.If using a rounded toroid, I wind enameled wire (thin insulation) directly on the ferrite, spaced evenly around the core.The current flows biased to the side of the diameter closest to the core. ( In fact rectangular turn section would be more ideal approach to a "current sheet")If using thick insulation, or Litz, I have not measured, but an airgap between winding and core implies leakage flux.The winding is indeed an inductive device, but the way a current transformer works, is that the secondary Amp.Turns is "exactly" the same as the primary.That assumes mutual inductance factor k = 1. The secondary current is not dependent on frequency, up to the limits of the low frequency model.So you need short leadouts to non inductive burden - which I see you will have, with 50 Ohm RF input.P6021The circuit diagram for the P6021 passive matching box seems to be published.(I know because I have copied into my notebook years ago. We used many these probes exclusively for FET, IGBT and SCR gate current measurements. The probes retain accuracy even in the presence of DC, as in scr gate pulses, the scope trace balances to the average DC )P6021 burden is a 68 Ohm shunted with two low pass filters plus some other components. The turns are 125 ( written on the side), the net burden is close to 62.5 Ohm. There are some frequency comp adjustments on the probe head.So for 200 mA measured, , the winding current is 200/125 = 1.6 mAThe burden voltage going to high impedance scope input is 1.6 * 62.5 = 100 mVThat is how the ratio is obtained ( 2mA / mV ) in the case of the P6021.The core area (visible) is about 2.5 by 4.5 mm, for nett core area approx 10e-6 m^2For the current measured of say, 2000 mA rms , 120 Hz is the lowest rating, worst case for B_peak. Highest rating is 60 MHzE_burden = 2000 *(1/2) = 1000 mVE = 4 * F * N * B_peak * A1.00 = 4 * 120 * 125 * B_peak * 10e-6B_peak = 1.6 TMost high frequency pulse measurements will be fractions of that value.For example 2 Amp 10 usec pulses @ 50 kHz, B peak = 4 mT approx

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 27, 2022, 06:05:53 pm
content: Hi Daz,I made a c/t with 30 turns of 24AWG on a 30mm ferrite toroid.The 50 Ohm burden was close to toroid, then cable to HP141T SAJust on wood block, it has about -70dBm of unwanted capacitive coupling and also inductance off the bnc connectors to the brass tacks.HP141T only goes to 110 MHzMeasuring from 1MHz to 99MHz with -10dBm ( 1.414 mA rms) from sig gen to the 1Turn input.The calculated output across 50 Ohm burden is -39dBmmeasured:The output is -40 dBm from 1MHz to 30 MHz, then a hump up to about -35dBm from 40 to 60MHz then flat at -42dBm again to 99MHz.I could make a test jig like yours, and I think a good hi Z diff amp is needed close to the resistor to reduce the common mode coupling,and lead inductance.https://app.box.com/s/s333n05r6pktw4uc6yb6jjuxom1e69xv

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 27, 2022, 10:17:37 pm
content: A lot of the fancy tricks with complicated compensation networks, distributed termination and nanocrystalline cores are utilized to get as wide bandwidth as possible, for example the discussed Tektronix P6021 has a bandwidth of half a million to one, and the Pearson 2878 has a bandwidth of over two million to one. I have several of either, but I find for EMI measurements something simpler will work just as well. A plain resistively terminated current transformer wound on a ferrite core will easily give a bandwidth of ten thousand to one with some care in the design, plus minus 3 dB that is. It doesn't beat a five hundred dollar Pearson that comes with a calibration report, but for EMI precompliance testing it's often more than good enough to be useful. There are even commercial EMI measurement CTs made with ferrite cores and lumped termination, like the Tekbox TBCP1-200 for example.To get a predictable sensitivity, you want to terminate the sense winding into a (non-inductive) resistor. The resistance along with the magnetizing inductance of the secondary gives you your low cutoff frequency, forming a single pole where the magnetizing reactance equals the terminating resistance. The resistance together with the number of turns gives you the sensitivity of the current transformer. More turns gives more parasitic capacitance however, compromising the HF cutoff of the coil. High end wideband transformers from the likes of Pearson, Stangenes and Bergoz tend to use very high permeability cores made from strip-wound nanocrystalline material, to increase the magnetizing impedance as much as possible without compromising HF performance by having a large number of turns.As an example, a Fair-rite 5975002701 core has an Al value of 5500 nH/n^2. If this is wound with 10 turns and terminated into 10 ohms, you will get a sensitivity of 1 V/A and a magnetizing inductance of 550 uH, giving a lower cutoff frequency of 3 kHz. The upper cutoff frequency will have to be determined experimentally, but a few tens of megahertz is not unrealistic. To match this to 50 ohms, you could put 40 ohms in series with the output. If you have a way to generate a test signal and monitor the current transformer output, I would highly encourage doing some experiments.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 28, 2022, 09:35:51 am
content: HiToday I wound the unknown type core ( I have two of them) with 9turns of wire wrap. Each turn was made of up 8x wire wrap as DIY Litz wire. The primary aim of the experiment was to measure the benefits (if any) of using Litz wire.The wire wrap was wound as a single layer with even spacing. Spacing the wire wrap minimises proximity effects. Winding the wire wrap close to the core minimises stray leakage.The results are attached.With rubbish hook up wire, the frequency response degraded with frequency. The hypothesis was that this was due to ferrite or copper losses.The use of DIY Litz wire shows no degradation of frequency response attributable to losses.The measured 3dB bandwidth is from 111kHz to 170MHz. There are no nasty spikes in the response. For most of the bandwidth, the response is within 1dB. Beyond the 3dB upper frequency, the coil is still usable.It seems reasonable to conclude that:using Litz wire provides measurable improvements in frequency response.hand winding Litz wire is surprisingly difficult. Next time I think I will make a simple jig to help.the performance of this coil as a current sensor is well beyond expectations.I will try more windings to improve the lower frequency response.I will also start looking at making a metal e-field shield enclosure. I will be specifically looking at the effects on frequency response (cavity resonance).If I see problems with cavity resonance, development will effectively be stopped for a few months while I order, then wait for a bag of ferrite beads to be delivered from China.They are available locally but are sold individually. Buying 500 or so would cost more than the national debt.

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 28, 2022, 11:37:07 am
content: Looks good.Litz wire in the past referred to strands woven so the each strand passes a period on each side and inner of the core ( my bad description.)From your photo , that winding used to be called "bifilar.. trifiar...multi-filar" where the strands are each in parallel flat on the bobbin or core. That is a good way to approximate a current sheet which reduces leakage ( ie keeping flux in core, not in air), and also keep strand currents balaced.I suppose your next test could be to run at various primary levels from low to high dBm to check the "gain" and phase over the range you need.I don't think I can do that accurately here, I can only go to +10dbm and 'scope is limited to 50 mV/cm @ 10:1 probe.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 29, 2022, 08:14:20 am
content: HiI don't have access to the exotic materials that might allow me to make a single really wide band current probe that I don't actually need. Two probes that cover the required bandwidth would be sufficient.I am also planning to make a lower frequency version. I can fit 160 turns of wire wrap on a single layer. If each winding is made of four strands of wire wrap, then I can make a current probe with 40 turns.Most commercial EMC standards have measurements made over the frequency range of 150 kHz to 30 MHz. I have already made a current sensor that measures over that frequency range so I could stop now.A lower frequency coil could then be used with my oscilloscope. Rather than using a burden resistor, I would use a 3dB attenuator to act as a burden resistor and matching to the coax.Before I test any more coils, I want to test a prototype enclosure. I have designed an enclosure that is like a donut with the hole offset. The offset will provide the space for the SMA panel connector. This shape will be the easiest for me to make because it only requires a lathe. I have a milling machine but milling out an enclosure would take a lot more time than a lathe. The plan is to print a 3D plastic prototype , and line it with aluminium foil for testing. If that works without wrecking the frequency response, then I will machine an enclosure from aluminium bar stock .

author: dazz1
author_rank: Frequent Contributor
timestamp: June 30, 2022, 03:25:36 am
content: HiToday I tested the 3D printed, aluminium foil lined enclosure. As expected, it definitely affected the frequency response.There is clear evidence that the enclosure acts as a tuned resonator.The Spec An shows two conditions:1. The coil is electrically isolated from the aluminium shield. This is not a "normal" connection. The effects on the frequency response are measurable, but too not bad.2. One side of the coil is grounded to the aluminium shield. In this condition, the enclosure clearly acts as a cavity resonator pushing the frequency response down by 40dB. This definitely affects the upper 3dB bandwidth of the coil.In both cases, the frequency response gains bumps and dips due to the presence of the enclosure.One of the plots below has two traces, one with the coil grounded to the enclosure, and the other with the coil floating.One of the things I have noticed is that the length of the leads from the coil have a significant effect on frequency responses. In some plots, you can see a small spike at about 23MHz. This is due to the wire from the end of the coax to the beginning of the coil winding tight against the core. The design of the enclosure specifically aims to minimise the unwound wire ends of the coil wire, but not on the prototype 3D printed enclosure. I don't have a socket fitted to the enclosure as I would with a final version.What to doThere are a few things I can do to improve the performance, by altering the cavity resonance:1. Not use an enclosure. Increases the risk of e-field noise polluting conducted current emissions. Not a good option.2. Increase the cavity resonance frequency by reducing the enclosed volume of the enclosed cavity.3. Dampen the resonance by filling the internal volume with very small ferrite beads. This may be detrimental to current probe sensitivity.4. Try adding radial cuts in the aluminium shielding to impede the surface currents generated by resonance. The effects of this are not yet known.It is obvious that the enclosure has a significant effect on the current probe performance. I am going to order a bag of ferrite beads from China. The lead time is about 2 to 5 months.In the mean time, I will try changing the shape and size of the enclosure, and anything else I think of.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 30, 2022, 12:05:21 pm
content: 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

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2022, 06:11:57 am
content: Rebonjour just returned to this topic...a few notes1/ 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 dimensions3/ termination should be at the winding and distributed4/ 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 thoughtsJon

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2022, 09:31:41 am
content: Quote from: jonpaul on July 01, 2022, 06:11:57 amThe 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?QuoteAgreed. 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.QuoteIf 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.QuoteThe 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.QuoteExperimental 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.QuoteIt 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 bandordampen 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.QuoteThere 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2022, 10:02:18 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2022, 10:36:55 am
content: Quote from: doktor pyta on June 30, 2022, 12:05:21 pmHiYes 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/869I 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2022, 10:47:48 am
content: HiLooking at Pearson's website, I found a test fixture.https://www.pearsonelectronics.com/applications/emi-testing-applicationsIt 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.

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2022, 11:22:50 am
content: 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/enSeveral other patents were granted over the years.All are long expiredThe "exotic" cores are easily available in USA, I will check a few sources,Bon Journee,Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 01, 2022, 12:56:57 pm
content: I remember the use of a high voltage rated square Pearson along with a scope, I think it was a small Philips with battery powerand 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 againI 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.

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2022, 01:13:02 pm
content: 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 WeekendJon

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2022, 08:59:20 pm
content: Quote from: dazz1 on July 01, 2022, 10:36:55 amWould conductive foam be a cost effective solution ?https://www.farnell.com/datasheets/1485187.pdf

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2022, 10:50:53 pm
content: Quote from: tautech on July 01, 2022, 08:59:20 pmI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2022, 12:19:30 am
content: HiHiI 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 experimentationI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2022, 03:22:41 am
content: Quote from: jonpaul on July 01, 2022, 11:22:50 amI'd be interested in what you find.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2022, 10:50:09 am
content: HiHaving read Pearson's patent, I did a bit of Googling and found Anderson's later patent 1972 here:https://patents.google.com/patent/US3701003A/enThe 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.

author: Hydron
author_rank: Super Contributor
timestamp: July 02, 2022, 12:20:28 pm
content: 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!

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 02, 2022, 06:31:44 pm
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2022, 11:09:42 pm
content: Quote from: Hydron on July 02, 2022, 12:20:28 pmI 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/] 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.

author: Hydron
author_rank: Super Contributor
timestamp: July 02, 2022, 11:26:35 pm
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2022, 11:30:27 pm
content: HiHere 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.

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2022, 12:15:49 am
content: Quote from: dazz1 on July 02, 2022, 11:09:42 pmIt'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.

author: nctnico
author_rank: Super Contributor
timestamp: July 03, 2022, 01:06:45 am
content: @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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 03, 2022, 03:29:25 am
content: Quote from: nctnico on July 03, 2022, 01:06:45 amI don't disagree with what you are saying but I don't have a network analyser either.QuoteIf 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 03, 2022, 03:31:44 am
content: Quote from: mag_therm on July 02, 2022, 06:31:44 pmI haven't tried cascading CTs but as I recall both Pearson and Anderson mention it in their patents.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 03, 2022, 03:43:50 am
content: Quote from: Hydron on July 02, 2022, 11:26:35 pmI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 03, 2022, 04:21:06 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 03, 2022, 04:31:04 am
content: Quote from: tautech on July 03, 2022, 12:15:49 amThe really great thing about antique test gear is that it is fixable with readily available parts. It was the first piece of equipment I have fixed with so many faults, I lost count.Call in when you are next down this way.

author: jonpaul
author_rank: Super Contributor
timestamp: July 04, 2022, 04:26:44 am
content: Hello again Dazz1, bravo for the progress.A few thoughts1/ use copper foil not aluminium, better shield, easily available and solderable3M and others make shielding tapes in various widths and thickness.2/ change winding method to reduce self capacitance to core and turn to turn.3/ The distributed term can be tested easily, just wind two or three sections, not 10..20 as Pearson.4/ Lowest abberation widest bandwidth with coil's natural Zo, not 50 Ohms. Try Optimization of Zo say 25..200 Ohms first.Afterwards use a resistive pad to match the result to desired 50 Ohm BNC/ cable.Bon courageJon

author: dazz1
author_rank: Frequent Contributor
timestamp: July 04, 2022, 09:10:04 am
content: Quote from: jonpaul on July 04, 2022, 04:26:44 amAll valid points but Cu foil is not available here at any sort of reasonable price.QuoteWhat you are looking at is a 9 turn coil. Early testing indicates that self capacitance is not a problem.Close coupling to the inner shield has proven to be essential to damp oscillations and improve bandwidth.QuoteThe mechanics of connecting resistors to a coil and to a ground would be time consuming.QuoteI haven't done any impedance/matching testing yet. It's on my list.Quoteyes.Quote

author: Hydron
author_rank: Super Contributor
timestamp: July 04, 2022, 10:38:16 am
content: If you need Cu foil tape in a hurry, try looking for "snail tape" in a gardening store. Will not have conductive adhesive, but you can still tack solder it to adjacent pieces. Also, it looks like trademe does have local sellers offering it affordably (including with conductive adhesive)?

author: dazz1
author_rank: Frequent Contributor
timestamp: July 04, 2022, 10:43:22 pm
content: Quote from: Hydron on July 04, 2022, 10:38:16 amHiThanks for the suggestions. I hadn't heard of "snail tape". I hate gardening.I suspect that gardening store and Trademe copper tape is most likely copper plated aluminium based on the very low price compare to 3M real Cu tape.There are quite a few sellers on Trademe that buy stuff from Aliexpress and then sell it locally. I buy a lot of stuff directly from Aliexpress.A lot of products with copper wire sold on Aliexpress are made with copper clad aluminium. It looks exactly like copper wire, but isn't.The final machined metal version of this current sensor is going to be made from aluminium. Even if I could buy a big enough piece of copper, the price would be many $So kitchen foil is a reasonable and practical (ie. low cost) proxy for solid aluminium.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 05, 2022, 12:15:42 am
content: HiThe next prototype No.8 now includes a BNC connector. I will not be using a BNC connector in the final metal version because of its size. It would require a larger diameter piece of bar to fit within an eccentric shape than an SMA connector.In this version, I also have shells of different diameter to investigate the resonance damping effects of the inner shield. If the inner shield diameter is too great, there is a lot of resonance. If the diameter is too small, maybe I am getting too much damping. That will degrade sensor sensitivity.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 05, 2022, 08:11:16 am
content: Hi!You're overcomplicating things...Incidentally, I made some EMC probes myself just lately.I do EMC-consulting as a business, and having really cheap probes allows me to have plenty of probes to leave several test-setups unchanged.If you read the standards, you'll want <1Ohms of Insertion Impedance. So 7 Turns are slightly too little; 8 is fine.Just use a simple coax. This gives you a nice electrostatic shield, is cheap and just works well enough for 150k-30MHzIn the attached photo, one uses a Epcos core, the other one sold by Würth.The 6 winding one was a test to see if the simple impedance transformation formula works out for RF (and this arrangement) as well.I plan to have this semi-open-source... ie. the 3d-printing files+plans for one make of core and a generator for arbitrary core dimensions (this is already finished in OpenScad) sold for cheap,Maybe I'll also get a cheap kit out with all parts included.I'm fully sure if there would be a "market" for this, but this may be doable for 100€ including international shipping... so I tend to give it a try in the near future.I still have to do some verification work.This will be mainly testing if the 3d prints hold up to common miss-use in a lab (ie. does the hole for the BNC survive or do I need to increase the wall thickness).Another thing to verify is the shielding effectiveness... I hope to get this done the next week.For the cal-jig, I opted to some PCB-"cage", as you can see in the other attachment (that's just a rendering for my documentation...).The cal-jig is fine up to about 100MHz and is useful to characterize random cores as long as they fit into the jigMy probe is fine up to 150MHz. So that's plenty good for conducted EMC testing.For your bi-conical antenna... I'd replicate the one in MIL STD-461.. there are plans in the standard and you get some reference antenna factors.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 05, 2022, 09:37:10 am
content: Quote from: wilhe_jo on July 05, 2022, 08:11:16 amYes I am. Over-engineering is funQuoteQuoteI did 9 turns but I haven't tested impedance yet.OK which standards? I haven't seen any for the current sensors.I started with the hope of to get something that would work 100k to +30MHz. I never expected to get to where I have.QuoteCore model numbers?QuoteAnd did it work out?QuoteNice.QuoteDo you think that the two legged connector holder will be strong enough?I have found that even a really short length of exposed end of the winding, not wrapped close around the core, is a problem.QuotePlease share the results. If coax shielding works well, Pearson is out of business.Do you just ground the coax shield at one end to avoid a shorted turn?Do you see any resonance?QuoteI like that.My uncharacteristically simple fixture is reasonably good I think but I will record a plot.QuoteHave you done a frequency response plot?QuoteI have based the dimensions on the MIL-STD but I have changed the mechanical construction to simplify manufacture and to make it snap assembly. I will make it to pack flat. The top of the antenna arms are curved so the ends enter the top junction at 90 degrees. It will be a lot easier to bend the arms and drill the junction sockets at 90 degrees. The curved arms will be closer to the ideal shape for a bi-conical antenna, but I think the most significant effect will be people asking me why they are curved.The antenna will be cheap and easy to make, so I may make several variants, depending on how well they work.I am planning to make two identical antenna for a start, because I will then be able to calibrate them.I haven't yet figured out what I am going to do for a balun. There are a few options.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 05, 2022, 10:10:47 am
content: Actually, the probe is defined in MIL-STD-462 and CISPR 16-1-2.In the end, using this probe is an established test procedure in the EMC world.So you get considerations for the measurement uncertainties and even construction plans right from the standardsI was a little wrong... it was Kemet ESD-R-57D-1 and Würth 74270097.The Würth one is especially interesting because of its price.The lower you get with your number of turns, the higher the resonance frequency gets.So 8 turns is what you want to get well below 1 Ohms of insertion impedance.And yes, the simple impedance transformation formula is good enough even in this crude coupling situation.So far, my 3d print held up well.However, I think, I'll add maybe one more perimeter to get some more stiffness when printing with PETG... PLA is plenty sturdy already.With regard to measurement results... they are in spec, but I haven't finished the documentation (I will have a presentation on a EMC symposium where this will get some closer discussion).A resonance is there, but that's somewhere around 180-200MHz IIRC.Using a coax is plenty good... to get the best bandwidth, just connect the shield on both ends and cut it at half the length (ie, you have half of the length from both sides).If I'm not totally wrong with my estimates, there's a resonance reflecting 1/4 lambda for the shield... so yes, doing this in this cheap/economical way has some drawbacks.There are some more things in the CISPR standard to consider: Transfer Impedance, shielding effectiveness against external electric fields and induction from external magnetic fields.So far, I'm happy with the results and I hope to get the rest of the measurements done in the next weeks.As I said, I have kind of a deadline because I'll use some of this stuff in a presentation about DIY stuff in an EMC lab73

author: dazz1
author_rank: Frequent Contributor
timestamp: July 06, 2022, 06:06:26 am
content: HiToday I did some testing with an RF bridge on the Spec An.The aim was to test the fixture and the effects of a coil in the fixture.I started off testing the Suhner 50ohm BNC load. This also showed the performance of my DIY RF balanced bridge. The one I use is shown here:https://www.epanorama.net/blog/2017/10/06/rf-bridge-for-antenna-measurements/. They are available from multiple suppliers. If fitted mine into a cast aluminium box that is a really good fit. Doing this creates problems with internal resonance that mangles the performance. I fixed this by gluing lots of small ferrite beads onto the interior surfaces of the enclosure.The first image (1) below shows the test setup laid out on the bench.The RF bridge was connected to the test fixture and not to the coil for all tests.All tests were done from 0 to 500MHzThe Spec An was setup to display reflected power and VSWR. 100% reflection from the load end gives a trace at 0dBm. When the load absorbs nearly all of the power, the reflected signal is very low. Much less than )dBm.The first trace below (2) shows how well the Suhner BNC 50ohm load works. This was connected directly to the DUT port of the RF Bridge. This is at least 40 years old so it was made long before Aliexpress, Bangood and others were around.For comparison, the next trace (3) shows the response of the cheap SMA Aliexpress 50ohm load. This was connected directly to the DUT port of the RF Bridge. The response of the cheap Aliexpress load is arguably better than the Suhmer. The Suhmer had the disadvantage of needing a cheap BNC to SMA adapter to connect to the bridge. This could explain the results. The test data shows that both test loads work well.The next trace (4) shows the effect of adding the coil fixture between the bridge and the 50 ohm load. No coil was fitted to the fixture. In this configuration, all of the energy from the tracking generator should pass through the fixture and be fully absorbed by the 50ohm load. Ideally, no RF should be reflected back from the fixture. The first tests showed the 50ohm loads reflected almost no RF. For practical purposes, all of the reflection is from the test fixture. The reflection trace shows the RF test fixture has exceptionally ordinary performance. By the time it gets to 250MHz, almost all of the RF is reflected. Less than 1dB makes it through to the test load. So when I am looking at a coil response trace at 200MHz, only about 2.5dBm is the coil under test.The next trace (5) shows the effect of adding a coil to the fixture. The coax connected to the coil is terminated with a 50 ohm load. This looks at the effect of wrapping a coil around a wire carrying current. Any voltage induced in the coil is going to be absorbed by the 50ohm load connected to the coil via the coax.To test the effects of the coil and connected load, I removed the test load from the coax. Any voltage induced in the coil would be reflected back by the open circuit at the end the coax.Spurs can be seen (6) in the trace where the resonated back to the coil. The resonance in the coil and coax was induced back to the wire in the test fixture. This shows that what is connected to the coil does affect the current passing through the middle of the sensor coil, but not by much. If there was no resonance, it would be less obvious that there is any difference.So I really should try and make a better test fixture. By the time this one reaches 100MHz, the results are unreliable. It works well enough to tell the difference between two coils, but not the absolute values.What I would do is adapt a TEM cell design like this one for coil testing:TEM cells are not suitable for coil testing because the central conductor is a sheet of metal that is far too big to fit through a coil. The sheet metal top and bottom connected to the coax shield are shaped to match the cell to the coax and reduce reflection. Achieving good matching includes the shape of the central conductor. A TEM cell based coil tester does not need to be as large as the one shown in the link.Replacing the central sheet conductor of the cell with a metal wire or tube should retain a reasonable match to the coax and to the load. It won't be perfect, but it would be better than my fixture or the Pearson version.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 06, 2022, 08:44:43 am
content: Well, IMHO a current-probe-cal jig can never get reasonably "well matched" (that's the wording from the standard) to 50 ohms...That's just physics... just get some of these coaxial line characteristic impedance calculators and enter some numbers...You'll get useless dimensions for an air-dielectricum at 50ohms.Even the jigs shown in the standard will never ever get close to a match.However, you can certainly some signal source and measure the voltage/power into some defined load (ie. do something like an S21 measurement).During this, you can investigate the power/voltage at the output of your probe.Now you know the current through the center contact at each frequency, and you know the output of the probe.This gives you the transfer impedance.That's the suggested method in the standard, btw.I see similar values like you. Up to around 100MHz, you get no problems.My intention is not to go way beyond that. So I don't really careThe nice thing with this is, you can terminate the jig with 50ohms and do a S21 measurement between the output of the probe and the jig.This gives you everything (considering the jig does not influence the match too much).However, I have everything to do a "correct" calibration.So at one point, I'll take all my current probes and do the correct setup with some power meters and a sig-gen (+ maybe some amplifier - just for fun.73

author: dazz1
author_rank: Frequent Contributor
timestamp: July 06, 2022, 09:51:36 am
content: HiI made my test jig before I had seen the Pearson version and when I thought I'd be struggling to get to 30MHz. I recognise that the flaw with both versions is the return signal has to make 4 right angle turns. At every turn there will be reflection. Also, the two abrupt transitions from coax to sheet metal are also reflective. So the theory of doing a TEM cell shaped like jig is that the transitions and corners are softened.Using a coax impedance calculator, I came up with some achievable dimensions for a coil tester. Making the central conductor with a larger diameter has a significant effect on impedance.Approximating a coax with a TEM cell shaped object should provide a better match above 100MHz. TEM cells are good to a few GHz. Maybe I can get to a couple of hundred MHz.Do I think your/ the standards method of measuring S parameters will work? YesDo I have the test gear to measure S parms? YesDo Ineedto measure above 100MHz? NoDo I have the materials and tooling to knock up a TEM-shaped test rig in less time than measuring the S-parms? YesWill anyone care ? I doubt it!

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 06, 2022, 01:27:07 pm
content: Well, I came up with similar numbers...The problem is, I need around 70mm outer diameter and I have a 25mm inner bore.Additionally, adding the Probe will change impedance drastically....After having some beers about this problem, I just tabled (US meaning!) it.I guess it is very reasonable to do something similar to CDN calibration.I.e.: calibrate your "forward power" to get some reasonable output at the termination and measure the probes output in a second step.My hope is to have higher dynamic range using power detectors (or even the receiver) than with my VNA... this would give me better values for the lower frequency ranges.Even though you get some different impedance along the line, the current (ie. the integral of the current density along the conductor area) will be the same - regardless of that impedance jumps.Hence, we're goldenA open TEM cell is limited by its length... mostly.... BTDTThere are nice plans out there.A 300MHz open TEM cell is nice, but my 750mm GTEM is nicer73

author: dazz1
author_rank: Frequent Contributor
timestamp: July 07, 2022, 11:06:54 am
content: Quote from: wilhe_jo on July 06, 2022, 01:27:07 pmI am going to give it a try and see what results I get.QuoteAgreed, unless there is a C storing energy in the middle.It would be a lot easier if the current is reasonably flat over the working frequency.I don't have a VNA so I can only measure s-parms magnitude, not vector values.QuoteMy collection of test equipment is very old (except for the Siglent spec-an) and modest.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 04:06:13 am
content: HiI am made and tested my new coil current test jig. It looks similar to a TEM cell, but it isn't. It is intended to approximate a short length of expanded coax cable.The outer conductor is two pieces of sheet aluminium that provide a relatively smooth path from one end to the other.In order to achieve a reasonably matched 50ohm impedance, the centre conductor, consisting of a bar of machined aluminium, tapers at both ends.The bar diameter is constrained by the diameter of the coil that it passes through. As a result, the bar diameter is not sufficient to achieve 50ohm impedance.The bar is dimpled at both ends. Bronze brazing wire was machined to fit into the BNC terminal and soldered in place. This left a 3mm short spike of brazing wire sticking out the back of the connector. The bronze spikes fit into the 1mm deep dimples. The bar is held in place by spring compression of the sheet metal. To remove/insert the bar, I just squeeze the sheet metal so the connectors spread apart. The bar drops out. Very simple to make and use.In the next few posts, I will attach test results.If it doesn't work, it will make a great coffee table conversation starter.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 04:42:59 am
content: HiAttached is the datasheet for the Pearson calibration fixture. I can't find a listed price for this item, so I guess the price probably comes with a health warning.Consult your doctor first. You might get a heart attack if you see the price.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 08:46:09 am
content: HiThe first two attachments are of the new and old test jigs out to 1GHz.SWR is marked at 4 different frequencies.The new TEM cell like jig is >3.5db better return loss than the first U shaped jig.The SWR is smooth across the frequency range with no sign of any resonance.The Pearson jig claims a frequency range of 10 kHz to 400 MHz with VSWR < 3.I am measuring VSWR 2.3 @ 800MHz with the new jig. At 800MHz, the old jig I am measuring VSWR = 14.So the new TEM-like jig is significantly better than my 1st attempt and at least as good as the Pearson jig.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 08:51:28 am
content: This is a repeat of the test in the previous post but out to a frequency of 500MHz.Again the new test jig is significantly better than the old version.Both tests are of the bare test jigs alone. No coil is fitted, so these tests are looking at the performance of the test jigs only.My DIY RF balanced bridge is used to measure the VSWR. The test jigs are terminated with 50ohm loads. Previous tests show these perform well with good VSWR measurements.Comparison of the VSWR plots for the Pearson and the new TEM-like test jig indicate my jig is better.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 08, 2022, 08:57:55 am
content: Tekbox has similar cal-fixtures for IMHO a reasonable (read: reasonable if you buy this for a business) price.The VSWR suggests, it will also have a resonance around 120MHz... so not "better" or worse than our tries.Anyways, I'm not sure if the VSWR really matters too much.The probe will definitely disturb the dielectric (less air - more conductive stuff).Do you have any programmable signal source?You could easily sweep the frequency and get the reading with your SA for both, the "output" of the fixture and the output of the probe.In the end, a SA is "just" a super selective power meter on steroids.Your SA has 50Ohme input.That could perfectly serve as the load for getting a reference reading to obtain the current while you terminate the probe with some 50Ohms terminator.When measuring the probes output, you could just move the terminator to the output of the fixture, and you're done.BTW: I'm also in the process of making a measurement automation software... I already have a similar measurement - so that would be easy to implement.73

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 09:13:43 am
content: This is a different test. The purpose was to directly measure the power that passes through the test jig across the frequency range.So rather than looking at reflected power (VSWR), this test measures forward power directly at the load side of the test jig.The tracking generator was set with the output going to one side of the test jig.On the other end of the test jig, the 50ohm load was replaced with a 10dB 50 ohm attenuator.In effect this is like putting a volt meter on the load to measure how much power gets through the test jig.With this setup, a perfectly flat output would show as -10dBm across the frequency range.The plots attached are across a 1GHz frequency range. Even the bad old test jig performs reasonably well.EDITED: Correct plots attached.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 09:22:03 am
content: Quote from: wilhe_jo on July 08, 2022, 08:57:55 amThis is a measurement of the test jig only. No coil is fitted to the jig. It will have no effect on the resonance of the coil.QuoteI think it does matter in that it is a measure of how much current actually gets through the test jig.See my latest post with the 50ohm load replaced with a 10dB attenuator.QuoteI think have already done this test, but given that VSWR measures reflected power, I could have just done this test with a calculator.QuoteThat would be really useful, especially if it could be connected to a stepper motor driving a turntable.Quote

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 09:50:54 am
content: Attached are the results of the same tests of VSWR.The marker table shows the measurements at specific frequencies. The measurements show that the TEM-like test jig is a significant improvement over the original. It will easily measure out to 500MHz with high confidence. There are no resonance or other bad characteristics. When compared to the Pearson test jig data sheet, the TEM-like jig is at least as good, and probably better. This is the Mk1 Mod0 version so I am sure the performance can be improved but I am not going to do that. One way to improve on this would be to use tube and hollow cones to actually make a full coaxial conductor pair. I think this would be unnecessary. The test show the TEM-like test jig is a reasonable approximation of an expanded coax conductor.The TEM-like test jig is simple and easy to make. This one can accommodate a coil with a max external diameter of 65mm. The centre conductor is 18mm diameter. It would be easy to make different diameter versions for smaller/larger coils.So in conclusion, the TEM-like test jig is a significant improvement over the first one I made. The frequency range extends far beyond what I need for testing current sensing coils.This line of experimentation has been a success.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: July 08, 2022, 11:11:47 am
content: I think my comment re the VSWR needs some explanation...Of course, it affects the power into the cal-system, but when you can measure the voltage at the load ( ie just use the SA as a 50 ohm load), it does not matter.However, if you put your probe around the septum of your stripline/tem cell/coax will see significant change as it modifies the dielectric.Having that said - as long as this mismatch is short compared to your min wavelength it shouldn't matter too much.As I said, I tried to get some nice values for my coax adapter but the closest I could get was abt 100ohms. So I completely ignored that...The only reason I see why a reasonably good VSWR would be nice ist that you could feed the jig from an amplifier.Some 10W into the load would allow for lower frequencies to be measured. However, I tend not to bother about everything <150kHz.Those commercial probes are often specified down to 100Hz. So every dB of input power helps lifting the output signal above the noise floor.But even here I see no problem. My PA can handle mismatch and a 6dB attn would solve this issue anyways (this commonly done for conducted immunity testing).Re my test software: I'm currently designing a manipulator for my GTEM.That's basically turning a rod with some motor by 120degree. So turntable control should be there soon as well.A cheap hacked combined with a stepper+some platform makes a nice antenna pattern measurement tool.I did this in the past in a hacky way to quickly get some plots.That will be handy for a current project, so I'll definitely add support for thisI hope everything will be "ready" and polished in september....74

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 08:34:23 pm
content: HiAll of what you say makes perfect sense.I don't think a 50ohm test jig is practical given the physics of the problem. I agree that VSWR isn't important.I was far more interested in building a jig that degraded gracefully as frequency increased. Specifically I didn't want any resonance.With a relatively flat VSWR curve, it won't be so necessary to account for the characteristics of the test jig when measuring the coil. Knowing that the current reaching the 50ohm termination is almost constant with frequency is useful. Of course inserting a coil will mess that up. Knowing that direct measurement of the termination current will give high quality data is another benefit. Based on the limited data for the Pearson version, I think the measured data shows the TEM-like jig has superior performance.Of course the most important thing is the looks. To a non-rf engineer, the purpose and how-it-works will be a complete mystery. That's the thing I like about RF. With some low tech tools and materials, I can make something with high tech performance.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2022, 08:53:23 pm
content: I just realised, I forgot to post the termination power measurements.These were made by replacing the 50ohm termination load with a 10dB attenuator. This directly measures the current that has passed through the test jig.The plots show power to 1GHz and 500MHz, for both the old jig and the new TEM-like jig.The new jig has a very flat curve, <1dBm to 500MHz. It is still usable out to 1GHz with no resonance or other issues.The attached image shows the test setup with a10dB attenuator replacing the 50ohm load.The other attached images shows the new and the old test jigs. The central conductor of the new jig is held between two spikes of bronze soldered into the BNC connectors. The sheet metal provides spring force that ensures good contact with the aluminium central conductor.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 10, 2022, 12:13:40 am
content: HiOK now that I am happy with the new test jig, I am back to testing coils.The latest coil has an inner split floating shield, which is to say it looks like a shield, but isn't. My theory is that when the inner shield is in very close proximity to the windings, eddy currents form in the shield from the current flowing in each wire-wrap winding. I think these eddy currents dampen the higher frequency resonance I, Anderson and Pearson see in the coils.Another feature I have consistently seen is spike around 23MHz. These looked about right for reflection along the cable connecting the spec-an tracking generator output to the test jig input. I have been using a 2m BNC cable.The only difference between the attached plots is the length of the cable. I used 15cm, 1m, 2m cables and the spike moved frequency with the cable length.Adding a 10dB attenuator did not affect the spike.I concluded that the spike is due to impedance mismatch by the coil. The coil in the fixture is causing reflection of the tracking generator signal along the coax. Experimentation indicates the spike is a result of e-field leakage through the outer shield through gaps in the foil. Clamping the 3 parts of the outer enclosure/shield together improved rejection of the spike.As a side note, the 9 turns on the coil provide an output exactly -20dB from the current passing through the coil (the tracking generator output was -10dBm). This is a convenient conversion factor to work with.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 10, 2022, 01:08:28 am
content: HiEven with flaws in the foil shielding, the frequency response is good out to 200MHz, but the response changes if the foil is moved.At the bottom end, the 3dB frequency response is 112kHz. It is no where near as good as the Pearson coils which go right down to a few Hertz. 112kHz is more than sufficient for my sort of work. I have the option of making a 2nd coil with a lot more turns to give a low frequency response.I have now reached the point where foil on 3D printed plastic isn't good enough to get reliable test results.I need to machine a version from solid metal. This might take me a few months because I will need a 2mm or 2.5mm thread cutting tap for the screws to hold everything together.I think a key parameter is the separation between the coil windings and the aluminium inner shield. If eddy current losses do provide the required damping, then the best way to achieve that is with a multi-strand, non-crossing, flat wound coil. This will keep the current close to the aluminium. That is exactly what I have done with the wire-wrap.Also the metal to make the coil enclosures is expensive here. I won't be making a lot of machined prototypes. I can hold the inner shield with a 3D printed insulating support. I may be able to do a little experimentation with the size of the inner shield. I will be aiming to make only one outer shield/enclosure. I will use a BNC connector if I can, but I think the available bar stock size will dictate a SMA connector.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 12, 2022, 09:42:47 am
content: HiAfter a search through the workshop, I found some suitable Al bar stock. The stock is big enough to include a BNC connector, rather than a smaller SMA.I have drawn up a cad design attached.The important unknown is the gap between the coil windings and the inner shield. There is no way of calculating this.If the gap is too narrow, the coil will be over-damped. If the gap is too wide, the coil will resonate.I have left enough thickness in the inner shield to increase the internal diameter to widen the gap.If I go too far, I can reduce the thickness of the entire assembly to reduce the gap on the top and bottom of the coil.What is not shown is that I will be using perforated overhead projector sheet as an insulator between the various metal parts.The internal diameter of the through hole is only 22mm, which is not large but sufficient for my needs. The offset hole simplifies manufacture. Very little milling will be required.The plan is to make this over the next couple of weeks.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 23, 2022, 10:56:55 am
content: HiI have started machining the inner shield, which looks like a shield but isn't. It is fully floating. It does act to dampen resonance. I think it works by the coil flux leakage inducing eddy currents in the aluminium. The effect increases with frequency, which is good because resonance occurs at higher frequency.I should note that the design is terrible. It requires trepanning a deep groove into the face of the bar. The coil is then pushed in. This will make removal of the coil very difficult. I did it this way because it uses the minimum length of bar stock. That stuff is expensive.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 23, 2022, 12:27:40 pm
content: Nice trepanning job, I have done that on medium format camera lens adaptors.I had a failure of the little HF 30 T c/t here. I might be lucky it was not on my new 'scope!. The old Eico 410 10 Mhz survived.The attenuator is ~ 4:1Previously:R11=R22 = 50 Ohm 1 watt precisionR21=R12 = 120 ohm 1/4 Watt.I had the c/t on primary of inverter transformerWhen the inverter started into low load, the transformer momentarily saturated ( separate problem)Resistor R11 blew to pieces and started arcing due to the higher resistance (290 Ohm) seen by c/tSo I changed toR11 = 56 Ohm 3 Watt paralleled with 470 Ohm to get near 50 OhmR22 still 1 Watt precision but with but added paralleled back to back zeners 4.7 V 1 Watt .I will make a board for that with a semicircle to glue the ferrite core and bnc onto.I am using these as pre-wound c/t, need to get some more @ $0.50 eaDon't know what the core is and they work as transformers up to 100 MHz, I used one as UnBal for the FM broadcast receiverhttps://www.surplusshed.com/pages/item/M2199.htmlEdit: ordering some of these for the burden/attenuator:CGS 355051RFTSMD Chip Resistor, 51 ohm, ± 1%, 5 W, 4320 [11050 Metric], Thick Film, High PowerMax 300 V rated working 111 VoltMaybe that size not necessary for RF use

author: dazz1
author_rank: Frequent Contributor
timestamp: July 24, 2022, 10:40:52 pm
content: HiA little more progress. The core and coil are fitted in the internal shield. Just a reminder that the coil is 9 turns. Each turn is 6 strands of wire wrap.I drilled a hole for the coil wires out the side.I drilled 4 holes in the back of the inner shield so I can push out the coil and core if needed.The metal on the internal bore is 0.5mm thick to maximise the hole diameter. It also has a tapered knife edge to reduce the capacitance between the cover.The thickness of the external metal is thicker than needed so I can adjust the gap between the windings and the aluminium if needed.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 25, 2022, 07:09:34 am
content: HiI did some testing with the inner shield. It look promising enough to go ahead and make the (actual) outer shield.The peaks under 100MHz are due to reflections down the cable. Changing the cable length changes the frequencies of the peaks. This is shown in the differences between the two overlaid traces. The two traces are different colours.The lower 3dB point is about 125kHz. Not even close to the Pearson specs, but adequate for my purposes.The 3dB bandwidth is at least 125MHz, usable with corrections out to 200MHz. This is reasonably close to the results seen with the plastic and foil prototypes.Tests with the prototypes indicate that the frequency response should flatten out once the outer shield is fitted. I hope that is the case with the solid aluminium version.I am not going to try to tune the response. I will just go ahead and make the outer shield and take it as it comes. It already performs far better than I expected it would when I started.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 04, 2022, 09:43:10 am
content: HiSome more progress.The main body of the current probe features the really bad design requiring trepanning a deep groove for the coil to sit in.I did the poor mans version of anodizing by boiling the inner shield in water. I discoloured to something that could easily be mistaken for titanium. It will never be seen once probe is assembled.I made a sacrificial fixture from a round piece of 6mm steel. The probe is clamped onto the fixture so I can hold the work machine the various surfaces in one pass.The fixture has the holes for the screws marked on it.Everything is clamped together to drill the holes for the screws. This will ensure all of the holes are perfectly aligned.If this probe doesn't work, it will be a very expensive and plain looking paper weight.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 04, 2022, 09:47:04 am
content: HiPhotos of the current probe ready to drill the holes for the screws.I will use 6x 6BA screws, because more screws give the impression of better quality, and because I have them in the drawer.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 06, 2022, 08:10:52 am
content: HiSome more progress with milling out the pocket where the BNC connector fits.Also I milled the flat where the BNC fits. This was done with the cover screwed on to ensure a perfect join between the cover and the main body. This is why the cover has six screws. It had to be strong enough to survive being milled in place.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 09, 2022, 08:26:03 am
content: HiToday I milled out the hole for the BNC connector. The threaded part is round with two flats. I wanted to minimise and twisting of the BNC connector when in use. This required a close fit and a hole that was far from round.I used a CAD programme to figure out the milling way points to approximate the hole shape/size I needed. I created a table of x/y coordinates I needed to reach. CAD told me I would need to file off some of the high points. The milling machine didn't listen. The hole ended up being the exact right size and shape. No fettling was required to allow the connector to slide into the hole.

author: tautech
author_rank: Super Contributor
timestamp: August 09, 2022, 08:58:10 am
content: Quote from: dazz1 on August 09, 2022, 08:26:03 amNice work Dazz.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 09, 2022, 10:52:01 am
content: HiI completed the final machining operation by turning down the external dimensions to size.With the enclosure screwed onto the fixture, I was able to machine the face and sides in one pass for best finish.Now I just need to assemble and test the current probe. Assembly includes cutting insulating sheet material (clear photocopy film) to separate the various parts where required.I think testing will be the most stressful task because of the risk that the whole thing is a failure.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 10, 2022, 09:21:14 pm
content: HiOK so I have finished assembly and testing.Attached are images of the final current sensor compared to the original CAD cross section.The machined current sensor all worked out as planned. I didn't have any disasters. Everything fits. It has a nice shine.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 10, 2022, 11:14:56 pm
content: HiI measured the performance. It was better and worse than the 3D printed versions with kitchen foil.It was better in that the resonance peak at about 23MHz is no longer there. This peak was related to the connected cable length and was not suppressed by an in-line 10dB attenuator. At the time my hypothesis was resonance was due to e-field pickup through gaps in the foil. The final version was fully e-shielded has no resonance peak.The frequency response is worse in that it is smooth but not flat. The prototype showed a flat response out to about 80MHz. The final version has a significant slope. This can only be due to the aluminium enclosure. The hypothesis is that the leakage flux from the coil windings is generating eddy currents in the aluminium, and these eddy currents cause losses. In effect, this forms a frequency dependent resistor. The higher the frequency, the lower the resistance. The positive benefit is that resonance of the coil is fully suppressed. It is clear that the damping is too high causing a relatively straight slope. The cure would be increase the clearance between the aluminium enclosure and the coil windings to reduce the damping effect. That is not a trivial task and I am not going to do that.The mitigation is that my spectrum analyser produces numerical CSV files that I can apply to measurements to correct for current probe errors. The good thing is that the frequency response is devoid of resonance which would be injected into the circuit under test, if it occurred. Too much damping is better than too little.So looking back at what has been achieved.I developed a current source that is in effect an open coax, which is easy to make and demonstrates good wide band frequency response.Proved that a 3D printer and kitchen foil can be used to produce reasonable prototypes for this application.Eddy currents are an effective method of damping coil resonanceIt is possible and practical to DIY build a current sensor coil up to at least 200MHzImproved low frequency response would require a nano-steel core. Not easy or cheap to source.Optimising the design of a current sensor with eddy damping would require experimenting with the clearances between the enclosures and the coil windings. That would require a significant R&D effort.I started this project hoping to get up to 30MHz for EMC testing. I have ended up with a working current sensor with a usable range >>30MHz but with sub-optimal performance. There is a lot of room for improvement, which the prototypes showed is achievable.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 12, 2022, 05:26:57 am
content: HiToday I did some investigation and experimentation trying to figure out why the 3dB bandwidth is down to 60MHz instead of the ~160MHz expected. I don't even need 60MHz but I'd still like to know why.I checked the current fixture. Even though the current probe is a tight fit within the fixture, it still works very well. I figured out that if I connected a 10dB attenuator where the 50ohm load is normally connected, I could normalise the injected current with the spectrum analyzer. I could then remove the small error of the fixture (<0.5dB).Analysis of previous measurements isolates the reduced performance to the outer shield/enclosure. There is something about the outer enclosure that is causing a significant loss, rising with frequency. It can't be the solid metal sections because at 100HHz, the skin depth is only ~7um. I was able to machine the outer enclosure to increase the gap to the inner shield. That had no effect.I checked that the inner and outer shields were not shorted together. No short and no change.The design offset the radial flux breaking grooves between the inner and outer shields. This offset improves e-shielding and eliminates any e-field break through.I tried aligning the two grooves but this only allowed a small and visible break through.I don't know what is causing the significant increase in losses as the frequency rises. Based on development with the 3D printed prototypes, I was expecting a flat response out to about >150MHz.The first image shows the frequency response of the fixture with the current sensor fitted in place. The fixture is good out to 600MHz, and usable well beyond that.The other plots show the frequency response of the current probe. The 3dB point is at 60MHz, well above my initial requirements for EMC work, but still not as good as the 3D printed prototypes indicate should be achievable.The current probe is entirely usable out to 100MHz to make relative current measurements. Absolute measurements will require error correction.The photograph shows the setup to measure the fixture error with the current probe fitted in place. The current probe is terminated with a 50ohm load to simulate connection to the spectrum analyser. This load will appear as a parallel load of about 450ohm to the current going through the fixture. The 10dB attenuator terminates the fixture current to 50ohm. The output of the attenuator measures the actual current passing through the current probe, which includes the effects of the current probe in the fixture. This is a good way of measuring the performance of the fixture and also the influence of the current probe on measurements. Having a big chunk of metal in the middle of the fixture has little effect on performance. It is also simple, easy and cheap to make.So in conclusion, the current sensor exceeded the original target of 30MHz bandwidth for EMC work but the 60MHz achieved on the final version is substantially less than the +150MHz bandwidth indicated by the 3D printed and foil wrapped prototypes. There is too much attenuation as frequency increases. Resonance is heavily suppressed, which is good, but sensitivity also suffers as frequency rises.The double skinned design completely eliminates e-field break through. There is no pickup of stray e-fields. This is good because the current sensor is only measuring the current through the probe, and not the voltage.So I now have a nice shiny paper weight that can measure currents out to >60MHz. More R&D would be required to figure out why the outer enclosure is reducing the bandwidth, but I am not going to go down that path.

author: garrettm
author_rank: Frequent Contributor
timestamp: August 15, 2022, 06:07:05 am
content: I wanted to say that I'm impressed with your workmanship dazz1!That said, I think the shield should be split near the center of the coil (rather than at the top edge) and a split should be added along the top and bottom plates (leaving only the outer edge as a continuous strip of metal). This is how the clamp type wideband current transformer I have is constructed.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 15, 2022, 08:56:07 am
content: HiI experimented to see if the position of the slot made any difference. I saw none. I did find that with just one slot, there was e-field leakage that appeared as a small resonance at around 20MHz, depending on the length of the cable between the current sensor and the spectrum analyser. Inserting an attenuator did not affect the resonance.I think it is more likely that placing the slot in the centre allows for two identical parts to be made for the e-shield. They would have been spin-formed. I did consider using metal spinning to make two halves of a shield but I would need to make a form. The other problem is that I would have needed to come up with some really ugly arrangement to make the connection to the BNC, just like yours.I decided if I was going to make a paper weight, it had to look good.If you have a look through the posts, I tested the finished un-earthed inner enclosure before I even started making the outer shield. It had heaps of bandwidth but also lots of e-field. I am just wondering if the bandwidth I thought I saw was the bandwidth of the e-field leaking down the coax shield. Maybe the "perfect" e-shield of the metal outer enclosure simply revealed the underlying H-field bandwidth of the coil. Maybe the e-field and h-field added together to give the flat frequency response I saw.The other issue I have is that the coil and core over damp resonance. That could be core losses. I don't know the history, make or material of the core. It is a junk-box special. That can't be eliminated as a root cause. Over-damping is much better than under-damping.The one thing I have not yet tried is testing with the inner enclosure not fitted. I might get around to trying that but I already know that will increase e-field leakage and might give false hope.Given that my original intention was to DIY a 150kHz to 30MHz EMC current sensor coil, making a 60MHz coil doubles that target. I have met the requirements. It is a great looking paperweight that can also measure RF conducted current.The current fixture is a success and I am considering making slightly larger, improved version, to normalise the current sensor with the spectrum analyser before each use. This will take out all of the errors.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 16, 2022, 05:40:05 am
content: HiBased on my test results, I suspect there may be sensor coils out there that fake their bandwidth because the coils are sensitive to both e-field (voltage) and h-field (current).As standard, sensor coils are calibrated with 50ohm source and load impedance. There is a simple test that could be done. Change the impedance seen by the coil as shown in the attached diagram. If the coil is only sensitive to current, then the current should be reduced in proportion to the increased impedance. If the coil is sensitive to e-field, then the output will increase based on the higher voltage.I think it is entirely possible that simpler, lower quality sensor coils might be able to claim a higher bandwidth than a fully e-shielded sensor coil.