impedance measurement with VNA using series, shunt/series through methods, graph

[coppercone2]

Thats what I see in the highest power density dc switchers

[joeqsmith]

If you are looking for best attenuation per turn at low frequency, it's tough to beat a toroidal tape-wound core, especially an amorphous or nanocrystalline one, e.g. Metglas or Finemet. For the most inductance per turn, avoid cut cores or "distributed gap" cores. Also avoid "square-loop" cores, and look for so-called flat-top annealed cores.

Here's an example: https://www.digikey.com/en/products/detail/T60006L2040W422/2258-T60006L2040W422-ND/12531985?itemSeq=363262270

It's not very tolerant of any DC, but CM DC should be negligible. The permeability starts rolling off below 10 kHz, but it's still pretty good at 100 kHz.

It will start to get quite lossy above that, but it's still CM loss so it helps. Combining with another ferrite core may help at the high end if needed.

Cheers,
John

As a matter of fact,  all of the transformers I have shown used tape wound nanocrystalline cores.  And yes, I have combined these with other materials to improve the common mode attenuation at the higher end.   #8 for example used Mn-Zn.   

As I mentioned, that last attempt used three different materials.   I suspect the insertion loss for this design would be very good but I want to see if I can improve the common mode before building it up. 

[coppercone2]

Tried 11/32 and the letter drills around it, it looks like 11/32 is the best fit (hard to slide on though), so I bought a 11/32 reamer and a 11/32 adjustable lap and diamond lapping paste, so I can ream a 11/32 hole then try to lap it to a fit I like before I start messing with the springs. I think that might work.

[joeqsmith]

I have some short pogos but they use a ball at the end.  I wonder if they make anything short enough that you could have just mounted them to a microstrip.   At 300MHz, the performance may be decent. 

I ended up remounting transformer #8 using SMA connectors and removed some of the excess wire which slightly improved the insertion loss. 

Shown is the data for the #8 after these changes along with the 2pc hybrid and the last tricore transformers compared with the Picotest.  I collected the data the same way virtualparticles did to try and get a realistic comparison.    In the 10kHz region, all three have a better common mode attenuation.  I wonder if that is a limit of the VNA.    That tricore just misses at 150kHz to 300kHz.   The Picotest's insertion loss beats everything I have shown so far.

[coppercone2]

well there has got to be some some problem to miniaturizing a plus sized rack box to a deck of cards, that could be it.


i stopped playing with my sdr but people have all sorts of weird problems with SDRs.

i can try it on a e5100 in the future one day, i am sure some one here will beat me to it

i always run into problems making anything smaller, even the most simple shit, something always gets you.


i say this somewhat jokingly, if the designer called it 'pico' when its the size of a modem and you got something the size of a deck of cards called nano, and pico is smaller then nano, but likely made with higher design effort, I assume the person that thought it his design is nano and you could still foreseeably go to a pico missed something, when the original is 20lb.

[joeqsmith]

I was wondering if running the VNA near it's lower limit was causing a problem, but our friend Brian is no beginner.   I would put more stock in their data than the graphs.     

https://www.electronicdesign.com/home/contact/21132621/brian-walker

The rounded pogo pins I mention are about half the length of these.   

[coppercone2]

the harwin ones have sleeves you can put alot spring loaded things into

[joeqsmith]

I believe Harwin made the short ones I mentioned.   The smaller ones in the last picture were made by Everett Charles.  They may offer something you could use as well.   

https://ect-cpg.com/spring-probes

[coppercone2]

the high frequency probe makes this design obsolete

at least i get to lap something

[joeqsmith]

I don't know what they charge for them.  They look pretty fancy.   Still, in that lower frequency range,  I would just continue with the home made setup.

[coppercone2]

yes I do expect to call the police on the quote, for some reason I think I can buy a shiny new VNA and a HP test fixture for the price, after all the goal of all this is cost.

if you just factor in the cost of making it, and you go with all USA and pay extra for things like USA brand lapping paste and machine tools

~45$ for the reamer, lapping tool and lapping paste, plus you get to keep the tools, you can get by with just the drill and a file most likely because it probobly is not measurable, I just want it to feel smooth
~20$ for the TeCu block
~15$ for the BeCu spring
~15$ for a ready made LMR400 cable, so you can cut it in half and use those
~20$ for the brass tubes and you get to keep a bunch (I think putting the seek thermal on the andonstar microscope mount is a very very expenisve tool, with the help of a shiny brass rod, it feels like a real thermal microscope, not that i have seen one). yes brass hobby tube is excellent stuff, not just for railroad hobbyists
~30$ for silver plate (kool amp) and you get to keep 99%
~15$ for aluminum soldering stuff
~15$ for polishing stuff

realistically a ham already has 1/2 of this stuff

and whatever probes you get

so its like $130 and most of it is otherwise useful other then the cable and the copper block, giving you alot of capabilities, the reoccurring cost is the probe you might break after a long time, and that is just 1/2 of the probe you can replace.

for someone else though, it seems that getting the full bandwidth of the VNA is most important. I have a 300MHz limit.

[coppercone2]

here is a picture, I think it came out nice actually with the silver plating, I am still learning about applying it, it looks like the tip needs a bit more work on closer inspection. btw I left one side of the tubing normal after a pipe cutter cut so it kind of conforms around the end of the coaxial cable, and there is a layer of DP460 glue on the top also.

I also thought about making a heavy braid braid that shorts out the two tubes with clamps so you can use it to probe something weird and big like an air variable capacitor.

[joeqsmith]

Are these for two different fixtures, or do you plan to butt them up against one another and place the component you want to test between the two pogos?

I assume you will mount these to some sort of ground plate.  How do you attache them to the VNA?

[coppercone2]

like this, if the explanation is hard, maybe this crude picture will help. the spring is like a head band, or perhaps a turtle neck that you put on your head so when the tube slides through it presses against the spring slightly so the connection is made on the very end, i am not sure if it will help, i think its proper if the spring is dense and small. i will turn that copper block in the backdrop into the U shape by cutting it into 3 pieces and brazing it together, and ream/lap the holes so it feels good. the T's are set screw knobs. I just tested it by putting it in a vise. you draw the springs back with tweezers, then put the part in with another tweezers and let it go. once it operates on set screws a pedestal can be made. Imagine someone glued a big fat o-ring to the outlet of the shaft hole but the o-ring is just a tightly wound spring that compresses/deforms on the outside of whatever you put through it. I think since the spring i got is silver plated Beryllium copper I can just mill a ball nose ring around the hole to seat the spring in there, and heat it on a hot plate with a ring of solder in there to adhere the spring to the big block. when i do the other side, I would just flip it and lump a bunch of heat sink compound all over the top of the fixture so the top spring does not de-solder when I heat it up.

I think a turtle neck sweater is the best explanation of how it works. its a step above wedging steel wool into a EM hole cable pass through (bootleg RF gland of some kind). If you can make really good colets it would be better, but thats so far outside of what I can do its not funny

the drawing is realistic because thats probobly what it will measure as with the tools/skill I have available, it is unlikely to have many square surfaces

[coppercone2]

http://www.sinocableglands.com/sale-11945562-emc-emv-stainelss-steel-cable-glands-ss304-ss316-ss316l-for-shielded-emc-cables.html

this might be an option if I can find it in the right size, I did not think of EMC glands for some reason.

someone with money might want to try this, but its $50 a pop. getting kinda heavy.
https://canada.newark.com/lapp-usa/53112037/cable-gland-3-4-npt-brass-9-17mm/dp/61AC3758

the manufactured glands in the interests of sanity and durability probobly have multiple contact resistance points (i.e. spring wedged in something, rather then fused to the main structure, so making your own might have impedance advantage).

I might also make a shield around it with a little lid with hinges to put the parts in and tighten down the lid around EMI gaskets if I get that far.

[coppercone2]

Oh right, that is just for thru measurement.

For reflection (i.e. component directly to ground), I thought of of making the same fixture but having only one side with a hole, the other would be the wall of the fixture with a pogo pin inserted directly into it.

For shunt measurement, I am not sure. This is a difficult one requiring alot of mechanical bullshit. I would probobly put a watch makers vise with flats instead of horns with dielectric jaws into the fixture to clamp the part with a pogo pin under neath, so I can press all 3 pins into the part without disturbing it. It would be clamped into the vise securely and all 3 pins would be inserted in all 3 axis. I suppose I can do that for all of them so the part is not floating, but I am satisfied with how 2 pogo pins hold small parts, it would only be necessary for something large like a big SMD inductor or capacitor.


I would need to remake the watchmakers vise out of plastic I think. It might be a significant parasitic capacitance. Maybe 3d printed with a nylon screw.


If anyone knows what I am calling a watchmakers vise, because of a ebay mis-name
https://www.esslinger.com/watch-case-opener-for-screw-on-style-backs-pocket-wrench-tool-square-pins/?gclid=CjwKCAjwkN6EBhBNEiwADVfya8e3HYAwocSj_Q63cwjbZArCKY7UxaCrfh994lWfYb5NdS_8zF3xMBoCgNcQAvD_BwE

its actually a removal tool. So that made out of plastic to hold the part in place with a pogo pin coming out the bottom and pogo pins coming from the sides.

I think that would be a good mechanism to clamp weird parts, if someone can make one for a 3d printer that is compatible with a nylon screw..

[coppercone2]

I am wondering since you have a 6GHz VNA, with the whole deoxit thing, can you try to put some adapters on it and then put a coupling thats really greased up (even with something else like wd40) and see if it makes a difference on the graph ? like really dirty with the oil being sloppy applied so it goes over the dielectric and stuff.

 in the interest of reducing mating cycle damage, no change would be beneficial, and be a bit of a mythbusters. its not defined

[joeqsmith]

I am wondering since you have a 6GHz VNA, with the whole deoxit thing, can you try to put some adapters on it and then put a coupling thats really greased up (even with something else like wd40) and see if it makes a difference on the graph ? like really dirty with the oil being sloppy applied so it goes over the dielectric and stuff.

 in the interest of reducing mating cycle damage, no change would be beneficial, and be a bit of a mythbusters. its not defined

I would never apply anything to an RF connector that was not what the manufacture recommends.  Now, if you found me an article published by Keysight, Copper Mountain Tech, Gore or manufactures of high quality RF connectors where they call it out as part of the maintenance and care, I would read it.

You are certainly free to buy your own test equipment and treat them as you wish.   I would even follow along if you decided to post about it. 

[coppercone2]

guess its going to have to wait an ebay deal then, i don't think its cobalt thorium g liquid

[joeqsmith]

For starts, you could buy the V2Plus and a few bags of cheap connectors.   Buy enough and maybe you can draw some sort of conclusion. 

If you are looking for low wear and long life from your $1000 metrology grade adapters,  you may want to consider including some of this in your testing:
 

[joeqsmith]

Distributors are slowing down.  The order for the last set of parts was finally processed.   Maybe next week.   

In the meantime here are a few posts on RF connector care.   I don't think you are going to find anything about WD-40 or other chemicals being recommended but again, I would certainly be up for reading about it if you do happen to find anything (from a reputable source).       

https://dl.cdn-anritsu.com/en-us/test-measurement/files/Manuals/Instruction-Sheet/10100-00031C.pdf

https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1ma99/1MA99_2E_CoaxConnectorHandling.pdf

http://na.support.keysight.com/pna/connectorcare/Connector_Care.htm

If you read this article, you may find a few problems:
https://www.microwaves101.com/encyclopedias/connector-care

Still some very good information on this site.

Here's an old paper from HP
https://www.ece.ubc.ca/~robertor/Links_files/Files/ConnectorCare.pdf

[coppercone2]

https://blog.solidsignal.com/reviews/using-dielectric-grease-weatherproof-coaxial-connectors/


Most coaxial connectors will not contain much air space in the active cable core area when correctly assembled. Filling the connector with a dielectric grease with dielectric properties matching the cable core dielectric material (commonly foamed polyethylene) will not cause any significant measurable shift in connector impendence or VSWR and in some cases will improve it.

thats for packing too

I assume they might mean analog VSWR meter on a rooftop antenna. when I zero my VNA just redoing the connection makes it drift though. I put deoxit on it long ago to protect it from humidity. just thinking about it, it might be safer on 75 ohm stuff for testing because it does not have the foam. i think your concerns are about the foam absorbing oil?

I thought to get a few adapters that are 75 ohm for easy cleaning and experiment on that. I suspect an ultrasonic bath in hot alcohol will clean them up so I can do proper before and after testing and see if cleaning has an effect. I think its solid teflon and not PEfoam in the 75 ohm connectors. I think they might be more stable, especially when soldering, so I tend to use 75 ohm on stuff that has a weird impedance anyway, since its harder to destroy, you can get aggressive with soldering the body. For something like attaching it to a high thermal dissipation object with solder (brass tube), the 75 ohm connector survives unscathed but a 50 ohm connector is delicate and if you are not super careful (and its difficult to be so this careful because its a difficult solder joint), they tend to bulge/deform and even smell bad. Shields also are more forgiving.

[joeqsmith]

I wouldn't turn to the cable or IT guy to be an expert on RF connector use.  You may find the ham and CB groups have a lot of wives tales as well.  You wouldn't turn to Facebook and Twitter for medical questions would you?     

https://youtu.be/Y0sg9G5BBVU?t=112

I suggest following the real experts for both your RF and medical questions.   

[JohnG]

If you are leaving your VNA out in the weather, then use of a dielectric grease might make sense to help prevent or reduce moisture infiltration and subsequent corrosion. I'm not exposing mine to the elements, though.

It also is used in applications where there might be high voltage. In the RF world, this would be in the transmitter output path, with a reasonably high power transmitter.

I'd be pretty dubious about it for signal measurement. SMAs might tolerate it, but most fancier connectors seem to have substantial air space. It's supposed to be air, or some kind of gas. Also, greases seem to have a bad tendency to migrate to where they are not wanted. It's a lot safer to use some connector savers.

I would not make any assumptions about cable dielectric formulation and cable impedance. I have seen all kinds of dielectrics for 50 ohm coax.

Cheers,
John

[coppercone2]

I think there is also something to  be said about the dollar value of the product, paying off <1uM precision lathes (apc7 is centered to 500nm), etc. I do know that metal things want oil. it good, even on gauge blocks you rub it off before you use it

my hunch is that there is a frequency where this begins to be noticeable, there is probobly an operating region where it does not matter, where it matters a little and when it screws with you

also people often dislike lubricants because it leads to over tightening or difficult torque setting adjustment (how do you compensate for oil), dry makes this more stable for the wrench. This can lead to a boondogle over oil stability, how to not oven oil stuff, etc. Steel has a 'slightly oiled' specification.. and thats for a bolt. try to translate that to a SMA torque wrench. Like with plastic piping, they say screw the wear, the worst thing is over tightening the plastic causing it to burst. in these coaxial connectors, spring deformation from over tightening can be catastrophic. I can see nothing by problems for a company that advocates use of oil but then tries to force people to use a particular oil that they categorized because its just oil, no one will really follow that because too many people have their own opinions on lubricants, and it gets rid of easy cleaning, since the oil might trap crap in it and make the connectors on average for some customers fail alot faster, whereas the life from just blowing it out with duster might be better unless the upmost care is taken, when you know things just wont happen correctly when like, non mechanically inclined people are asked to fuck around with grease in really posh laboratories, and since we are mixing RF and light so much in laboratories now, its a menace to optical systems.