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#50 Reply
Posted by
joeqsmith
on 20 Apr, 2021 21:42
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But you can make the pogo pin protrude only like 1mm,
I mean on the PCB you have, the traces leading to the resistor, in the series config, you scrape the solder resist off of them, lay the pogo pins down on the PCB so they are touching across the SMD pad facing each other, like a bridge over a valley, so its naturally short circuited. Put the part in there to unshort circuit it. I don't think this will effect things very much, you might need to put a tiny copper finger under the moving part of the pogo pin, right up a mm from the head. I think it will be good to quite a high frequency. If its gliding on top of a bed of copper fingers it should still be considered part of the 50 ohm stripline right?
Pins I have are longer than the boards (think Evert Charles bed of nails). For me, it's not a big deal to solder the parts down. If I added pogos, I would need to come up with a way to cal out their effects.
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#51 Reply
Posted by
joeqsmith
on 20 Apr, 2021 21:48
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Hi Joe,
Didn't have time to do a chart but you can pull what you need from the S-Parameters that one of our Apps guys created for me. I hope this helps!
BW
It does and answers the reason I had asked that last question. I would still like to see if you could replicate their data if you get a chance.
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you can get pogo pins that are about 1 inch long and very thin,longer if you use them with the housing to make it replaceable.
you can't do the same cal procedure? I assume you solder in a solid SMD jumper (high current solid metal)
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#53 Reply
Posted by
joeqsmith
on 20 Apr, 2021 23:08
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you can get pogo pins that are about 1 inch long and very thin,longer if you use them with the housing to make it replaceable.
you can't do the same cal procedure? I assume you solder in a solid SMD jumper (high current solid metal)
The boards are about and inch long total. For a thru jig, you have about 1/4" for the pogo pins.
When I use the boards you mention, I have a set with standards mounted to them. The parts I test are mounted the same board and mounted just like the load. I use the same connectors as well. Basically, I want to remove much of the errors. Adding pogos would toss that out the window and I wouldn't gain anything. I'm not suggesting you shouldn't do this. As you said, you don't want to solder SMD parts.
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well its mainly for MLCC, I am not sure about inductors, I assume they are pretty robust, but I know ceramic caps break ALOT with fracture. i figure resoldering makes this worse potentially.
I do wonder if press fit is better though the way you do it because is really beefy, you have piezo and magnetostriction, i wonder if it can do something with the springs to form some kinda pendalum. it might need another tiny clamp, it is possible it might be causing the pogopins to vibrate back and forth kinda like a shaker, your method is truely massive in comparison to the transducer, this might effect anything with L&C,.
though, that giant fixture might make it easy to over pressure a capacitor also.
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#55 Reply
Posted by
joeqsmith
on 21 Apr, 2021 00:32
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I am not aware that I have ever damaged a capacitor with these jigs, or any part for that matter. No doubt, it could happen. The bigger problem is getting the parts inserted and aligned. Tweezers. Now that I finally broke down and bought a small set of Pace tweezers, I'm about as fast soldering and removing them from the test boards. Those were one of my better investments.
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i have a pair of those, still the problem is the capacitors can fail down the line. i dunno why i really care but if I wanted to sift through say an entire box of them to test everything, the solder would get old, even with pace tweezers, since you are supposed to clean all that stuff with braid when you are done, and testing a 100 strip is difficult to solder that much
all the proper work always shows after desoldering for resoldering you are supposed to start with a clean part that had all old solder stripped off as much as possible, presumably because its contaminated.
maybe you can use a solder pot after pace tweezers if that works good enough to remove anything excess but its too much for me, i would feel like im in china
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#57 Reply
Posted by
joeqsmith
on 21 Apr, 2021 03:09
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Wow, you are actually testing all your capacitors? I can't think of a time I have sorted through a reel of ceramic capacitors. I normally pitch any parts after testing them. I have some of these boards with parts permanently mounted to them for demo'ing the Nano. I also have some parts mounted that I keep for my other VNAs to as a sanity check.
Sounds like you are reading about mixing solders. That's nothing I am concerned with at home. You may be working more in production environment where I am just playing around with one of my hobbies for the fun of it.
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i wanted to test things like the old IBM experiments where you put like a whole bunch of decoupling capacitors on a board for ground plane things, generally I dont measure anything, and I am interested in things like vibration, shock resistance, etc. just experiments for myself to get to use/make cool gear like shaker tables
I see alot of new rules of thumb being made after a few VNA experiments and people shouting all about them but the sample size is small, its new and popularity is very high, potentially dangerous (1uF ceramic madness spam) , I feel like someone needs to look at those new theories with a bit of practical things in mind to see if its good advise to be shoving down peoples throat all the time) , there has been a bit of a 'decoupling revolution' on electronics forums in the last 5 years, some might call it new testament, that too many old timer engineers were doing things by rules of thumb and that its all wrong or useless etc (often suspiciously pitched with parts count reduction or miniaturization, in line with what the fab would want you to do for the sake of their packaging costs). when i showed some stuff i found to very skilled / long time people they were often a little suspicious, but where I found them in their point in life they had little financial support, personal drive, etc (understandable that you are not amped up about some place you left in the 90's and that you wanna do something else with your life without having a impedance laboratory in your basement)
generally at a company they only investigate this if sales fails to bullshit out of it so its non commercial
i feel like using a few classic approaches can recently get your circuit 'reamed' in the comments because of a few pictures and graphs on a few threads here and there (feels a little extreme, feels like saying that you pray to roman gods or something)
i have too much interest in grounding/electrical bonding. im not saying its not true but there is alot of stuff to look at, and alot of it is related to manufacturers providing more data and cheaper equipment, but with more advanced datasheets there is more confidence and less testing and more complex theories formed , which is something that i don't always like
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#59 Reply
Posted by
joeqsmith
on 21 Apr, 2021 12:12
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EEVBLOG is the only social forum I follow. I must have missed out on this 1uF ceramic madness spam and decoupling revolution you mention. The super low cost VNAs has certainly caused a bit of a trend for the RF hobbyist but more so for the antenna and 3D printer groups. They need their 3 places beyond the decimal SWR readings and plastic cases. Outside of all the noise, there are a few that have actually started to experiment with them. IMO, that's a good thing.
No doubt that as we continue to increase pushing for faster edge rates, more details become important. That doesn't mean that the beginner need place these same constraints on their first 555 timer circuit.
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Which data am I trying to replicate? It's hard to doubt the Steve. Just look at this testimonial.
"He's not just a man, he's a Universe."
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#61 Reply
Posted by
joeqsmith
on 21 Apr, 2021 15:32
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Which data am I trying to replicate? It's hard to doubt the Steve. Just look at this testimonial.
"He's not just a man, he's a Universe."
Originally I was asking if you could replicate what I believe is the common mode attenuation of the transformer. See their data here:
https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/msg3548742/#msg3548742That may be asking too much as you would need to set up the test to run it. The easiest thing that would pretty much clear everything up now is just measure the inductance with an RLC meter between the two coax shields. Then, measure the DC resistance with a basic ohm meter. I would expect that the DC resistance will be sub ohm and the inductance will be VERY high. Just let me know what RLC meter you used and what the settings were.
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#62 Reply
Posted by
joeqsmith
on 21 Apr, 2021 18:33
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This was the 8th attempt, again coaxial design using multiple core materials.
By far, it's the highest performance (most common mode attenuation from 10Hz to 1MHz) of what I have been able to come up with but it pales compared to the numbers Picotest has published. Assuming that virtualparticles is able to measure the inductance of the shield, and we are able to validate their plots then I am going to have to give some more thought as how to pull this off.
Because I am out of ideas for now, I went ahead and mounted it in a box and will try making a few measurements with it. No, not with the Nano....
Although....
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Hi JoeQ
Measured from connector ground to connector ground. I used a pair of pigtails and a copper plate as a low inductance common ground as you can see in the picture. I calibrated to the end of the test cables, attached the cheezy pigtails and then used port extensions to get to the end of them. The attached S-Parameter file should be what you want.
Best,
BW
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Here is zoomed in to 1MHz so you'll have enough points to compare the charts
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#65 Reply
Posted by
joeqsmith
on 22 Apr, 2021 01:08
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Hi JoeQ
Measured from connector ground to connector ground. I used a pair of pigtails and a copper plate as a low inductance common ground as you can see in the picture. I calibrated to the end of the test cables, attached the cheezy pigtails and then used port extensions to get to the end of them. The attached S-Parameter file should be what you want.
Best,
BW
Thank you very much for running this test. This data is very helpful.
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#67 Reply
Posted by
joeqsmith
on 22 Apr, 2021 03:33
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I believe with the data virtualparticles has provided, combined with clips from the webinar, we have everything required to replicate it if that was a goal. During the webinar, Steve Sandler talks about the trade off when designing the transformers. You may want to design a custom one for your particular application.
Attached is some data I took tonight using my four test resistors (100mOhm 1%, 25mOhm 2%, 1mOhm 1% and 100uOhm 1%). The data was collected both with and without the transformer. We can clearly see the effects of adding the transformer at these lower frequencies.
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#68 Reply
Posted by
joeqsmith
on 22 Apr, 2021 23:46
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I was curious about the 1mOhm part reading 30% high. I tried changing the cable from RG400 to one of my good HP cables, no effect. I moved the DUT right to Port 1 with a single adapter, no effect. I increased the source 10dB, no effect. I raised the frequency, no effect. The noise floor is about 109dB where I am testing now which seems like plenty of margin. It just likes that 85.6dB figure.
So I tossed the old HP VNA aside and switched to an old RF generator and my SA. No surprise, I got very similar results. The noise floor for the SA is much better so I tossed in the 100uOhm.
While it would be interesting to try and sort it out, I'm beyond the limit of what I can measure with the equipment I own.
It's possible my test boards are the source of the problem. I doubt that the 1mOhm 1% 50ppm part is out 30%.
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how do you calibrate these things? I imagine the problem might be when you move the coaxial cable around or unscrew it after calibration.
if you have a DC or close microohm meter you might want to put your test fixture to that by making a kelvin that terminates in the SMA connector and fiddle with it
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#70 Reply
Posted by
joeqsmith
on 23 Apr, 2021 00:54
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For the old HP and the SA I get transmission only so I inserted a thru. Everything is torqued. It's very reproducible, even with two completely different setups. The thru is the same PCB so we should be removing much of the error.
With these big power resistors, I can insert two banana to BNC and BNC to SMA on the two sides of the PCB. Drive the one side with a current source and read the voltage on the other.
1 mOhm
With 1.6716 Amps I measure 1.987mV or 1.189 mOhms.
With 3.9334 Amps, I measure 4.666mV or 1.186 mOhms.
100 uOhm
With 5.078 Amps, I measure 1.083mV or 213.3 uOhms.
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I think you will need something like a 4 terminal resistor to measure that accurately , 100uohm is just not happening normally with any kind of high precision that you want with a normal part regardless of the setup I think
https://www.globalspec.com/FeaturedProducts/Detail/OhmiteManufacturing/FKGK_Series_4Terminal_Resistor/312325/0Maybe you need to make another VNA pcb with sense traces that are there for verification with a microohm meter or discrete microohm meter setup and not used with the VNA, they will effect it at some frequency but if you stay low you can maintain the correlation between both measurements. that part has a special pad btw, I suppose you can put a shorting jumper there to short the little trace to the big trace when you do a VNA measurement but you dont wanna solder near that thing
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#72 Reply
Posted by
joeqsmith
on 23 Apr, 2021 11:47
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Agree, as I said it's possible the test boards are the problem. For the 1mOhm, I may be able to improve what I have. Agree, the 100uOhm would be a more difficult problem. If we ignore that for now and say the DC measurements are reasonable, the VNA readings, while closer, are still off a fair amount. There may be something else limiting the measurement.
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Maybe ultrasonic cleaning the contacts and deoxit gold?
Are you using high end VNA cables? I do wonder if they are the best type of cable for low resistance measurement, i.e. they have foam dielectric in them.. I only have 18GHz or more cables but they are thin and I don't have the parts for making the PCB yet unless I want to do it with waveguide launcher connectors (I found like 100 sma waveguide launchers in the trash a long time ago).
Now I am not sure what dielectric hard line solid sheath uses, but I have a feeling for that kind of stability you don't want foam in the circuit, and for some reason I think a higher wire gauge conductor is better.
I do have some hard line however, if I do this I could try to make BNC hardlines that go to the PCB if you feel like waiting a month. I think my rg141 is solid core and solid teflon dielectric
I need RG400 or whatever its called for my pogo pin fixture, the brass tubes should get here but when I think about it I think I need to redo the plan with a peice of hardline rather then a flex cable because mechanically some kind of bullshit will go down with anything but solid teflon insulation
Also, if you make your own cables, since it turns out bondable (not sure, still need to experiment), you can try to glue the pin into the connector after soldering or crimping to the center conductor by using a primer and loctite glue (medical) to bond the center pin to the connector with more then press fit, i suspect as you are bending the cable the center pin is moving around inside of that plastic sleeve and causing a problem
of course what i recommend is going to go into full hardline for the fixture that is well bonded into place some how (maybe stainless zip ties on a metal frame) that goes to the VNA and then use all APC-7 connectors and retrofit the VNA with APC-7 so it can be put into exactly the same space every time and connected with genderless rotational sleeves so you can put it into a fixture bonded to the analyzer every time then tighten the sleeves without fiddling with anything, but that is like a 1000$ mod, because if you use bigender/genderless (what is PC now lol?) connectors you can put the fixture on machine pins and then do the same thing every single time. I am imagining a 1/4 inch plate or better bolted to the bottom of the VNA with pins on it to fit the fixtures*
*its been a while since I used APC-7 connectors so I am not sure they are truely machine pin compatible, because you might need to tilt them, don't do this unless you figure out a connector that will work, I don't feel like looking for one to see how flat the face is right now, or if there is any spring loaded stuff that you don't want rubbing on a slide fit in there. I think it can work if that is a problem if you put the pins orthaganol to the plate so the fixture slides on from the front rather then slides to sit on top of the plate, or you bend them, but I feel like hard line would be best left as strait as possible without bends introduced into it, ready made coupler/extensions might be the best to use
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#74 Reply
Posted by
joeqsmith
on 23 Apr, 2021 17:59
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I have modified both the 1mOhm and 100uOhm boards to try and improve the DC measurements.
I'm sure the cables contribute some error, along with everything else. The question is which ones are causing the majority of the error.
Maybe ultrasonic cleaning the contacts and deoxit gold?
.....that goes to the VNA and then use all APC-7 connectors and retrofit the VNA with APC-7 so it can be put into exactly the same space every time ....
I'll leave you to do your thing.