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how to meas Zo of FT caps?
I have a bunch of small feedthru caps that I need to measure their Zo
to assure it's 50 +/-10 ohms. Any thoughts on how to do it? I'm going to try to put a 50 ohm load on them and look at them with a network analyzer. I should be able to get it's Zo from the VSWR, no? Any other ways? |
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Not much more info. These are definetly feedthru caps and they want
them verified to be Zo of 50 +/- 10. ( We're a small parts testing house) Maybe they're meant to be signal feedthrus? I assumed they were for power lines ,but you're right; being Zo = 50 they must be for signals? On Tue, 06 Jul 2004 16:46:25 -0500, Bob Liesenfeld wrote: wrote: I have a bunch of small feedthru caps that I need to measure their Zo to assure it's 50 +/-10 ohms. Any thoughts on how to do it? I'm going to try to put a 50 ohm load on them and look at them with a network analyzer. I should be able to get it's Zo from the VSWR, no? Any other ways? Hmmmmm....Are you sure you mean feedthrough capacitors? My understanding of FT caps is that they are a very low Z from the terminal(s) to ground while allowing a DC voltage to pass through. I don't see where you are getting the 50 Ohm Z value. But, perhaps I have missed something. More info? Bob WB0POQ |
wrote in message ... I have a bunch of small feedthru caps that I need to measure their Zo to assure it's 50 +/-10 ohms. Any thoughts on how to do it? I'm going to try to put a 50 ohm load on them and look at them with a network analyzer. I should be able to get it's Zo from the VSWR, no? Any other ways? Feedthrough caps are designed to be low inductance caps to ground- typically used to ensure no RF gets through on DC or bias lines. Never heard of a 50 Ohm designed feedthrough. My junkbox is full of 0.001 and 0.01 feed throughs. To be 50 Ohms it would need to be coaxial in nature- they may exist but never saw one. Bulkhead barrel connectors would fulfill that requirement. Dale W4OP |
We're a small parts testing
house) I assume you do testing for profit, this is an Amateur Radio newsgroup. But, since you asked :-). These are definetly feedthru caps and they want them verified to be Zo of 50 +/- 10. The Zc of a capacitor (capacitive reactance) is frequency dependent. Not sure what Zo of a cap. refers to. At some frequency the capacitor will have a Zc=1/2*pi *f*c. I am sure you know that. An ideal capacitor will have a Zc=0-jZc. A real world capacitor will have a non-zero real part of R+-jX. To have the real part 50 ohms( Zo), must mean the frequency is in the VHF region or higher. To measure what you want, you need a Z meter with the capability of measuring the complex impedance of the feed-thru capacitors at the frequency of interest. I have an antenna analyzer that I use for that sort of thing, but it only reads capacitive reactance at the frequency of interest. Not sure if this helps? 73 Gary N4AST |
Thanks for all the responses. There's not much info here. The lab
manager gave me about 10 feedthru caps to verify the impedance. The "official" looking tag in the bag says "Zo=50+/-10 ohms". That's it. No freq, no capacitance. For sure they're feedthru caps, built like you're typical feedthru cap. I figured, for testing purposes, treat it like a small section of coax.I'm not even sure what freq they want it checked at. I'll have to get more details. |
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To determine whether you really have a feedthrough capacitor, use a
simple capacitance meter to measure the shunt C. (Surely an electronic parts measuring company has the capability to measure capacitance.) A feedthrough capacitor will probably measure somewhere between 100 pF to 0.1 uF, and the Z0 specification is meaningless, as others have pointed out. But if it's a coaxial transmission line feedthrough, the shunt C will be a pF or so at most. To measure the Z0 of a coaxial transmission line feedthrough, you'll probably need a high speed TDR, like the Tek TDS 820, 11801, or older gear with high speed TDR plugins. HP makes similar gear. The problem with using a network analyzer is that the line is probably too short to make a good measurement, unless you have a 40 GHz or higher frequency analyzer, a really good termination, and know how to interpret the results. The advantage of a TDR is that it graphically gives you the Z0 as a function of physical position, so you can see the Z0 of the feedthrough itself, independent of the connecting cable and the termination. However, to get the necessary resolution so you can see the Z0 of a physically short line like the feedthrough, you need a high speed TDR. That's exactly the sort of job that high speed TDRs are designed to do. There's also a cost issue. The last time I checked -- some time ago, admittedly -- a new good high speed TDR was about $25k, and a good 50 GHz network analyzer was roughly $500k. This price ratio would be about the same for rental units, also. Roy Lewallen, W7EL |
In article , Bill Turner
writes: On Tue, 06 Jul 2004 21:46:37 -0500, Bob Liesenfeld wrote: Curious. Very curious. From my experience, measuring the 'Zo of a feedthrough cap' would be something like measuring the 'color' of a miles/gallon rating of an automobile. Not an applicable unit of measurement. _________________________________________________ ________ Not at all. A feedthrough cap is essentially like a very short piece of coax. Think about it. If the capacitance is small, it could well "look" just like a short piece of 50 ohm coax. If the capacitance is large - 1000 pf or so - that's different. It would then look like a piece of coax with very low Zo. It all depends. No. 10 pFd at 1 GHz has a reactance of 15.9 Ohms. That tosses the VSWR in the bucket if that is part of a "line section." A "small capacitor" of 100 pFd at 100 MHz has the same reactance of 15.9 Ohms. Feed-throughs were designed to be SHORTS on purpose... |
Well here's what I did: I treated it like a peice of coax. I made a
fixture to hold the "feedthru" with semi-rigid coax on one side and a 50 ohm chip resistor on the other. I dropped in a short peice of coax and cal'ed to that. Then checked VSWR, was about 1.00something. Then I took out the coax, dropped in a few feedthrus and got VSWRs about 1.04 - 1.06, which works out to about 53 - 55 ohms or so. How's that sound? |
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Maybe one of the network analyzer experts will jump in at this point --
I'm not one. If I were your client, I certainly wouldn't be satisfied with your methodology. But I'm not your client, so what satisfies me isn't important. Good luck. Roy Lewallen, W7EL wrote: Well here's what I did: I treated it like a peice of coax. I made a fixture to hold the "feedthru" with semi-rigid coax on one side and a 50 ohm chip resistor on the other. I dropped in a short peice of coax and cal'ed to that. Then checked VSWR, was about 1.00something. Then I took out the coax, dropped in a few feedthrus and got VSWRs about 1.04 - 1.06, which works out to about 53 - 55 ohms or so. How's that sound? |
Could you suggest some way I could do it?
On Wed, 07 Jul 2004 18:29:31 -0700, Roy Lewallen wrote: Maybe one of the network analyzer experts will jump in at this point -- I'm not one. If I were your client, I certainly wouldn't be satisfied with your methodology. But I'm not your client, so what satisfies me isn't important. Good luck. Roy Lewallen, W7EL wrote: Well here's what I did: I treated it like a peice of coax. I made a fixture to hold the "feedthru" with semi-rigid coax on one side and a 50 ohm chip resistor on the other. I dropped in a short peice of coax and cal'ed to that. Then checked VSWR, was about 1.00something. Then I took out the coax, dropped in a few feedthrus and got VSWRs about 1.04 - 1.06, which works out to about 53 - 55 ohms or so. How's that sound? |
In article , Bill Turner
writes: On 07 Jul 2004 20:39:22 GMT, (Avery Fineman) wrote: No. 10 pFd at 1 GHz has a reactance of 15.9 Ohms. That tosses the VSWR in the bucket if that is part of a "line section." _________________________________________________ ________ A section of coax has both capacitance and inductance. Together, they form the Zo of the line. You are quoting only capacitance while ignoring the inductance. "Ignoring?" No. Spare me a lecture. 10 puffs IS a "small capacitor. You were not talking "inductance." 10 pFd IS a 15.9 Ohm reactance at 1 GHz. That's also a small reactive shunt on this mythical "line section." I grant you, a typical feedthrough capacitor is designed with large capacitance and is used for filtering, but that does not preclude one from being designed to "look" like a piece of coax if the designer wishes. And that was the original question. Sorry, no. The original question concerned testing of what LOOKED LIKE "feedthrough capacitors." The tester was thinking of "treating it like a small section of transmission line." Based on the response of actual testings, such as very low VSWR measured, the gadgets are probably just wire feed- throughs intended for something like passing through a hermetic seal. Those would have very little capacitance and thus would have very little discontinuity...the VSWR would not be raised very much. If those "feedtrhoughs" had really been capacitors, then, in all probability, they would have tested as capacitances on a bridge such as a 1 MHz bridge. A handheld C or L/C meter could have read that on a 1 nFd or 10 nFd scale. Without further ado, based on what was written by the person who has the devices, my opinion is that they are just wire feed- throughs with minimum capacitance and the inductance of a very short piece of straight wire. Those MIGHT work as "line sections" going through a hermetic seal wall of something...but not a very practical way to run a transmission line through that wall. Feed-through CAPACITORS were NEVER rated by "characteristic impedance" (Zo). Not in 1964, not in 2004, not in years between. Wire feedthroughs for sealing-type bulkheads are seldom rated for anything but current carrying capacity and withstanding or flash-over voltage. Those have just conductors going through with minimal capacity to "ground" and minimal inductance straight through. |
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Bill Turner wrote:
But think about this in relation to your statement: We both know of at least one person (and his boss) who wants to know the Zo of a feedthrough capacitor. Do you really think in forty years there has not been ONE OTHER person who might have actually designed such a thing? I've worked with at least ten "engineers" and dozens of bosses who, given a similar bag of feedthroughs containing a slip of paper saying "RPM = 2400 +- 1%", would work for a week designing a test to verify or disprove the spec. -- Spammers are people who are too lazy and cowardly to rob liquor stores, but still want to make money by stealing instead of working. -- Morely Dotes, The Open Sourceror's Apprentice |
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Wes wrote,
Message-id: On Wed, 07 Jul 2004 18:34:15 -0400, wrote: |Well here's what I did: I treated it like a peice of coax. I made a |fixture to hold the "feedthru" with semi-rigid coax on one side and a |50 ohm chip resistor on the other. I dropped in a short peice of coax |and cal'ed to that. Then checked VSWR, was about 1.00something. Then I |took out the coax, dropped in a few feedthrus and got VSWRs about 1.04 |- 1.06, which works out to about 53 - 55 ohms or so. | How's that sound? Like you don't know what you're doing. Your DUT is a two-port device. If you really have a network analyzer, why aren't you using it to measure the device as a two port? Is two port theory even being taught in engineering schools these days? 73, Tom Donaly, KA6RUH |
They're 50 ohm RF and DC feedthrus. Here's the company:
http://www.shp-seals.com/products_and_Services.htm Click on bulletin 100. Most50 ohm types gave a VSWR of 1.05ish, giving about 53 ( or 47 ) ohms. I also had a few of the DC type that gave a VSWR of 1.4ish, leading to about 25 ohms ( or 75 ohms, could I check the phase to see which?). |
On Thu, 08 Jul 2004 17:41:07 -0400, wrote:
They're 50 ohm RF and DC feedthrus. Here's the company: http://www.shp-seals.com/products_and_Services.htm Click on bulletin 100. Most50 ohm types gave a VSWR of 1.05ish, giving about 53 ( or 47 ) ohms. I also had a few of the DC type that gave a VSWR of 1.4ish, leading to about 25 ohms ( or 75 ohms, could I check the phase to see which?). suppose they are 50ohm DC with 45° phase angle ? --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm |
On 08 Jul 2004 18:05:51 GMT, (Tdonaly) wrote:
|Wes wrote, |Message-id: | |On Wed, 07 Jul 2004 18:34:15 -0400, wrote: | ||Well here's what I did: I treated it like a peice of coax. I made a ||fixture to hold the "feedthru" with semi-rigid coax on one side and a ||50 ohm chip resistor on the other. I dropped in a short peice of coax ||and cal'ed to that. Then checked VSWR, was about 1.00something. Then I ||took out the coax, dropped in a few feedthrus and got VSWRs about 1.04 ||- 1.06, which works out to about 53 - 55 ohms or so. || How's that sound? | |Like you don't know what you're doing. | |Your DUT is a two-port device. If you really have a network analyzer, |why aren't you using it to measure the device as a two port? | | | | |Is two port theory even being taught in engineering schools these days? Beats me Tom. I'm retired, I know nuttin' 'bout engineering school. I do know that if someone had brought me these devices to test, the first thing I would have done would be to try and talk them out of doing it. I did that often. I used to see things like big LC power filters with a solder lug on one side and two feet of shield wire coming out of the other side with a pigtail on the far end and an attenuation spec that went from 10 Hz to 10 GHz. Damn fool engineers would think, well I've got an AMRAAM missile with a 10 GHz transmitter, so I better have a filter that rejects 10 GHz. Trying to explain that the device can't be measured to the design engineer and the spec writer was almost a pointless exercise. I can say that after my first retirement and subsequent rehire to manage a components engineering group that was doing the spec writing, we didn't do that crap anymore. Specing and measuring feedthru filters was one of the most pointless things we ever did. The specs all measure attenuation in a 50 ohm system and the filters are never used in a 50 ohm system. In fact, nobody ever knows what the source and load Z are at the test frequencies so they don't know what they need or what they get. Oh well, it was a pretty good living. Thank you taxpayers. |
Wes wrote:
Specing and measuring feedthru filters was one of the most pointless things we ever did. The specs all measure attenuation in a 50 ohm system and the filters are never used in a 50 ohm system. In fact, nobody ever knows what the source and load Z are at the test frequencies so they don't know what they need or what they get. Wes makes a valid point about the source and load impedances being unknown in practical applications. The filter performance is going to be different from the performance measured in a 50-ohm system. But that doesn't make a 50-ohm measurement completely "pointless". In order to select a filter, you have to make *some* attempt to characterize the performance of the available options, under some kind of standard conditions. If the source and load impedances are totally unknown, a 50-ohm environment is actually not a bad choice for a standard test, since it's neither extremely high nor extremely low. If you have a better handle on the practical source and load Z, then it makes sense to define a different standard test environment - for example, the standard Line Impedance Simulating Network (LISN) used for AC line/mains filters has a much lower source Z. The common-sense solution is to have standard tests, but understand their limitations. Unfortunately - as Wes knows, all too well - military spec writers aren't noted for either common sense or understanding. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
In article , Bill Turner
writes: On 08 Jul 2004 03:42:41 GMT, (Avery Fineman) wrote: massive snip Feed-through CAPACITORS were NEVER rated by "characteristic impedance" (Zo). Not in 1964, not in 2004, not in years between. _________________________________________________ ________ How dare I disagree with someone who knows about every feedthrough capacitor ever made over a forty year period by every manufacturer in the world. Shame on me. Why? But think about this in relation to your statement: We both know of at least one person (and his boss) who wants to know the Zo of a feedthrough capacitor. Do you really think in forty years there has not been ONE OTHER person who might have actually designed such a thing? ...on April 1st? The famous "Write-Only Memory" or WOM comes to mind. :-) The original question involved "Zo of a feedthrough CAPACITOR." Capacitors rated in "Zo" of a transmission line? I don't think so. I've never seen that in 55 years. Maybe you have? Capacitors are rated first in Farads. Next in working voltage. In "characteristic impedance?" I doubt it happened even once. The word "never" is powerful. Be careful with it. Always. :-) Right yer are, guv'nor, beggin yer pardon, yer lordship. bows respectfully, backs out through castle entrance... |
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