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?

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

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.

wrote:

Thanks for all the responses. There's not much info here. The lab
manager gave me about 10 feedthru caps to verify the impedance.

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.

The
"official" looking tag in the bag says "Zo=50+/-10 ohms".

And no manufacturer or part number on said official looking tag?

If it were April, and you were a new employee, I might suspect a bit of
chicanery here. ;) ;)

Keep us posted. I'm always willing to learn new stuff. :)

Bob WB0POQ

On Tue, 06 Jul 2004 20:02:28 -0400 wrote:

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.

Sounds to me like they are just feedthrus, not feedthru caps.

I don't think there's any way that a feedthru cap could have a Zo of
50 Ohms (pure resistive.) They are simply not made to pass RF; their
purpose is to serve as a short to RF.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

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...