measure characteristic impedance of feedthrough panel connectors.

Gents,

I have an application where there is 10Mbit ethernet that must go through 20 hermetically sealed feedthrough connectors over a 200 foot run. each connector is ~ 4" long. These are custom made feedthroughs due to the environment of the application. The feedthroughs were specified to have a characteristic impedance of 100 - 150 ohms to match the 120 ohm twisted pair wires carrying the ethernet signal. After we got them there was no documentation showing a characteristic impedance test. When we asked the manufacturer they said the didn't have the capacity to measure them but built them to equations and they should be fine.

How can I measure the characteristic impedance of these feedthrough connctors to verify they meet spec?

The obvious fallback plan is to monitor the network signl quality with and without the connectors installed but I'd like a more quantitative measurement of them.

Or am I way off base and the characterictic impedance of the connectors will have a miniscule affect. My RF experience is pretty light but I notice that some coax connectors are rated for 50 or 75 ohms impedance. Don't know if that is the impedance of the connector itself or some termination resistance inside the connector.

You help is appreciated.

thanks

Reply to
mook johnson
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I am not an expert but I think that the preferred method would be to use an instrument called a Network Analyzer. But a Spectrum Analyzer with a tracking generator output could probably be used instead. The impedance will vary versus frequency. So you need to know over what frequency range the impedance should be within spec. As a guess, I'd say that would be from 10 MHz up to maybe 100 MHz. But that's just a guess.

You should probably also be worrying about the total insertion loss from going through 20 of them, not to mention all of the reflections that might occur on the line. Maybe sealed cable pass-throughs would have been better.

Reply to
Tom Gootee

Time Domain Reflectometer (TDR). Terminate the "far" end with a 120 ohm non-inductive resistor, and you're off to the races! ;-)

Have Fun! Rich

Reply to
Rich Grise

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10Mbit Ethernet was compatible with four different wiring schemes; AUI had power and separate shielded pairs, 10baseT is the familiar two- twisted-pairs scheme, 10base500 was multiple shielded coax (thickwire), and 10base200 was a single shield coax cable. I'd have used the easy scheme, 10base200, with 50 ohm feedthroughs; it sounds like this is 10baseT instead.

The wavelength of the third harmonic of 10 Mbit signals is about 10 meters; your connectors are much shorter, so a reflection test is enough. Pulse generator-> long cable -> feedthrough -> long cable ->

terminator and look for reflections at the generator end.

Reply to
whit3rd

Differential TDR would work. A Tek 11801/SD24 if you can get access to one. I could do it for you.

John

Reply to
John Larkin

Yes, many RF connectors are rated by their characteristic impedance. The commonest (most BNCs, N connectors, SMA etc.) is 50 ohms. One can buy BNCs with a 75-ohm characteristic impedance. In either case, if the connector is a terminator it'll have only one accessible terminal... the termination resistor takes up the other side.

The characteristic impedance of a connector or feedthrough can become a significant issue if the connector is a significant fraction of a wavelength. If your feedthroughs aren't fairly close to 120 ohms, then at a 4" length they might be long enough to cause enough of a reflection in the line to affect signal reliablity, I suspect.

What I'd suggest that you do, in order to determine signal quality, is to use a TDR (time-domain reflectometry) measurement system. Basically, it's a combination of a pulse generator with a well-controlled source impedance (in your case, 120 ohms) and an oscilloscope, connected to the input of your twisted-pair line. The other end of the line should be terminated by the line's 120-ohm characteristic impedance. By looking at the trace on the oscilloscope screen, you can actually see the effect of any impedance discontinuities in the line, and figure out how far from you they are (figure roughly 1 foot of wire between you and the glitch, for every nanosecond between the point at which the pulse starts and the point at which you see a squiggle or jump in the oscilloscope trace).

There are industrial TDR setups made specifically for scoping out Ethernet (and other network) wiring problems. Or, you can improvise one with a 'scope, a fast pulse generator, a resistor, and some cable and connectors.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

thanks guys!

Reply to
mook johnson

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Pretty much, it may be handy to have a few meters of known good cable is useful places so that every thing can be seen clearly.

Reply to
JosephKK

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