Thanks Ken - And therein lies the problem ( - that is: Differentiating between a "good" and a "bad" cable is difficult unless the "bad" cable is at the extreme i.e. o/c. I have run radiated experiments using fixtures and did see a delta between cable manf. However (due of course to the complex test environment) the deltas did not translate into meaningful results at the site..
I suppose that - like many other shielded cables - there is no meaningful way of determining a "bad" cable other than inspection. Thanks Charles Grasso (w) 303-706-5467 From: Ken Javor [mailto:[email protected]] Sent: Friday, April 20, 2018 3:16 PM To: [email protected] Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements This message originated outside of DISH and was sent by: [email protected]<mailto:[email protected]> Yet another thing I meant to mention but forgot is that since the HDMI cable with all conductors connected together does not represent a 50 ohm controlled impedance transmission line, if the cable is electrically long what you measure by connecting a 50 ohm receiver to one end is going to depend on just how long the cable is. Just another reason to keep things electrically short. And assuming that the need for cable shield SE is from 30 - 1000+ MHz, you want to use the highest frequency at which the cable is electrically short. Obviously you could use 10 kHz, but at that frequency you might not be able to tell the difference between a pigtail and a 360 degree peripheral termination. If my brain were analogized to an engine, we could say it was fine but that my transmission keeps popping out of gear... Ken Javor Phone: (256) 650-5261 ________________________________ From: Ken Javor <[email protected]<mailto:[email protected]>> Date: Fri, 20 Apr 2018 14:43:17 -0500 To: "Grasso, Charles" <[email protected]<mailto:[email protected]>>, "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Conversation: [PSES] Using a TDR for Shielding Effectiveness measurements Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements One thing I should have mentioned as well is that if the cable is electrically long, then in addition to an ugly waveform, the current measured by the probe is probe-position dependent. That means what you measure as ZT isn't accurate if you pick any one probe position. You need the average. But you don't need accurate ZT, just repeatable. So if you pick a probe position (must be) near the cable end connected to the receiver, then IFF (if and only if) each cable tested is identical in length and construction, you ought to get repeatable results. But if the cables are of varying length and electrically long, you can only compare against identical lengths. If you keep the cables electrically short, these problems vanish. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Grasso, Charles" <[email protected]<mailto:[email protected]>> Date: Fri, 20 Apr 2018 19:36:40 +0000 To: Ken Javor <[email protected]<mailto:[email protected]>>, "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Conversation: [PSES] Using a TDR for Shielding Effectiveness measurements Subject: RE: [PSES] Using a TDR for Shielding Effectiveness measurements Thank you Ken and Istvan for your insightful comments. As can be seen from the minimal response set - the problem of evaluating incoming inspection is an " opportunity" to excel! I doubt that the TDR will be the tester of choice - or even if there is an easy solution to the problem. Thanks Charles Grasso (w) 303-706-5467 From: Ken Javor [mailto:[email protected]] Sent: Friday, April 20, 2018 12:00 PM To: [email protected]<mailto:[email protected]> Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements This message originated outside of DISH and was sent by: [email protected]<mailto:[email protected]> Talk about cluelessness, I didn't even realize the OP was about cable shield SE... Totally ignorant of the standard Mr. Woodgate is citing so this post is an addition to, not in lieu of or in contradiction to whatever that standard is about. I also don't know what HDMI cable looks like in terms of shield configuration. I'm assuming here that there are several conductors within an overall shield. If it were me, just looking for quick incoming quality check, I'd build a fixture that provided for proper termination of the cable connectors, and have that mating connector bonded to a bracket that was in turn bonded to a ground plane. I would inject using a BCI method a signal whose spectrum was such that the cable was electrically very short. The mating connectors would be wired so all interior conductors were tied together, and terminated in 50 ohms on one one side and in an EMI receiver on the other. Using a current probe to monitor injected current, and an arbitrary waveform generator (these days) as the injection stimulus, I would select the aforementioned frequency range (or frequency) where the cable is electrically short. The resultant transfer impedance measurement would be the metric of interest. This could be considerably simplified using a spectrum analyzer with tracking generator. And if the cable SE isn't very high (high ZT) you could get by with a high level impulse such as from a MIL-STD-461 CS115 generator (easily cobbled together if you don't have one or want to buy one) and an o'scope to read the current coupled signal. With CS115, you can get 5 amps on the cable and be able to pick up say 50 mV peak, for a transfer impedance of 10 milliohms. But the problem to watch for in a TD measurement is that if the stimulus is fast relative to the electrical length of the cable, then the induced current (response) will "wash out" and you get an ugly current waveform, and hence an ugly coupled potential, and thus difficult to take the ratio. So the risetime and duration of a usable impulse are dependent on how long the cable-under-test is. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Grasso, Charles" <[email protected]<mailto:[email protected]>> Reply-To: "Grasso, Charles" <[email protected]<mailto:[email protected]>> Date: Fri, 20 Apr 2018 15:48:57 +0000 To: <[email protected]<mailto:[email protected]>> Conversation: [PSES] Using a TDR for Shielding Effectiveness measurements Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements Thanks John.. From: John Woodgate [mailto:[email protected]] Sent: Friday, April 20, 2018 9:40 AM To: Grasso, Charles <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements This message originated outside of DISH and was sent by: [email protected]<mailto:[email protected]> Go to: http://www.iec.ch/dyn/www/f?p=103:22:0::::FSP_ORG_ID:1247 and look for publications in the 62153 series. There are several. John Woodgate OOO-Own Opinions Only J M Woodgate and Associates www.woodjohn.uk<http://www.woodjohn.uk> <http://www.woodjohn.uk><http://www.woodjohn.uk> <http://www.woodjohn.uk><http://www.woodjohn.uk> Rayleigh, Essex UK On 2018-04-20 16:05, Grasso, Charles wrote: Thanks to all that replied. The task at hand is to devise a (relatively) simple measurement of the shielding quality at incoming inspection - hence the thought of using a TDR to qualitatively assess (in this case) the shielding of HDMI cables. There would be measurements of the sample cables compared to the performance of a known good cable. So I am not at specific SE number rather the measurement of the construction that affects the SE. I realize now that my ask was poorly worded. I am (well!) aware of the difficulties of this approach (for example : false positives and false negatives) so maybe I should restate the question: How does one measure samples of a manufactured lot of cables for shielding quality? I'd be interested in suggestions! Thanks Charles Grasso (w) 303-706-5467 From: Ken Javor [mailto:[email protected]] Sent: Thursday, April 19, 2018 11:44 PM To: [email protected]<mailto:[email protected]> Subject: Re: [PSES] Using a TDR for Shielding Effectiveness measurements This message originated outside of DISH and was sent by: [email protected]<mailto:[email protected]> Given the few responses to date, I'll fess up to cluelessness and curiosity. Aside from the obvious dynamic range limitation working in the time domain, if you limit yourself to the time domain, how do you map SE vs. frequency? And given the inherent wide band nature of a TDR signal, I can't see it being radiated properly. I'm thinking you would have to use something like an ASTM 4936 coaxial test fixture in order to main signal integrity up to the reflection point where the sample material puck is installed. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Grasso, Charles" <[email protected]<mailto:[email protected]>> Reply-To: "Grasso, Charles" <[email protected]<mailto:[email protected]>> Date: Thu, 19 Apr 2018 15:23:27 +0000 To: <[email protected]<mailto:[email protected]>> Conversation: Using a TDR for Shielding Effectiveness measurements Subject: [PSES] Using a TDR for Shielding Effectiveness measurements Hello all, In lieu of using shielded chambers/spectrum analyzers and such, has anyone used a TDR for performing shielding effectiveness measurements? The simplicity is tempting - but I am dubious about the dynamic range? I have poked about in the usual place - but didn't find any definitive studies. (I suppose that is a clue in and of itself!) Thanks Charles Grasso (w) 303-706-5467 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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