[SI-LIST] Re: Length of an Impedance Discontinuity in order for it to be noticeable
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: otter30@xxxxxxxxxxxxx
- Date: Thu, 16 Jul 2009 18:08:20 -0400
Jim
At 30 MHz, this will not be an issue. Just make sure that there are
ample grounds in the bulkhead connector to reduce interference to/from
other signals that might be passing through the same connector.
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
otter30@xxxxxxxxxxxxx wrote:
> Hello,
> I am trying to decide if a 2" long impedance discontinuity in a coax cable
> will noticeably distort my video
> signal.
>
> This is an unusual situation where I am trying to bring video from a
> periscope into a submarine. At
> the point where the signal actually passes through the hull of the sub, it
> leaves the comfort of coax cable.
> The shield and the center conductor each connect to pins on a hull
> penetrating fitting and travel about 2"
> before re-entering coax inside the ship.
>
> The video BW is 30MHz and this works out to a wavelength of about 6m in the
> cable. Thats about 236 inches. The extent of the discontinuity relative to
> the wavelength is roughly .0085. My question is, will this
> cause any noticeable effect on the received signal?
>
> I've seen some TDR formulae on the impedance discontinuity extent relative to
> the TDR pulse rise time and read papers on how hard it is to detect spatially
> small (relative to rise time) discontinuities. The significant case that
> comes to mind is that of right angle bends in pcb traces.
>
> Does anyone know of similar rules for the extent of dicontinuities relative
> to wavelength? We've all seen the
> lambda / 20 criteria used in different arguments, but if the discontinuity
> extent is less than lamda / 20
> ( or lamda * .05) does that mean it is not an issue?
>
> There was a nice article on the Extron Electronics website where the author
> put 50ohm bnc connectors on a 12ft run of 75ohm coax. He them made TDR and
> step response measurements that were indistinguishable from those made with
> 75ohm bnc connectors on the same cable. I'm hoping my situation is similar.
>
> Any insight and/or experiences with this would be appreciated.
>
> Thanks,
> Jim
> run of 75ohm cable
>
>
>
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
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