[SI-LIST] Re: Length of an Impedance Discontinuity in order for it to be noticeable
- From: "Bowden, Ivor" <ibowden@xxxxxxxxxxxxxxxxx>
- To: <otter30@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Jul 2009 17:09:05 -0700
And this is analog video(?), so it isn’t a strictly "make or break" kind of
thing. 30MHz is around VGA (640x480 non-interlaced) pixel rate, or the
bandwidth of sharp RS-170 (monochrome, interlaced). Impedance discontinuities
tend to show up on the screen as visible "ghosts" (reflections), while
bandwidth limits tend to show as left to right smearing of sharp vertical
edges. If you know the characteristics of your bulkhead adapter you could run a
Spice simulation?
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Tom Dagostino
Sent: Thursday, July 16, 2009 4:31 PM
To: 'Scott McMorrow'; otter30@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Length of an Impedance Discontinuity in order for it to
be noticeable
I'll agree with Scott but want to add that lead dress may make or break
this. The ground should be adjacent to the signal and the loop of exposed
center conductor/ground should be as small as possible on both sides of the
bulkhead.
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Thursday, July 16, 2009 3:08 PM
To: otter30@xxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Length of an Impedance Discontinuity in order for it
to be noticeable
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
TeraspeedR 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
TeraspeedR is the registered service mark of
Teraspeed Consulting Group LLC
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