[SI-LIST] Length of an Impedance Discontinuity in order for it to be noticeable
- From: otter30@xxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 16 Jul 2009 20:33:46 +0000
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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