[SI-LIST] Re: Length of an Impedance Discontinuity in order for it to be noticeable
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: tom dogastino <tom@xxxxxxxxxxxxx>, Scott McMorrow <scott@xxxxxxxxxxxxx>, <otter30@xxxxxxxxxxxxx>, si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Jul 2009 23:11:57 -0700
Jim
A 1VP-P analog video signal like I am assuming it is then it only takes 1
Millivolt of noise ( like Tom mentioned ) before it starts to show as noise (
small snow ) in the picture on the screen. One Millivolt of noise relative to
1VP-P is 60db signal to noise ratio which in old NTSC days is considered a good
S/N ratio. A 2VP-P signal or bigger would obviously improve signal to noise
ratio while <1VP-P would decrease this ratio.
Richard Jungert
> From: tom@xxxxxxxxxxxxx
> To: otter30@xxxxxxxxxxxxx; scott@xxxxxxxxxxxxx
> CC: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Length of an Impedance Discontinuity in order for it
> to be noticeable
> Date: Thu, 16 Jul 2009 18:26:14 -0700
>
> Jim
>
>
> Iâ??m more concerned with the discontinuity causing reflections and bandwidth
> loss. If there is a large loop before the bulkhead it will look inductive
> and be a low pass filter. This filter does what Ivor mentioned, causes lost
> of detail from reduced bandwidth and if you have a bunch of reflections
> bouncing around, ghosts. The more controlled the impedance the fewer
> problems with ghosts and BW loss. But yes, loops will increase crosstalk and
> the amount will be dependent on the loop area and the risetime (bandwidth) of
> the aggressor signal.
>
>
>
> The digital SI rule of thumb is discontinuities less than 1/6 of the signal
> will probably not be an issue but you are not dealing with a digital signal
> here. It is analog and you are dealing with video signal that can have a
> high sensitivity to noise. Good video is usually digitized to 10 bit
> resolution so any noise of 1/1000 of the peak signal can show up. It is best
> with your situation to do some simulations to see if you are going to run
> into problems or not.
>
>
>
> 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
>
>
>
> From: otter30@xxxxxxxxxxxxx [mailto:otter30@xxxxxxxxxxxxx]
> Sent: Thursday, July 16, 2009 5:22 PM
> To: tom@xxxxxxxxxxxxx; 'Scott McMorrow'
> Cc: si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Length of an Impedance Discontinuity in order for
> it to be noticeable
>
>
>
> Thanks for your replies Scott and Tom.
>
> The spacing for the pins mating with the shield and center conductors is very
> close to the separation
> of the shield and center conductor within the coax itself. The loop area
> would be pretty tight. I gather that you are both more concerned with
> interference as a result of the signal being unshielded than with the effect
> of the impedance discontinuity.
>
> Do either of you know of a general relation for how large an extent the
> discontinuity could span relative to
> signal wavelength before it becomes an issue? Every SI book I have says that
> the severity of the effect depends on the extent of the discontinuity
> relative to wavelength, but none of them quantify the relationship.
>
> I can see that the extent is well under lambda / 20, but I don't have a clear
> understanding of what this means.
>
> Thanks,
> Jim
>
> -------------- Original message from "Tom Dagostino" <tom@xxxxxxxxxxxxx>:
> --------------
>
>
> > 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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