[SI-LIST] Re: Length of an Impedance Discontinuity in order for it to be noticeable
- From: olaney@xxxxxxxx
- To: r_jungert@xxxxxxxxxxx
- Date: Fri, 17 Jul 2009 04:50:53 -0700
After all is said and done, the short answer to the question of whether a
two inch discontinuity will mess up your video signal is "No". That is,
unless you go out of your way to mess it up in the ways mentioned. I'd
allocate more ground pass through pins than one, and I'd choose them all
adjacent to the signal pin, assuming you have the option. It amounts to
the same tricks pulled on PCB vias to approximate coax.
Other than that, the situation is not as bad as it might appear for
several reasons peculiar to video. One is that video signal to noise as
perceived by the viewer is weighted by the aperture function of the human
eye, which acts like a low pass filter. Video S/N calculations take this
into account, along with viewing distance and other things. Basically,
the short discontinuity will affect high frequency picture detail most,
and this is where it matters least. Perceived S/N will be 10 to 15 dB
better here than simple calculations would indicate.
The second thing in your favor is that the gray scale discrimination of
the eye isn't better than about 8 bits. You can perceive contouring
artifacts at that precision if you have a stationary image, know what to
look for, and use the more critical portion of the brightness range under
ideal viewing conditions. Under normal conditions you won't notice. The
reason for 10 bits (and sometimes more) is to ensure you have at least 8
bits of precision after gamma correction and other things that stretch
the signal. If your system has 10 bit precision clear to the screen,
errors of a few LSB are invisible.
The effect of a discontinuity much shorter than wavelength can be thought
of as proportional to the product of the length and how bad the mismatch
is. The shorter the length and less the mismatch, the less it matters.
On the other hand, even a millimeter of severe mismatch cannot be
ignored. An inch or so of 50 versus 75 ohms is far from severe at
baseband video bandwidths, and the app note results you mentioned are
correct.
Orin Laney
On Thu, 16 Jul 2009 23:11:57 -0700 Richard Jungert
<r_jungert@xxxxxxxxxxx> writes:
> 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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