[SI-LIST] Re: Length of an Impedance Discontinuity in order for it to be noticeable
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: Richard Jungert <r_jungert@xxxxxxxxxxx>, tom dogastino <tom@xxxxxxxxxxxxx>, si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Jul 2009 08:40:58 -0700
I need some help here, and will give you my $0.02.
1) Are you folks implying that you can detect a 1/1000th error in the video
signal? Yes, some video cards will allow you ridiculously fine granularity,
but I don't know that it's worthwhile to ensure your system can support less
than -60dB from driver to receiver. It seems to me that, by the time you do
that, you'll have no time left for issues that really matter.
This rule doesn't seem reasonable. If it is, can you point me to a specific
procedure that outlines exactly how you check it?
Even if it did exist, I would think that even if it's detectable by measurement
devices, the human eye can't tell the difference. How do we put some sanity
into this?
I've played around with analog video a bit and have concluded (so far) that the
specifications far exceed what humans can detect on the best monitors
available. For instance, I've had systems with 2 video outputs, one that fails
the spec. and the other that doesn't. The difference was indistinguishable.
Gross errors, on the other hand, (wrong termination) are readily apparent. But
this is for servers, where it's not as critical as a super-gamer system.
I don't think I live alone in the world of finite resources, having to pick my
battles. I'd love to hear insight into what's reasonable to worry about.
2) While I'm asking questions - doesn't the 1/6 rule of thumb apply to "When do
I have to consider an element 'lumped' vs. 'distributed'?" I think the answer
to the question "When do I have to worry about a discontinuity?" is less strict
- about the risetime of the signal (Bogatin's "Simplified" book, page 330).
3) For my back-of-the-envelope calculation, I would say:
a) If the signal really is a 30MHz sinewave, the risetime is 0.35/30MHz
= 11.7ns, or >>2" (360ps, worst-case, probably much faster than that, 200ps, in
coax). No worries.
b) If the signal has a fast rise-time, say 1/10 the period, or 3.3ns,
the 2" (360ps) is only 1/9 of that. No worries.
c) If the signal has a really, really fast rise-time, say 500ps, the 2"
(360ps) is about equivalent to that. Discontinuity may be detectable on a
scope, but probably not on the monitor. No worries.
I'd conclude that this isn't worth worrying about, time to move onto something
more critical (perhaps like the grounding scheme, as Scott mentioned).
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Richard Jungert
Sent: Thursday, July 16, 2009 11:12 PM
To: tom dogastino; Scott McMorrow; otter30@xxxxxxxxxxxxx; si list freelist
Subject: [SI-LIST] Re: Length of an Impedance Discontinuity in order for it to
be noticeable
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
>
>
> I’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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