[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 15:56:58 -0700
"I have been around way too many picky video engineers."
I'm one of them, and I stand by my remarks. The weighting factor for
high frequency detail is part of the research-based science of the
business. What the optics guys call modulation transfer function applies
to the human eye, and industry standard curves are used for noise
weighting in instruments that measure video S/N. It may vary somewhat
between individuals (usually on the lower bandwidth side; think
astigmatism, glasses, etc.), but even picky video engineers can't
compensate for their own eyeballs. It's much harder to see high frequency
impairments on a monitor than on a scope trace. Regarding impedance
mismatch, in my experience the impedance between adjacent signal pins in
a typical mil-type connector is usually in the 85 to 100 ohm range. At a
17 dB worst case return loss, this is not a huge mismatch to 75 ohms at
any frequency. We know that length matters. It's hard to believe that a
2" connector equivalent to less than lambda/100 at 30 MHz can contribute
any meaningful internal ringing for risetimes commensurate with that
frequency or even double that. Then consider that the spectrum of
baseband video for real scenes is typically much heavier on the low
frequency end than at the upper band edge, so there isn't much energy to
ring.
But the real bedrock is that a properly designed video system with a
single short-length mismatch between the video source and load, even a
bad one, will not ring. At most the signal level will drop. If there
is visible impairment, either the driver lacks a proper backmatch or the
monitor is misterminated. I don't see a problem here for the setup as
described. Therefore, the short answer to the original question remains
"No", or perhaps "No" to the fifth power. The periscope users will never
notice.
There is the potential for crosstalk where the shield goes through a pin
and other things implied by the physical setup, but those are separate
matters already mentioned.
Those who view their monitors through magnifying glasses get no sympathy.
If that's what it takes to see impairments, the design is pretty good.
There comes a time to shoot the engineer and ship the product.
Orin
On Fri, 17 Jul 2009 13:31:21 -0700 Richard Jungert
<r_jungert@xxxxxxxxxxx> writes:
> Olaney
>
> A super fast risetime signal can produce ringing and this ringing
> can easily show up in your picture. Slow down the risetimes of
> signals on the output and mask the problem. Hide it and dog gone it
> we will get it into production next week. Wait till a fussy
> customer complains about it and then we will fix it.
>
> Its easy to get caught up in all the subjectivity of video and
> perception and noise etc etc.
>
> In 8 bit systems its relatively easy to see the random noise and
> quantizing noise in the picture with a keen eye and a good
> magnifying glass.
>
> I have been around way too many picky video engineers.
>
> Richard Jungert
>
>
>
>
>
> > To: r_jungert@xxxxxxxxxxx
> > CC: tom@xxxxxxxxxxxxx; scott@xxxxxxxxxxxxx; otter30@xxxxxxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> > Date: Fri, 17 Jul 2009 04:50:53 -0700
> > Subject: Re: [SI-LIST] Re: Length of an Impedance Discontinuity in
> order for it to be noticeable
> > From: olaney@xxxxxxxx
> >
> > 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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