Re: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES

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From: Chandan (chandan_career@yahoo.com)
Date: Thu May 24 2001 - 07:16:02 PDT


Hi,

Here are my comments:
For a transmission line with several CMOS receivers, I
am convinced that the input capacitance of the CMOS
receivers can be considered as a distributed load on
the transmission line. In which case, equation 4.77 on
page 173 of " A Handbook of Black magic" is
applicable. My concern was for a net topology in which
a trace is connected to a SINGLE receiver at the end
of the line. My understanding is that the single
receiver should be considered as a lumped load for the
following reasons:
1. The SINGLE receiver is placed at one point of the
line.
2. There is an element of non-uniformity that I cant
put in words; The SINGLE receiver
represents a good deviation in capacitance. (Typically
CMOS receivers have an input capacitance of 10pF,
whereas typical line capacitances are in the range of
2 to 3pF - assuming a line with an unloaded
characteristic impedance of 60 ohm & a velocity of 6
inch/ns).
In fact I imagined TWO LUMPED loads; the driver's
output capacitance & the receiver's input
capacitance.
Kindly comment, since I am not fully convinced that a
single receiver on a transmission line can be
considered as a distributed load.

I also acknowledge that capacitive loading on a
transmission line will reduce its characteristic
impedance. However, I was curious to understand the
relationships involved for lumped loads. The equation
that I presented in my initial e-mail (from page 10 of
Motorola's AN1051) as well as eqn. 4.77 of Howard
Johnson's book are one & the same and speak of the
effect of distributed capacitance on a transmission
line. Howard Johnson also speaks about
"distributing the capacitance uniformly". In the case
of a single receiver, the designer
cannot distribute capacitance uniformly as there is
only one (or 2) capacitance to distribute.

Too some extent, I agree with Dave's
(david_instone@uk.xyratex.com) analysis. I believe
that
if the load at the end of the TL is not visible to
the driver during its transition (H to L
or L to H), then the driver "sees" an unloaded
characteristic impedance. PLease comment.

ANOTHER QUESTION:
If the net topology in which a trace is connected to a
SINGLE receiver at the end is indeed a
distributed-load transmission line, do you think that
I would get a good approximation of the
value of loaded characteristic impedance by
substituting N = 2 in equation 4.77 (page 173 of
Black magic)?

Thanks,
Chandan

--- David Instone <david_instone@uk.xyratex.com>
wrote:
> Well if Chandan isn't confused I am!
> A simple experiment with a TDR shows that if the
> line is long enough for
> a pulse edge to have left the driver and not yet
> reached the capacitor
> the line looks like a TL of Z0 (ignoring the cap)
> and the capacitor acts
> as a capacitive load. In this case the series
> resistor+Rdriver should
> match Z0. If the line is short enough or the rise
> time long enough that
> the pulse has not completely left the driver before
> it hits the
> capacitor then it should probably be considered as a
> distributed
> capacitance when calculating the series resistor.
> The problem comes
> when the range of risetimes possible spans both
> conditions!
> Or am I being over simplistic?
> Now on the same theme, I believe that if lumped caps
> are distributed
> along a TL and the distance between them is greater
> than a rise time
> then the TL should be treated as a TL of unloaded Z0
> with capacitive
> discontinuities and the terminators should match
> unloaded Z0 (unless
> they are closer than a risetime to a cap), only if
> the caps are closer
> together than a risetime should Z0 be considered to
> have been lowered
> and the terminators matched to loaded Z0 (unless
> they are further from
> the last/first cap than a risetime).
> Again am I right or over simplistic/just plain
> wrong.
>
> Dave
>
> Degerstrom, Michael J. wrote:
> >
> > Ken,
> >
> > No problem - I was trying to do the same thing for
> Chandan
> > as my feeling was that he was trying to apply this
> > Zoprime formula when it wasn't apparent that he
> understood
> > its utility. I'm sure many of us have difficulty
> interpreting
> > the original posts to this bulletin board.
> Hopefully one
> > or both of our posts were beneficial.
> >
> > Mike
> >
_______________________________________________________________
> > Mike Degerstrom Email:
> degerstrom.michael@mayo.edu
> > Mayo Clinic; 200 1st Street SW ; Rochester, MN
> 55905
> > Phone: (507) 538-5462 FAX: (507) 284-9171
> > WWW:
> http://www.mayo.edu/sppdg/sppdg_home_page.html
> >
______________________________________________________________
> >
> > > -----Original Message-----
> > > From: Ken Cantrell
> [mailto:Ken.Cantrell@srccomp.com]
> > > Sent: Wednesday, May 23, 2001 12:02 PM
> > > To: Degerstrom, Michael J.
> > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD
> ON TRANSMISSION LINES
> > >
> > >
>
> > > Mike,
> > > What I was getting at, if you re-examine at
> Chandan's
> > > message, is that he
> > > doesn't understand the basic material yet. Note
> his formula
> > > for Zoprime. I
> > > thought your answer was more advanced than the
> question that
> > > he asked, and I
> > > didn't want him to miss the fundamentals.
> > > Ken
> > >
> > > -----Original Message-----
> > > From: owner-si-list@silab.eng.sun.com
> > > [mailto:owner-si-list@silab.eng.sun.com]On
> Behalf Of
> > > Degerstrom, Michael
> > > J.
> > > Sent: Wednesday, May 23, 2001 9:37 AM
> > > To: Ken.Cantrell@srccomp.com;
> si-list@silab.eng.sun.com
> > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD
> ON TRANSMISSION LINES
> > >
> > >
> > > Ken,
> > >
> > > I'm not sure why you would recommend to use an
> approach that
> > > takes more effort to implement and then provides
> less accurate
> > > SI results. You can use this technique where
> lumped load
> > > capacitance is included into the transmission
> line capacitance
> > > only for certain net topologies. But using this
> approach will
> > > not allow you to see the capacitive reflections
> and any stubbing
> > > effects from package leads. Also, you may not
> be predicting
> > > the delay from your source to your load
> accurately.
> > >
> > > Mike
> > >
> > >
>
_______________________________________________________________
> > > Mike Degerstrom Email:
> degerstrom.michael@mayo.edu
> > > Mayo Clinic; 200 1st Street SW ; Rochester, MN
> 55905
> > > Phone: (507) 538-5462 FAX: (507) 284-9171
> > > WWW:
> http://www.mayo.edu/sppdg/sppdg_home_page.html
> > >
>
_______________________________________________________________
> > >
> > >
> > > > -----Original Message-----
> > > > From: Ken Cantrell
> [mailto:Ken.Cantrell@srccomp.com]
> > > > Sent: Wednesday, May 23, 2001 9:55 AM
> > > > To: Degerstrom, Michael J.;
> chandan_career@yahoo.com;
> > > > si-list@silab.eng.sun.com
> > > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD
> ON TRANSMISSION LINES
> > > >
> > > >
> > > > Chandan,
> > > > The receiver capacitance is modeled as part of
> the toal distributed
> > > > capacitance where the new capacitance (Cprime)
> is equal to the [line
> > > > capacitance plus (the load capacitance * the
> number of loads
> > > > divided by the
> > > > length of the transmission line)]. Zo prime
> is then equal to sqrt
> > > > (L/Cprime). It's effect will be to lower Zo.
> I refer you to Howard
> > > > Johnson's book, page 173, section 4.4.3.1.
> The only time I
> > > treat the
> > > > receiver capacitance as lumped is in a
> bi-directional mode where the
> > > > receiver capacitance introduces a group delay
> equal to 2*Zo*Cl.
> > > > Ken
> > > >
> > > > -----Original Message-----
> > > > From: owner-si-list@silab.eng.sun.com
> > > > [mailto:owner-si-list@silab.eng.sun.com]On
> Behalf Of
> > > > Degerstrom, Michael
> > > > J.
> > > > Sent: Wednesday, May 23, 2001 7:39 AM
> > > > To: chandan_career@yahoo.com;
> si-list@silab.eng.sun.com
> > > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD
> ON TRANSMISSION LINES
> > > >
> > > >
> > > > Model the transmission line as a transmission
> line and the
> > > > load should be modeled as a lumped
> capacitance. You
> > > > may also want to model the package inductance
> and capacitance
> > > > depending on your application. Some of the
> load capacitance
> > > > may have series resistance due to ESD filter
> circuitry but
> > > > I rarely see vendors supplying this
> information unless it
> > > > it is factored into the IBIS model package
> resistance.
> > > >
> > > > Mike
> > > >
_____________________________________________________________
> > > > Mike Degerstrom Email:
> degerstrom.michael@mayo.edu
> > > > Mayo Clinic; 200 1st Street SW ; Rochester,
> MN 55905
> > > > Phone: (507) 538-5462 FAX: (507) 284-9171
> > > > WWW:
> http://www.mayo.edu/sppdg/sppdg_home_page.html
> > > >
>
_______________________________________________________________
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Chandan
> [mailto:chandan_career@yahoo.com]
> > > > > Sent: Wednesday, May 23, 2001 4:00 AM
> > > > > To: SI FORUM
> > > > > Subject: [SI-LIST] : EFFECT OF LUMPED LOAD
> ON TRANSMISSION LINES
> > > > >
> > > > >
> > > > > Hi,
> > > > >
> > > > > Consider a long trace - long enough for it
> to be
> > > > > considered as a transmission line. Let this
> trace be
> > > > > connected between a CMOS driver & a CMOS
> receiver. The
> > > > > receiver can therefore be modeled as a
> capacitive
> > > > > load at the end of the line.
> > > > >
> > > > > 1. Should this capacitor be considered as a
> lumped
> > > > > element or distributed?
> > > > > This question assumes importance when
> point-to-point
> > > > > clock traces are routed between a
> synthesizer & a
> > > > > receiver. Ideally, the value of series (or
> source)
> > > > > termination resistance that must be used is
> the
> > > > > difference between the characteristic
> impedance and
> > > > > the driver's internal resistance. I was
> wondering if I
> > > > > should use the loaded characteristic
> impedance or
> > > > > unloaded characteristic impedance. I then
> began to
> > > > > wonder if I should consider the load as
> distributed or
> > > > > lumped. Kindly comment.
> > > > >
> > > > > 2. Does the lumped capacitance affect the
> properties
> > > > > of a transmission line? If so, how?
> > > > >
> > > > > 3. The following approximation describes the
> > > > > relationship between the loaded
> characteristic
> > > > > impedance, unloaded characteristic impedance
> and
> > > > > the value of DISTRIBUTED capacitance:
> > > > >
> > > > > Z0' = Z0/ROOT OF (1 + CL/CO)
> > > > >
> > > > > where Z0' = Loaded characteristic impedance
> > > > > Z0 = Unloaded (Intrinsic) characteristic
> impedance
> > > > > CL = Load capacitance/unit length
> > > > > C0 = Intrinsic capacitance/unit length
> > > > >
> > > > > Is there any relationship between loaded
> > > > > characteristic impedance, unloaded
> characteristic
> > > > > impedance and the value of LUMPED
> capacitance?
> > > > >
> > > > > Thanks,
> > > > > Chandan

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