[SI-LIST] Re: Diff line

  • From: Ben Rothchild <benrothchild@xxxxxxxxx>
  • To: scott@xxxxxxxxxxxxxxxx
  • Date: Mon, 18 Feb 2002 20:41:29 -0800 (PST)

Hi Scott,

Thank you Sir for patiently clarifying
my doubts.

Regards,
Ben

--- Scott McMorrow <scott@xxxxxxxxxxxxxxxx> wrote:
> 
> Ben,
> 
> Here are the answers to your questions.
> 
> >How is impedance control dependent on whether
> >they are loosely/tightly coupled.
> >
> With loosely coupled differential pairs,
> manufacturing etch contol 
> affects the width of the traces, which alters the
> impedance.  For 
> tightly coupled differential pairs, the width
> between the pairs is also 
> greatly altered by the etch dimensions.  Impedance
> of a differential 
> pair trace is controled by the width of the trace
> and the separation of 
> the edges.  The percentage change for both is higher
> with a tightly 
> coupled differential pair.
> 
> >What is even mode return loss and how is coupling
> >affecting this.
> >
> Even mode return loss is S11 for the common mode
> signal (the part of the 
> signal that is not differential and is caused by
> driver and delay skew) 
> and is the reflected energy that returns back to the
> driver.  Higher 
> return loss means more common mode energy is
> bouncing around.   This is 
> usually due to impedance mismatch in the system. 
> Tighter trace coupling 
> causes a larger divergence in odd and even mode
> impedances.  If the 
> system is matched for odd mode (differential
> transmission) then the even 
> mode suffers greater mismatch and return loss.  This
> can become a 
> problem when the even mode return loss sets up
> unwanted resonance 
> conditions in the system.
> 
> >How is insertion loss affected by coupling.
> >
> To achieve the same differential impedances,
> conductor width must be 
> smaller for tightly coupled differential pairs, when
> compared to loosely 
> coupled differential pairs.  This smaller width
> causes higher DC 
> conductor loss and higher AC losses at high
> frequencies due to skin 
> effect.  In addition, a larger percentage of the
> fields for tightly 
> coupled differential pairs are concentrated towards
> the edge of the 
> pair, increasing the field concentration here and
> thereby increasing the 
> conductor losses.
> 
> >Are you experts there driving gigabit links
> >through 2mm open pin field connectors!
> >Its very hard to believe. Or is it controlled
> >impedance connectors. But then are there any
> >controlled impedance 2mm connectors. Any 
> >inputs on what connectors you are using and
> >the highest data rate.
> >
> Teradyne, Molex, Tyco, FCI, Erni and many others
> make a number of 
> varieties of 2 mm connectors which have controlled
> impedance for 50 ohm 
> single-ended and 100 ohm differential signals. 
> We've used them 
> successfully for 2.5 and 3.125 Gbps system
> backplanes.  Some of the 
> newer connectors are capable of supporting nearly 10
> Gbps data transmission.
> 
> >How is impedance variation at connector, BGA
> >escapes and via transitions affected by 
> >coupling.
> >
> Tightly coupled differenential pair separation must
> be widened at the 
> point of insertion into these strucures.  Changing
> the separation of the 
> pairs causes a change in impedance.
> 
> >What resonance are you refering to, could you
> >please elaborate.
> >
> Whenever an impedance mismatch occurs a resonance
> structure is formed. 
>  Quarter-wave and half-wave resonance structures are
> created by these 
> mismatches.  These resonance modes can cause
> degradation in signal 
> waveshape and can couple to other similar resonance
> structures across 
> the board.  I'd refer you to the book: Microwave
> Engineering by Pozar 
> for more information on resonance.
> 
> 
> Good questions, Ben.
> 
> 
> 
> regards,
> 
> scott
> 
> -- 
> Scott McMorrow
> Principal Engineer
> SiQual Interconnect Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR  97062-3090
> (503) 885-1231
> http://www.siqual.com
> 
> 
> 
>
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