[SI-LIST] Re: ååï Re: differential signaling (common-mode)
- From: Larry Miller <ldmiller@xxxxxxx>
- To: mike_bihan@xxxxxxxxxxxx
- Date: Sun, 31 Jul 2005 08:26:22 -0700
The common mode impedance of CAT-5 UTP cable is about 1000-3000 Ohms =20
(cable-to-earth). This is sufficiently high and lossy enough that no =20
significant resonance results. This is achieved by the twisting in =20
the pairs and is just "leakage". At the ends the transformers have =20
common-mode chokes (coupled windings that pass differential signals =20
but "cancel" common-mode signals) to keep the impedance high.
The idea of having a "floating" system is to allow for significant =20
ground potential differences between the two ends of a link. This =20
often happens running between buildings powered by different AC mains =20=
systems. The signal transformers are required to provide up to 1500 =20
Volts DC of isolation with no adverse effects (arcing, excessive =20
currents); this can be something of a challenge for PCB layout.
For Ethernet up to 100 Mbps, only 4 of the 8 wires in the cable are =20
used. The other 4 have the so-called "Bob Smith" termination (50 Ohms =20=
line-to-line in a pair, 100 Ohms pair to pair) to prevent parasitic =20
radiation of signals coupled from the active signal lines. Without =20
this termination the cable will not meet EMI radiation specification =20
limits. For 1Gbps Ethernet all 8 wires are used and this termination =20
is not used.
Larry Miller
On Jul 31, 2005, at 4:53 AM, Bi Han wrote:
> Steve:
>
> In twisted pair, is there any predictable common mode return path?
>
> If there is not, how they control the CM reflection? In Dr. =20
> Howard's book,
> there is transformer and common mode choke between Tx and cable. I =20
> guess that there might be huge CM resonance in the cable, however, =20
> they are rejected at Tx/Rx side, right?
>
> thanks,
> Han
>
>
>
> steve weir <weirsi@xxxxxxxxxx> =D0=B4=B5=C0=A3=BA
> Bi,
>
> Whether you model it or not in a real chip and board there will be =20
> a CM
> return path. You would be well advised to control it within the =20
> package so
> that it can match what is achievable on the board. Then you can =20
> terminate it.
>
> In unshielded twisted pair applications the transformer buys a huge =20=
> CMR.
>
> Steve.
> At 11:32 AM 7/30/2005 +0800, Bi Han wrote:
>
>> Hi, SI-listers:
>>
>> The design task is to design on-chip differential transmission =20
>> line to
>> send signal. According to a lot of papers, edge coupled stripline =20
>> is used.
>>
>> The cross-section is as below graph(might be lost). One key thing =20
>> is that
>> there is no underneath common-mode return path.
>>
>> ___________________________________________________
>>
>> ______ ______
>> dielectric |__+__| |__-___|
>> ___________________________________________________
>>
>>
>> Hi-Z substrate
>>
>> ___________________________________________________
>>
>>
>> Then what is going to happen to common-mode signal? Common mode =20
>> should not
>> be a propagating mode, however, its S21 parameter after several
>> reflections should be as below.
>>
>> S21=3DA*Hx*T/(1-R2*R1*Hx*Hx)
>>
>> R1 is source reflection coefficeint;
>>
>> R2 is load reflection coefficeint;
>>
>> Hx is propagation function, Exp(-gamma*L);
>>
>> Common mode impedance will be quite high if there is no good =20
>> return path.
>> Therefore, both source and load are not well terminated. Source =20
>> could be
>> very like short, and load could be very like open.
>>
>> Then R1=3D-1, R2=3D1; the S parameter could go infinity if Hx(w)=3D-1; =
=20
>> It could
>> resonate;
>>
>> It looks that twisted pair could survive this condition and goes =20
>> well,
>> why? Also several IEEE papers also used above configuration on-=20
>> chip and
>> goes fine, why?
>>
>>
>>
>>
>> ---------------------------------
>> DO YOU YAHOO!?
>> =D1=C5=BB=A2=C3=E2=B7=D1G=D3=CA=CF=E4=A3 =
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>>
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>
>
>
> ---------------------------------
> DO YOU YAHOO!?
> =D1=C5=BB=A2=C3=E2=B7=D1G=D3=CA=CF=E4=A3 =
=D6=D0=B9=FA=B5=DA=D2=BB=BE=F8=CE=DE=C0=AC=BB=F8=D3=CA=BC=FE=C9=A7=C8=C5=B3=
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