[SI-LIST] Re: Equations for creating twisted pair transmission lines?
- From: <rolf_harjung@xxxxxxxxxxx>
- To: <tom_cip_11551@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 5 Aug 2004 13:22:35 +0200
Hello Tom,
here is a formula I found in the reference book:
Brian C. Wadell: Transmission Line Design Handbook, chapter 3.3.3
-------------------------------------------------------------------
A symmetric twisted pair
_______ _______
/ e_r \ air / e_r \ =20
/ ___ \ / ___ \ =20
/ /XXX\ \ / /XXX\ \=20
| |XXXXX| | |XXXXX| |
| |XXXXX| | |XXXXX| |
\ \XXX/ / \ \XXX/ /=20
\ / \ / =20
\_______/ | \_______/ |
| |
| | | | =20
| d | | D |
|-----| |-------------|
=20
with=20
d =3D outer diameter of signal wire
D =3D outer diameter of dielectric
er=3D relative dielectric constant
T =3D number of twists per length
has the characteristic impedance
___________ =20
377 Ohm / D _ | D^2 \
Z0=3D ------------- * ln | --- + \ |----- -1.0 |
PI*sqrt(e_eff) \ d \| d^2 /
=20
e_eff=3D1.0+q*(e_r-1.0)
q=3D0.25+0.0004*(theta)^2
theta=3Datan(T*PI*D)
Note: For the softer PTFE insulation the eqation
q=3D0.25+0.001*(theta)^2=20
should be used.
-------------------------------------------------------------------
Kind regards,
Rolf
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of tom_cip_11551
Sent: Mittwoch, 4. August 2004 19:19
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Equations for creating twisted pair transmission
lines?
Hi,
I have been creating transmission lines on PCBs for longer than most=20
people that use this group have been around. However, I now have the=20
need to use a twisted pair cable for the transmission of a 270 Mbps=20
digital data stream (SMPTE 259M digital video to be specific). I am=20
sure that there must be an easy way to account for characteristic=20
impedance in terms of cable types and thicknesses and twists per=20
inch, etc. I just have not been able to find a good reference (can't=20
cut through the advertiesments for cable sales).
Thank You
Tom
tom_cip_11551@xxxxxxxxxxx
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=20
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