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Request clarification on signal transmission on coupled paired microstrips

Could some electronics guru please clarify the following ? I am working on some SPICE simulations of signal propagation down coupled parallel microstrips.Microstrips are modelled as lossy transmission lines.

As we know there are two propagation modes.

  1. Even - signals in the same direction. No problems at all.

  1. Odd - signals propagate in opposite directions. My questions are with regard to how to correctly model this case in SPICE, specifically, applying the signals. First,for two coupled parallel microstrips, there are two input ports(1, 2) and two output ports(3, 4). Q1: To model the odd mode, is one input applied to one input(port 1), but the second input applied to port 4, to ensure signals travelin opposite directions ? Q2 Is the signal applied to port 3 in phase with that applied to port 1 ?

Is there an alternative way to model thi?

Any hints/suggestions would be of immense help. Thanks in advance.

Reply to
dakupoto
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You've misunderstood that. A pair of odd mode signals still propagate together in the same direction. They just have opposite polarities.

'Odd mode' is RF jargon for 'differential'.

Jeroen Belleman

Reply to
Jeroen Belleman

I have a project now where the customer insists on using a shielded twisted pair but the signal will not be well balanced. It's ugly. The even and odd modes both matter.

This is my model for the diff pair:

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It takes three Spice txlines to model both modes.

In my case, the lines have to wind up 100 ohm differential, but there can be various common-mode impedances, depending on how the cable is actually constructed. One extreme is two separately shielded 50 ohm coaxes; the other extreme is a tight twisted pair with no shield except the uinverse.

Phil: this will be a bootstrapped photodiode at the end of a shielded twisted pair. I said it was ugly!

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Right. Even mode is basically both wires connected together at the ends, working against the shield like a coax. Odd mode is a differential signal applied symmetrically to the wires of the pair.

They can have different impedances and different velocities.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

They always do.

What, you design your differential signaling paths without consideration of common mode dynamics?

For shame.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Yikes. If you need me to yell at your customer, let me know. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

And they want it to be fast. It is very weird.

I can send you the sim privately, just for fun, if you are interested and will run it well between meals. You are NDA'd with these guys already.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

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