[SI-LIST] Re: Power plane / VCC plane as reference for differencial microstrips
- From: Riaz Sobrany <r.sobrany@xxxxxxxxxxxxxxxx>
- To: Niki Steenkamp <niki@xxxxxxxxxxxxxx>
- Date: Mon, 23 Aug 2004 16:13:12 +0100 (BST)
On Mon, 23 Aug 2004, Niki Steenkamp wrote:
> I have another question (of a more theoretical nature): If you look at =
> a
> differential signal, then the ground connection is only really needed to
> carry the even mode currents since odd mode currents cancel out. So if =
> you
> ignore the even mode currents (lets say we have perfectly balanced =
> drivers),
> then you do not need a reference plane for return current.
That is correct. A ground plane is not required for a balanced
transmission line and it will successfully convey odd mode signals
without one. Unshielded twisted pair or the 300 ohm flat antenna downleads are
examples of balanced transmission lines without a ground plane.
However, such transmission lines will continue to convey even mode signals
because in practice some grounded structure will be nearby and will act as
a ground plane for even mode signals. The even mode characteristic
impedance will be very high which will reduce the amount of even mode
signals at the load end but will result in them radiating due to lack of
confinement between the transmission line and (unintentional) ground
plane.
> With a =
> microstrip
> on a PCB, the reference plane is however rerquired to get the impedance =
> right
> - the net current in the reference plane is however 0. Is this sort of
> right?
Not in reality. When a balanced transmission line with a ground plane is
driven in the odd mode then an eddy current circulates in the ground
plane. With some geometries such as shielded twisted pair then ground
plane currents almost cancel out but with two parallel PCB traces then
much of the current doesn't cancel out. There have been a few papers
published about this bizarre effect.
> Would you still be able to get a 100ohm differential impedance =
> if you
> did not have a reference plane (say with a single layer PCB)?
In theory yes.
> I would =
> guess
> that it is possible, but you would have to place the traces extremely =
> close
> together to get enough coupling. =20
That is true to create the desired odd mode capacitance per unit length.
A controversy exists amongst engineers whether balanced transmission lines
on a PCB should have their own ground plane or not. I myself think they
should have their own ground plane.
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