# Controlled Impedance Trace-reference plane

#### hoyyoth

Joined Mar 21, 2020
273
Dear Team,

May I know why do we need to place a reference plane immediately below a controlled impedance trace.
What will happen if it is not placed

Regards
HARI

#### Deleted member 115935

Joined Dec 31, 1969
0
Yes. Controlled impedance.

If you have a single conductor, then to have an impedance, you need a reference plane.
that could be a copper plane, or free space.

When you have two conductors, as in a differential pair, then you have two impedance, one between the pair, the differential impedance, and one to the reference plane, as above,

A differential pair in free space, the dominant impedance is the differential impedance, as there is little reference to a plane,
on a PCB, then there are both single ended and differential impedances on the pair, if the two tracks are far apart, but clos to a ref plane, then they impedance might be constrained by the single ended impedance.

For instance, things like ethernet after the transformer , or differential microphone inputs typically are differential , with no plane as reference.

I'd say, if your asking, don't stray from the "standard" traces over a solid reference plane,

https://en.wikipedia.org/wiki/Stripline

https://en.wikipedia.org/wiki/Microstrip

• hoyyoth

#### hoyyoth

Joined Mar 21, 2020
273
Yes. Controlled impedance.

If you have a single conductor, then to have an impedance, you need a reference plane.
that could be a copper plane, or free space.

When you have two conductors, as in a differential pair, then you have two impedance, one between the pair, the differential impedance, and one to the reference plane, as above,

A differential pair in free space, the dominant impedance is the differential impedance, as there is little reference to a plane,
on a PCB, then there are both single ended and differential impedances on the pair, if the two tracks are far apart, but clos to a ref plane, then they impedance might be constrained by the single ended impedance.

For instance, things like ethernet after the transformer , or differential microphone inputs typically are differential , with no plane as reference.

I'd say, if your asking, don't stray from the "standard" traces over a solid reference plane,

https://en.wikipedia.org/wiki/Stripline

https://en.wikipedia.org/wiki/Microstrip
Thank you